Mass spectra and strong decays of charmed and charmed-strange mesons
Abstract
A semi-relativistic potential model is adopted to calculate the mass spectra of charmed and charmed-strange meson states up to the excitations. The strong decay properties are further analyzed with a chiral quark model by using the numerical wave functions obtained from the potential model. By using the strong decay amplitudes extracted from the chiral quark model, we also systematically study the coupled-channel effects on the bare masses of the -wave states, since the masses of and cannot be explained with bare -wave states within the potential model. Based on our good descriptions of the mass and decay properties for the low-lying well-established states, we give a quark model classification for the high mass resonances observed in recent years. In the -meson family, can be classified as the radially excited state ; and can be classified as the second orbital excitations and , respectively; may be a candidate of or ; while may favor the high mass mixed state ; however, there still exist puzzles for understanding the natures of and , whose decay properties cannot be well explained with either pure and states or their mixing. In the -meson family, favors the assignment; and may favor the mixed states and via the - mixing, respectively; may favor or , or corresponds to a structure contributed by both and ; the newly observed resonance as an assignment of , by including coupled-channel effects the mass of is close to the observed value, however, the width cannot be well understood in the present study. Many missing excited - and -meson states have a relatively narrow width, they are most likely to be observed in their dominant decay channels in future experiments.
I Introduction
In the past 15 years, significant progress has been achieved in the observations of the and meson spectra. More and more higher excitations have been found in experiments. In the -meson family, several new signals , , and were observed for the first time by the BaBar collaboration in 2010 BaBar:2010zpy , and were confirmed by the LHCb collaboration with slightly different masses in 2013 LHCb:2013jjb . The decay angular distributions show that both and should have an unnatural spin parity, while both and favor a natural spin parity. Furthermore, the LHCb collaboration observed two new higher -meson excitations, and , with natural and unnatural parities, respectively LHCb:2013jjb . In 2015, LHCb observed a new state with spin-parity numbers in the channel by analyzing the decay LHCb:2015eqv . In 2016, LHCb also observed a new state with in the channel by analyzing the decay Aaij:2016fma . The resonance parameters of are inconsistent with the previously observed resonance in Ref. LHCb:2013jjb . In 2019, a four-body amplitude analysis of the decay is performed by the LHCb collaboration Aaij:2019sqk . The spin-parity numbers for , , and were systematically determined to be , , and , respectively. The determined spin parity numbers for and are consistent with the determinations in the previous experiments Aaij:2016fma ; Aaij:2015sqa . In the Review of Particle Physics (RPP), , , and are labeled as , , and , respectively, by the particle data group (PDG) Zyla:2020zbs .
In the -meson sector, two new resonances/structures and observed in the channel by BaBar in 2006 BaBar:2006gme . The was confirmed by Belle one year later Belle:2007hht , and its spin-parity numbers were determined to be . In 2009, BaBar observed the decays and , and measured their branching fractions relative to the final state BaBar:2009rro . Meanwhile, a new broad higher -meson excitation was also reported by BaBar. In 2012, the existence of and was further confirmed by using the collision data at LHCb LHCb:2012uts . In 2014, by an analysis of decays, the LHCb collaboration found two resonances with and with in the final state LHCb:2014ott ; LHCb:2014ioa , which indicates that the structure previously observed by BaBar BaBar:2009rro ; BaBar:2006gme and LHCb LHCb:2012uts consists of at least these two resonances. In 2016, the resonance was observed in the channel by LHCb Aaij:2016utb , its resonance parameters and spin-parity numbers are consistent with the determinations for in Refs. LHCb:2014ott ; LHCb:2014ioa . Furthermore, LHCb also found weak evidence of consistent with an unnatural parity assignment Aaij:2016utb . Very recently, the LHCb collaboration observed a new excited meson with in decays LHCb:2020gnv . More experimental information about the excited charmed and charmed-strange mesons is collected in Table 1.
The experimental progress provides us good opportunities to establish an abundant and -meson spectrum up to the higher orbital and radial excitations. In theory, to understand the nature of the charmed and charmed-strange mesons, especially the newly observed states, and to establish the charmed and charmed-strange meson spectra, in the recent years a lot of studies have been carried out within various phenomenological models from several aspects, such as the mass spectrum Ebert:2009ua ; Liu:2013maa ; Liu:2015lka ; Liu:2015uya ; Liu:2016efm ; Badalian:2011tb ; Allosh:2021biq ; Patel:2021aas ; Chen:2018nnr ; Zhou:2014ytp ; Chen:2009zt ; Zhang:2009nu ; Godfrey:2015dva ; Song:2015nia ; Song:2015fha ; Gandhi:2019lta ; Sun:2013qca ; Ferretti:2015rsa ; Segovia:2015dia ; Li:2010vx ; Lu:2014zua ; Kher:2017wsq ; Eshraim:2014eka ; Shah:2014caa ; Yu:2020khh , strong decays Godfrey:2015dva ; Song:2015nia ; Song:2015fha ; Gandhi:2019lta ; Sun:2013qca ; Ferretti:2015rsa ; Segovia:2015dia ; Li:2010vx ; Lu:2014zua ; Kher:2017wsq ; Eshraim:2014eka ; Shah:2014caa ; Zhong:2008kd ; Xiao:2014ura ; Zhong:2009sk ; Zhong:2010vq ; Sun:2009tg ; Sun:2010pg ; Wang:2021orp ; Song:2014mha ; Colangelo:2012xi ; Colangelo:2010te ; Yu:2016mez ; Wang:2010ydc ; Wang:2013tka ; Yu:2014dda ; Wang:2014jua ; Wang:2016hkf ; Chen:2015lpa ; Chen:2011rr ; Zhao:2016mxc ; Tian:2017okw ; Li:2017sww ; Li:2009qu ; Godfrey:2013aaa ; Godfrey:2014fga ; Gandhi:2020vap ; Gupta:2018zlg ; Wang:2016enc ; Zhang:2016dom ; Li:2017zng ; Tan:2018lao ; Wang:2018psi ; Yu:2020khh , etc., some previous works can be found in Refs. Godfrey:1985xj ; Zeng:1994vj ; Gupta:1994mw ; Lahde:1999ih ; DiPierro:2001dwf ; Godfrey:2005ww ; Close:2005se ; Close:2006gr ; Vijande:2006hj ; Zhang:2006yj ; Li:2007px ; Wei:2006wa . Furthermore, spectroscopic calculations on the lattice are making steady progress Mohler:2011ke ; Moir:2013ub ; Kalinowski:2015bwa ; Cichy:2016bci . In the -meson family, the , , and resonances listed in RPP Zyla:2020zbs are usually considered to be the states, apart from a few disputes about and in recent works Gayer:2021xzv ; Albaladejo:2016lbb ; Du:2017zvv ; Du:2020pui . The newly observed resonance may be classified as the radially excited () state Kher:2017wsq ; Badalian:2011tb ; Liu:2015uya ; Yu:2014dda ; Chen:2011rr ; Gupta:2018zlg ; Wang:2010ydc ; Wang:2013tka ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua , although the width is underestimated in some works Li:2010vx ; Ferretti:2015rsa ; Zhong:2010vq ; DiPierro:2001dwf ; Sun:2010pg . The may favor the the state Ferretti:2015rsa ; Kher:2017wsq ; Badalian:2011tb ; Liu:2016efm ; Liu:2015uya ; Ebert:2009ua ; Gupta:2018zlg ; Yu:2014dda ; Chen:2015lpa ; Wang:2010ydc ; Wang:2013tka ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua , or a mixture via the mixing Li:2017sww ; Chen:2011rr ; Yu:2020khh ; Zhong:2010vq ; Xiao:2014ura ; Sun:2010pg ; Li:2010vx ; Chen:2015lpa . The resonance can be assigned to the state , while may correspond to a mixed state with Badalian:2011tb ; Liu:2016efm ; Liu:2015uya ; Liu:2015lka ; Zhong:2010vq ; Xiao:2014ura ; Wang:2010ydc ; Wang:2013tka ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua ; Wang:2016enc ; Li:2010vx ; Gandhi:2019lta ; Kher:2017wsq ; Yu:2016mez ; Yu:2014dda ; Gupta:2018zlg . The resonance is a good candidate for the state Godfrey:2015dva , while a few components of may exist via the mixing. The quark model classification of and is still controversial in the literature. The unnatural parity state is explained with the state Chen:2018nnr ; Godfrey:2015dva ; Lu:2014zua ; Song:2015fha , states with Yu:2014dda ; Xiao:2014ura ; Sun:2013qca ; Gupta:2018zlg ; Li:2017zng , or states with Liu:2016efm ; Liu:2015lka ; Liu:2015uya ; while the natural parity state is explained with Chen:2018nnr ; Song:2015fha , Gupta:2018zlg ; Sun:2013qca , Gandhi:2019lta ; Liu:2013maa ; Liu:2015uya ; Liu:2016efm , Yu:2014dda ; Godfrey:2015dva ; Xiao:2014ura , or Yu:2014dda , and so on.
On the other hand, in the -meson family, it is not controversial to classify the and resonances as states (high mass mixed state) and , respectively, however, the other two states and (low mass mixed state) classified in the quark model are not well established. Considering the positive parity resonances and first reported by BaBar BaBar:2003oey and CLEO CLEO:2003ggt as the and assignments, one finds their masses are too low to be comparable with the theoretical expectations. Some studies suggest that and are the mixtures of bare core and component Ortega:2016mms ; Yang:2021tvc . The new resonance with is suggested to be a strong candidate of the radial excitation by the collaboration LHCb:2020gnv , however, its measured mass and width are inconsistent with the recent theoretical predictions in Ref. Wang:2021orp . The and resonances may be identified as the and , respectively Ferretti:2015rsa ; Badalian:2011tb ; Liu:2016efm ; Liu:2015uya ; Ebert:2009ua ; Wang:2014jua ; Zhou:2014ytp ; Song:2014mha ; Segovia:2015dia ; Chen:2009zt ; Zhang:2009nu ; Godfrey:2015dva , or their mixtures Li:2017sww ; Li:2010vx ; Li:2007px ; Close:2006gr ; Zhong:2009sk ; Chen:2011rr ; Song:2015nia . The resonance can be classified as the state Liu:2015uya ; Ferretti:2015rsa ; Kher:2017wsq ; Chen:2009zt ; Badalian:2011tb ; Liu:2016efm ; Liu:2015lka ; Wang:2014jua ; Godfrey:2014fga ; Zhou:2014ytp ; Wang:2016hkf ; Wang:2016enc ; Song:2014mha ; Segovia:2015dia ; Godfrey:2015dva ; Xiao:2014ura . It should be mentioned that there still exist puzzles for the structures around 2.86 GeV, it may be contributed by all of the -wave states with Segovia:2015dia ; Gandhi:2020vap ; Zhong:2009sk . The higher resonance may be a candidate for the states with Ferretti:2015rsa ; Badalian:2011tb ; Kher:2017wsq ; Segovia:2015dia ; Chen:2009zt ; Liu:2016efm ; Liu:2015uya ; Ebert:2009ua ; Li:2010vx ; Song:2015nia ; Sun:2009tg ; Zhong:2009sk ; Li:2017zng ; Godfrey:2015dva ; Xiao:2014ura . More information about the status of the charmed and charmed-strange meson study can be found in the recent review work Chen:2016spr .
The recent LHCb experiments LHCb:2013jjb ; LHCb:2015eqv ; Aaij:2016fma ; Aaij:2019sqk ; Aaij:2015sqa ; LHCb:2014ott ; LHCb:2014ioa ; LHCb:2012uts ; Aaij:2016utb ; LHCb:2020gnv have demonstrated the capability of both discovering the and mesons and determining their properties. Thus, more and more progress in the observations of the excited - and -meson states will be achieved in forthcoming LHCb experiments. Stimulated by the recent progress in experiments, we have systematically analyzed the strong decay properties of the excited - and -meson states within a chiral quark model in Refs. Zhong:2008kd ; Zhong:2009sk ; Zhong:2010vq ; Xiao:2014ura , where the wave functions for the excited meson states are adopted the simple harmonic oscillator (SHO) forms, while their masses are referred to the quark model predictions in the literature. To deepen our study and more reliably understand the and -meson spectrum, in this work we carry out a combined analysis of both mass spectrum and strong decays. First, we calculate the mass spectrum within a semi-relativistic potential model, where the relativistic effects from the light quarks can be reasonably included. With this model the masses for the observed and -meson states can be described successfully. Then, by using the available wave functions and masses from the potential model, we calculate the OZI-allowed two-body strong decays of the excited and mesons with the chiral quark model. This model has been successfully applied to describe the strong decays of the heavy-light mesons and baryons li:2021hss ; Xiao:2020oif ; Wang:2019uaj ; Wang:2018fjm ; Xiao:2020gjo ; Wang:2020gkn ; Xiao:2018pwe ; Xiao:2014ura ; Zhong:2010vq ; Zhong:2008kd ; Zhong:2009sk ; Liu:2012sj ; Zhong:2007gp ; Xiao:2013xi ; Nagahiro:2016nsx ; Yao:2018jmc ; Wang:2017kfr ; Xiao:2017udy ; Wang:2017hej ; Liu:2019wdr . Based on our good descriptions of the mass and decay properties for the well-established states, we give our quark model classifications of the newly observed resonances/structures. Finally, according to our assignments for the newly observed resonances, we attempt to predict the properties of the missing resonances, which may be useful for future investigations in experiments.
Resonance | Mass (MeV) | Width (MeV) | Observed channel | Ref. | Time | ||
UN | BaBar BaBar:2010zpy | 2010 | |||||
UN | LHCb LHCb:2013jjb | 2013 | |||||
LHCb Aaij:2019sqk | 2019 | ||||||
, | N | BaBar BaBar:2010zpy | 2010 | ||||
N | BaBar BaBar:2010zpy | 2010 | |||||
N | LHCb LHCb:2013jjb | 2013 | |||||
LHCb Aaij:2016fma | 2016 | ||||||
LHCb Aaij:2019sqk | 2019 | ||||||
UN | BaBar BaBar:2010zpy | 2010 | |||||
UN | LHCb LHCb:2013jjb | 2013 | |||||
LHCb Aaij:2019sqk | 2019 | ||||||
? | BaBar BaBar:2010zpy | 2010 | |||||
? | BaBar BaBar:2010zpy | 2010 | |||||
, | N | LHCbLHCb:2013jjb | 2013 | ||||
N | LHCb LHCb:2013jjb | 2013 | |||||
LHCb Aaij:2015sqa | 2015 | ||||||
LHCb Aaij:2016fma | 2016 | ||||||
LHCb Aaij:2019sqk | 2019 | ||||||
LHCb LHCb:2015eqv | 2015 | ||||||
UN | LHCb LHCb:2013jjb | 2013 | |||||
? | LHCb LHCb:2013jjb | 2013 | |||||
? | LHCb LHCb:2013jjb | 2013 | |||||
LHCb Aaij:2016fma | 2016 | ||||||
LHCb LHCb:2020gnv | 2020 | ||||||
? | BaBar BaBar:2006gme | 2006 | |||||
Belle Belle:2007hht | 2007 | ||||||
N | BaBar BaBar:2009rro | 2009 | |||||
? | LHCb LHCb:2012uts | 2012 | |||||
BaBar Lees:2014abp | 2014 | ||||||
LHCb Aaij:2016utb | 2016 | ||||||
? | BaBar BaBar:2006gme | 2006 | |||||
N | BaBar BaBar:2009rro | 2009 | |||||
? | LHCb LHCb:2012uts | 2012 | |||||
LHCb LHCb:2014ott ; LHCb:2014ioa | 2014 | ||||||
LHCb LHCb:2014ott ; LHCb:2014ioa | 2014 | ||||||
LHCb Aaij:2016utb | 2016 | ||||||
? | BaBar BaBar:2009rro | 2009 |
This work is organized as follows. In Sec. II, the mass spectra for the charmed and charmed-strange mesons are calculated within a semi-relativistic quark model. In Sec. III, the strong decays are estimated within the chiral quark model. In Sec. IV, some discussions based on our numerical results are carried out. Finally, a summary is given in Sec. V.
II mass spectrum
II.1 model
In Ref. li:2021hss , we adopt a nonrelativistic linear potential model to calculate the - and -meson mass spectrum. It is found that the masses for the - and -meson states can be reasonably described within the nonrelativistic quark model li:2021hss , however, the effective harmonic oscillator parameters, , which are obtained by equating the root-mean-square radius of the harmonic oscillator wavefunction for the specified quantum numbers to the root-mean-square radius of the wavefunctions, are notably smaller than those from the relativized quark model Godfrey:2015dva . To consistently include the relativistic effects on the wavefunctions, the nonrelativistic Hamiltonian is replaced with the relativistic one
(1) |
where is the quark momentum in the center-of-mass system, is the distance between two quarks; and are the masses of light and heavy quarks, respectively; and the reduced mass .
The effective potential includes the spin-independent part and spin-dependent part . The spin-independent part is adopted the standard Cornell form Eichten:1978tg
(2) |
which includes the color Coulomb interaction and linear confinement, and zero point energy . The spin-dependent part is adopted the widely used form Godfrey:1985xj ; Swanson:2005 ; Godfrey:2004ya
(3) |
where
(4) |
is the spin-spin contact hyperfine potential. The tensor potential is adopted as
(5) |
The spin-orbit interaction can be decomposed into symmetric part and antisymmetric part :
(6) |
with
(7) | |||
(8) |
In these equations, is the relative orbital angular momentum of the system; and are the spins of the light and heavy quarks, respectively, and . The running coupling constant in the coordinate space is adopted a parameterized form as suggested in Ref. Godfrey:1985xj
(9) |
The parameters and are free parameters which can be fitted to make the behavior of the running coupling constant at short distance be consistent with the coupling constant in momentum space predicted by QCD. In this work we take , , , , , , which are the same as those adopted in Refs. Godfrey:1985xj , except that the parameter is slightly adjusted to better describe the mass spectrum. It should be mentioned that in the spin-dependent potentials we have replaced the light quark mass with to include some relativistic corrections to the potentials as suggested in Ref. Liu:2013maa . The parameter set { , , , , , } in the above potentials is determined by fitting the mass spectrum.
For the heavy-light meson system, the antisymmetric part of the spin-orbit potential, , can cause a configuration mixing between spin triplet and spin singlet defined in the - coupling scheme. Thus, the physical states and are expressed as
(10) |
where , and the is the mixing angle. In this work corresponds to the higher mass mixed state as often adopted in the literature. The mixing angle is perturbatively determined with the non-diagonal matrix element . It should be mentioned that the coupled-channel effects can cause a configuration mixing as well, we neglect these effects on the the mixing angle in our calculations.
II.2 numerical method
In this work we use the Gaussian expansion method Hiyama:2003cu to solve the radial Schrodinger equation for a meson system with quantum numbers of the orbital angular momentum and its component,
(11) |
The spatial wave function is expanded with a set of Gaussian basis functions,
(12) |
The Gaussian function with given range parameters is writhen as
(13) |
Transforming to the momentum space, one has
(14) |
The size parameters are set to be a geometric progression form Hiyama:2003cu
(15) |
There are three parameters . These parameters, the eigenenergy , and the expansion coefficients can be determined with the Rayleigh-Ritz variational principle by solving the generalized eigenvalue problem
(16) |
where and .
II.3 parameters
The model parameters adopted in this work are listed in Table 2. The parameter set {, , , , , , } for the -meson spectrum is determined by fitting the masses of the well established states , , , , and . While for the -meson sector, the parameter set is determined by fitting the masses of the well established states , , and together with the newly observed state . In the present work, the slope parameter and the running coupling constant for the -meson spectrum are set to be the same as those for the -meson spectrum, considering that they may be independent on a specific quark flavor. It should be pointed out that the zero-point-energy parameter is taken to be zero for the , , heavy quarkonium systems in the literature Deng:2016ktl ; Deng:2016stx ; Li:2019tbn ; Li:2019qsg . For these heavy quarkonium systems, the zero point energy can be absorbed into the constituent quark masses because it only affects the heavy quark masses slightly. However, if the zero point energy is absorbed into the meson systems containing light quarks, it can significantly change the light constituent quark masses, which play an important role in the spin-dependent potentials. Thus, to obtain a good description of both the masses and the hyperfine/fine splittings for the meson systems containing light quarks, a zero-point-energy parameter is usually adopted in the calculations. The slope parameter of the linear potential GeV2 determined in the present work is consistent with that of the relativized quark model Godfrey:1985xj , while is slightly larger than GeV2 adopted in the non-relativistic quark model Deng:2016ktl ; Deng:2016stx ; Li:2019tbn ; Li:2019qsg .
It should be mentioned that we cannot obtained stable solutions for some states due to the singular behavior of in the spin-dependent potentials. To overcome the singular behavior, following the method of our previous works Deng:2016ktl ; Deng:2016stx ; Li:2019tbn ; Li:2019qsg ; Li:2020xzs , we introduce a cutoff distance in the calculation. Within a small range , we let . By introducing the cutoff distance , we can nonperturbatively include the corrections from these spin-dependent potentials containing to both the mass and wave function of a meson state, which are crucial for our predicting the decay properties. It is found that the mass of the state is more sensitive to the cutoff distance due to its relatively larger factor than the other excited meson states. Thus, the cutoff parameters for the - and -meson spectra are determined by fitting the masses of the and . Note that when the other parameters are well determined, the masses of these states can be reliably worked out with the perturbation method li:2021hss without introducing the cutoff distance , although the wave functions obtain no corrections from the spin-dependent potentials containing . We obtain the masses and MeV for and , respectively. These masses calculated with the perturbation method are comparable with the predictions in Refs. Zeng:1994vj ; Lahde:1999ih . By fitting the masses and MeV of the and states obtained with the perturbation method, we determine the cutoff distance parameters to be and fm for the - and -meson spectra, respectively.
(GeV) | (GeV) | (GeV) | (GeV) | (GeV) | ||
(GeV) | (MeV) | (fm) | ||||
State | Ours | Exp Zyla:2020zbs | GM Godfrey:2015dva | EFG Ebert:2009ua | ZVR Zeng:1994vj | LJM Li:2010vx | LNR Lahde:1999ih | |
---|---|---|---|---|---|---|---|---|
State | Ours | Exp Zyla:2020zbs | GM Godfrey:2015dva | EFG Ebert:2009ua | ZVR Zeng:1994vj | LJM Li:2010vx | LNR Lahde:1999ih | |
---|---|---|---|---|---|---|---|---|
II.4 results
With the determined model parameters listed in Table 2, by solving the radial Schrödinger equation with the Gaussian expansion method Hiyama:2003cu we obtain the masses of the and meson states, which are listed in Table 3 and Table 4, respectively. For comparison, some other model predictions in Refs. Zyla:2020zbs ; Godfrey:2015dva ; Ebert:2009ua ; Zeng:1994vj ; Li:2010vx ; Lahde:1999ih and the data from RPP Zyla:2020zbs are listed in the same table as well. Furthermore, for clarity, the spectra are also shown in Figs. 1 and 2. It is shown that the masses for the well-established states together with the newly observed states can be reasonably described within the semi-relativistic quark model. Our results are also in good agreement with other quark model predictions, although there are some model dependencies in the predicted masses for the higher - and -wave states.
To compare the meson wave functions obtained in the present work with those obtained with the relativized quark model Godfrey:2015dva , we also extract the effective harmonic oscillator parameters of the harmonic oscillator wave functions by equating the rms radius of the harmonic oscillator wave function for the specified quantum numbers to the rms radius of the wave functions calculated from our potential model. Our obtained parameters together those from the relativized quark model Godfrey:2015dva are given in Table 5. It is found that the parameters of the harmonic oscillator wave functions estimated in this work are consistent with those determined with the relativized quark model Godfrey:2015dva .
State | Ours | GM Godfrey:2015dva | Ours | GM Godfrey:2015dva | |
---|---|---|---|---|---|
0.597 | 0.601 | 0.616 | 0.651 | ||
0.499 | 0.516 | 0.514 | 0.562 | ||
0.451 | 0.450 | 0.461 | 0.475 | ||
0.424 | 0.434 | 0.432 | 0.458 | ||
0.403 | 0.407 | 0.409 | 0.424 | ||
0.390 | 0.399 | 0.395 | 0.415 | ||
0.538 | 0.516 | 0.549 | 0.542 | ||
0.459,0.460 | 0.475, 0.482 | 0.468,0.469 | 0.498, 0.505 | ||
0.459,0.460 | 0.475, 0.482 | 0.468,0.469 | 0.498, 0.505 | ||
0.421 | 0.437 | 0.431 | 0.464 | ||
0.427 | 0.431 | 0.436 | 0.444 | ||
0.405,0.414 | 0.417, 0.419 | 0.413,0.420 | 0.433, 0.434 | ||
0.405,0.414 | 0.417, 0.419 | 0.413,0.420 | 0.433, 0.434 | ||
0.391 | 0.402 | 0.398 | 0.420 | ||
0.473 | 0.456 | 0.478 | 0.469 | ||
0.416,0.420 | 0.428, 0.433 | 0.424,0.428 | 0.444, 0.448 | ||
0.416,0.420 | 0.428, 0.433 | 0.424,0.428 | 0.444, 0.448 | ||
0.397 | 0.407 | 0.405 | 0.426 | ||
0.419 | 0.410 | 0.425 | 0.419 | ||
0.390,0.391 | 0.396, 0.399 | 0.396,0.398 | 0.408, 0.410 | ||
0.390,0.391 | 0.396, 0.399 | 0.396,0.398 | 0.408, 0.410 | ||
0.374 | 0.385 | 0.381 | 0.400 | ||
0.422 | 0.423 | 0.426 | 0.432 | ||
0.396,0.398 | 0.404, 0.407 | 0.402,0.404 | 0.417, 0.419 | ||
0.396,0.398 | 0.404, 0.407 | 0.402,0.404 | 0.417, 0.419 | ||
0.388 | 0.390 | 0.394 | 0.405 |
III STRONG DECAY
III.1 model
In this work, the Okubo-Zweig-Iizuka (OZI)-allowed two-body strong decays of the excited and meson states are calculated within a chiral quark model. The details of this model can be found in Refs. Zhong:2008kd ; Zhong:2007gp ; Zhong:2009sk ; Xiao:2014ura . In the chiral quark model Manohar:1983md , the low-energy quark-pseudo-scalar-meson interactions in the SU(3) flavor basis are described by the effective Lagrangian Li:1994cy ; Li:1997gda ; Zhao:2002id
(17) |
While the quark-vector-meson interactions in the SU(3) flavor basis are described by the effective Lagrangian Zhao:1998fn ; Zhao:2000tb ; Zhao:2001jw
(18) |
In the above effective Lagrangians, represents the th quark field in the hadron, is the pseudoscalar meson field, is the pseudoscalar meson decay constant, and represents the vector meson field. Parameters and denote the vector and tensor coupling strength, respectively.
To match the nonrelativistic wave functions of the heavy-light mesons, we should adopt the nonrelativistic form of the Lagrangians in the calculations. The nonrelativistic form of Eq. (17) is given by Li:1994cy ; Li:1997gda ; Zhao:2002id
(19) |
in the center-of-mass system of the initial hadron, where we have defined . On the other hand, from Eq. (18), the nonrelativistic transition operators for the emission of a transversely and longitudinally polarized vector meson are derived by Zhao:1998fn ; Zhao:2000tb ; Zhao:2001jw
(20) |
and
(21) |
In the above equations, q is the three-vector momentum of the final state pseudoscalar/vector meson; is the energy of final state pseudoscalar meson; is the internal momentum operator of the th quark in the heavy-light meson rest frame; is the spin operator for the th quark of the heavy-light system; and is a reduced mass given by with and for the masses of the th quark in the initial and final mesons, respectively. and represent the energy and mass of the final state heavy hadron, is the mass of the emitted vector meson. The plane wave part of the emitted light meson is , and is the flavor operator defined for the transitions in the SU(3) flavor space Li:1997gda ; Zhao:2002id ; Zhao:1998fn ; Zhao:2000tb ; Zhao:2001jw . The parameter in Eq. (20) is defined as . The chiral quark model has been successfully applied to describe the strong decays of the heavy-light mesons and baryons li:2021hss ; Xiao:2020oif ; Wang:2019uaj ; Wang:2018fjm ; Xiao:2020gjo ; Wang:2020gkn ; Xiao:2018pwe ; Xiao:2014ura ; Zhong:2010vq ; Zhong:2008kd ; Zhong:2009sk ; Liu:2012sj ; Zhong:2007gp ; Xiao:2013xi ; Nagahiro:2016nsx ; Yao:2018jmc ; Wang:2017kfr ; Xiao:2017udy ; Wang:2017hej ; Liu:2019wdr . It should be mentioned that the nonrelativistic form of quark-pseudoscalar-meson interactions expressed in Eq. (19) is similar to that of the pseudoscalar emission model DiPierro:2001dwf ; Godfrey:1985xj ; Koniuk:1979vy ; Capstick:2000qj ; Goity:1998jr , except that the factors and in this work have an explicit dependence on the energies of final hadrons.
For a light pseudoscalar meson emission in heavy-light meson strong decays, the partial decay width can be calculated with
(22) |
where is the transition amplitude, and and stand for the third components of the total angular momenta of the initial and final heavy-light mesons, respectively. as a global parameter accounts for the strength of the quark-meson couplings. Here, we take the same value as that determined in Refs.Xiao:2014ura ; Zhong:2007gp ; Zhong:2008kd , i.e., . While, for a light vector meson emission in heavy-light meson strong decays, the partial decay width can be calculated with
(23) |
To be consistent with the parameters of the mass calculations within the potential model, the masses of the component quarks are adopted as GeV, GeV and GeV. The decay constants for , and mesons are taken as MeV, MeV, respectively. For the quark-vector-meson coupling strength which still suffers relatively large uncertainties, we adopt the values extracted from vector meson photoproduction, i.e. and Zhao:1998fn ; Zhao:2000tb ; Zhao:2001jw . The masses of the mesons used in the calculations are adopted from RPP Zyla:2020zbs if there are observations, otherwise, the meson masses are adopted our predictions.
IV Discussion
State | Channel | Br (%) | Zyla:2020zbs | ||
---|---|---|---|---|---|
keV | MeV | ||||
as | Total | keV | MeV | ||
keV | keV | ||||
as | keV | keV | |||
Total | keV | keV | |||
as | |||||
Total | |||||
as | |||||
Total | |||||
Total | |||||
Total |
State | Channel | (MeV) | Br (%) | (MeV) Zyla:2020zbs | |
---|---|---|---|---|---|
Total | |||||
as | |||||
Total | |||||
Total | |||||
Total |
IV.1 -wave vector states
In the and families, the ground vector () charmed and charmed-strange states, and , are well established. The strong decay transition is kinematic forbidden. The charged state can decay into both the and final states. While the decays of the neutral are governed by the channel, however, the is kinematic forbidden. With the numerical wavefunctions determined from the potential model, the strong decays of and are calculated within the chiral quark model. As shown in Table 6, our predicted decay partial width of keV is consistent with the observation. While for , the predicted width of keV and the partial width ratio
(24) |
are in remarkable agreement with the experimental data keV and , respectively Zyla:2020zbs .
IV.2 -wave states
IV.2.1
In the -meson family, our predicted mass for the state is MeV, which is comparable with the predictions in the literature Zeng:1994vj ; Lahde:1999ih ; Ebert:2009ua ; Liu:2013maa ; Liu:2015lka ; Liu:2015uya ; Liu:2016efm ; Badalian:2011tb ; Allosh:2021biq ; Patel:2021aas ; Chen:2018nnr ; Godfrey:2015dva ; Song:2015fha ; Gandhi:2019lta ; Sun:2013qca ; Ferretti:2015rsa ; Li:2010vx ; Lu:2014zua ; Kher:2017wsq . The may dominantly decay into the and channels with a width of MeV. The partial width ratio between and is predicted to be
(25) |
Our predicted width of with the chiral quark model is close to the predictions within the models Chen:2011rr ; Yu:2014dda ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua , however, in these works the predicted decay rate into the channel is tiny.
The listed in RPP Zyla:2020zbs may be classified as the radially excited state in the -meson family. Its average measured mass and width are MeV and MeV, respectively Zyla:2020zbs . This state was first observed by the BaBar collaboration in the channel in 2010 BaBar:2010zpy , and was confirmed by the LHCb collaboration with significance by using collision data LHCb:2013jjb ; Aaij:2019sqk . As the assignment of , the mass of is consistent with various quark model predictions Kher:2017wsq ; Badalian:2011tb ; Liu:2015uya ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua . In Refs. Zhong:2008kd ; Zhong:2010vq ; Xiao:2014ura we have studied the strong decays of the as the state by using the SHO wave function, the obtained width, MeV, is too narrow to be comparable with the data. In the present work, with the genuine wave function determined by the potential model our predicted decay width of ,
(26) |
is close to the data MeV measured by BaBar BaBar:2010zpy . The is also explained as the state based on the strong decay analyses in the literature Yu:2014dda ; Chen:2011rr ; Gupta:2018zlg ; Wang:2010ydc ; Wang:2013tka ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua . According to our chiral quark model predictions, the has large decay rate () into the channel. Thus, to better understand the nature of the state and to test various model predictions, further observations of the missing channel are needed in future experiments.
In the charmed-strange sector, our predicted mass for the state is MeV, which is comparable with the predictions in the literature Zeng:1994vj ; Lahde:1999ih ; Ebert:2009ua ; Liu:2013maa ; Liu:2015lka ; Liu:2015uya ; Liu:2016efm ; Badalian:2011tb ; Allosh:2021biq ; Patel:2021aas ; Chen:2018nnr ; Godfrey:2015dva ; Song:2015nia ; Ferretti:2015rsa ; Segovia:2015dia ; Li:2010vx ; Kher:2017wsq . The decay channel is the only OZI-allowed two body strong channel for . With the wave function obtained from our potential model calculations, its width is predicted to be
(27) |
The state is also predicted to be a narrow state with a width of 10s MeV in the literature Godfrey:2015dva ; Song:2015nia ; Colangelo:2012xi ; Wang:2012wk ; Zhong:2008kd ; Xiao:2014ura .
Recently, the LHCb collaboration observed a new excited state, , in the invariant mass spectrum of the decay LHCb:2020gnv . Its mass, width and the spin parity numbers are measured to be MeV, MeV and , respectively. The is suggested to be a candidate of the missing state LHCb:2020gnv . However, considering as the state, it is found the observed mass is about MeV lower than most potential model predictions.
The coupling of the core to the two hadron final states was considered within the model in the literature Xie:2021dwe ; Ortega:2021fem . It is found that when taking into account the loop correction to the bare state, the physical mass will be close to that of . In this work, we also estimate the mass shift of by including the coupled-channel interaction within our chiral quark model. The details about the coupled-channel quark model are given in Appendix. A. The mass shift of is shown in Fig. 3. Our result shows that the coupled-channel interaction induces a mass shift of MeV. The bare mass MeV of will be shifted to the physical mass MeV, which is consistent with the measured mass of . Our coupled-channel calculation within the chiral quark model are consistent with that in Refs. Xie:2021dwe ; Ortega:2021fem .
Assigning the newly observed resonance to the state, the higher mass problem can be overcome by taking into account the loop correction, however, the width of cannot be well understood within our chiral quark model. Adopting the observed mass MeV, our predicted width,
(28) |
is about a factor of smaller than the center value of the data MeV. Our predicted width is consistent with the recent predictions with the relativistic wave functions obtained by solving the full Salpeter equation Wang:2021orp and the model Xie:2021dwe . To establish the state and uncover the nature of the , more observations are needed in future experiments.
IV.2.2
In the -meson family, our predicted mass for the state is MeV, which is comparable with the predictions in the literature Zeng:1994vj ; Lahde:1999ih ; Ebert:2009ua ; Liu:2013maa ; Liu:2015lka ; Liu:2015uya ; Liu:2016efm ; Badalian:2011tb ; Allosh:2021biq ; Patel:2021aas ; Chen:2018nnr ; Godfrey:2015dva ; Song:2015fha ; Gandhi:2019lta ; Sun:2013qca ; Ferretti:2015rsa ; Li:2010vx ; Lu:2014zua ; Kher:2017wsq . According to our chiral quark model predictions, the may be a narrow state with a width of
(29) |
and dominantly decays into and channels with branching fractions about and , respectively. However, the decay rate into the channel is tiny (). In Refs. Song:2015fha ; Godfrey:2015dva ; Lu:2014zua ; Li:2017sww ; Chen:2015lpa , the is predicted to be a broader state with a width of MeV. Combined with our previous study Zhong:2010vq , we find that the strong decay properties of are very sensitive to the details of the wave function due to the nodal effects.
From the point of view of mass, the resonance listed in RPP Zyla:2020zbs may be a candidate of the state. This resoancne was first observed by BaBar in the and decay channels in 2010 BaBar:2010zpy . The measured mass and width are MeV and MeV, respectively, and the measured partial width ratio between and is . In 2013, in the final state the LHCb collaboration observed a similar resonance with a mass of MeV and a width of MeV LHCb:2013jjb . In 2016, LHCb collaboration carried out an amplitude analysis of the decays, they extracted a resonance with mass and width of MeV and MeV Aaij:2016fma . Recently, from the decays, the LHCb collaboration also extracted a resonance with mass and width of MeV and MeV Aaij:2019sqk . The resonances observed in different experiments might be the same state, which is denoted by in RPP Zyla:2020zbs , although there are some differences in the observations of different experiments.
Considering as the assignment, the strong decay properties have been analyzed in the literature. The strong decay analyses in Refs. Ferretti:2015rsa ; Kher:2017wsq ; Gupta:2018zlg ; Yu:2014dda ; Chen:2015lpa ; Wang:2010ydc ; Wang:2013tka ; Song:2015fha ; Godfrey:2015dva ; Lu:2014zua support this assignment. However, with the assignment our predicted width MeV is too small to be comparable with the average measured value MeV. To well explain the decay properties, the is also suggested to be a mixed state via the mixing in the literature Li:2017sww ; Chen:2011rr ; Yu:2020khh ; Zhong:2010vq ; Xiao:2014ura ; Sun:2010pg ; Li:2010vx ; Chen:2015lpa . In Ref. Colangelo:2012xi , the study within effective Lagrangian method indicates that it is impossible to explain the ratio BaBar:2010zpy measured by BaBar with a pure state.
In the charmed-strange sector, our predicted mass for the state is MeV, which is comparable with the predictions in the literature Zeng:1994vj ; Lahde:1999ih ; Ebert:2009ua ; Liu:2013maa ; Liu:2015lka ; Liu:2015uya ; Liu:2016efm ; Badalian:2011tb ; Allosh:2021biq ; Patel:2021aas ; Chen:2018nnr ; Godfrey:2015dva ; Song:2015nia ; Ferretti:2015rsa ; Segovia:2015dia ; Li:2010vx ; Kher:2017wsq . According to our chiral quark model predictions, the may be a narrow state with a width of
(30) |
and mainly decays into the and final states. Our predictions are consistent with those predicted with a model Zhang:2006yj . However, in other works Godfrey:2015dva ; Close:2005se ; Chen:2015lpa ; Song:2015nia the is predicted to be a relatively broad state with a width of MeV.
From the point of view of mass, the resonance listed in RPP Zyla:2020zbs can be assigned to the state. The was first observed in the final state by the BaBar collaboration in 2006 BaBar:2006gme , and one year later its quantum numbers were determined by the Belle collaboration Belle:2007hht . The average measured mass and width are MeV and MeV, respectively Zyla:2020zbs . More experimental information about is collected in Table 1. Some phenomenological analyses in the literature Ferretti:2015rsa ; Badalian:2011tb ; Liu:2016efm ; Liu:2015uya ; Ebert:2009ua ; Wang:2014jua ; Zhou:2014ytp ; Song:2014mha ; Segovia:2015dia ; Chen:2009zt ; Zhang:2009nu ; Godfrey:2015dva support as the assignment. However, considering as , the measured width MeV and ratio cannot be explained within our chiral quark model. To well explain the observations, the is also suggested to be a mixed state via the mixing in the literature Li:2017sww ; Li:2010vx ; Li:2007px ; Close:2006gr ; Zhong:2009sk ; Chen:2011rr ; Song:2015nia . The mixing in the - and -meson families will be further discussed later.
IV.3 -wave states
IV.3.1
In the -meson family, the mass for the second radial excitation is predicted to be MeV within our potential model calculations. The mass gap between and is estimated to be MeV. Our predictions are consistent with those in Refs. Godfrey:2015dva ; Ebert:2009ua ; Zeng:1994vj . The may be a broad state with a width of
(31) |
and dominantly decays into the (29%), (24%) and (21%) final states. More details can be seen in Table 6. It should be mentioned that there exist large model dependencies in the decay properties predicted in the literature.
In Refs. Lu:2014zua ; Song:2015fha , the authors suggested that the unnatural parity state observed in the final state by the LHCb collaboration LHCb:2013jjb might be explained with according to their strong decay analysis within the model. However, our predicted width of is too broad to be comparable with the measured width MeV, although the predicted mass is consistent with the data. To establish the , more observations of the other main decay channels, such as and , are needed in future experiments.
In the -meson family, the mass for the second radial excitation is predicted to be MeV within our potential model calculations, which is about 100 MeV larger than that of the charmed partner . Our prediction is consistent with that of the relativized quark model Godfrey:2015dva . From Table 7, one sees that the state may have a width of
(32) |
and dominantly decays into the and final states. The main decay mode predicted within our chiral quark model are consistent with that predicted within the model Godfrey:2015dva , however, our predicted width is about a factor of 2.2 larger than that within the model Godfrey:2015dva . To look for the missing state, the final state is worth to observing in future experiments.
IV.3.2
In the -meson family, the mass for the second radial excitation is predicted to be MeV within our potential model calculations, which is consistent with the predictions in Refs. Godfrey:2015dva ; Ebert:2009ua ; Zeng:1994vj . The mass splitting between and is estimated to be MeV. From Table 6, it is seen that the may be a broad state with a width of
(33) |
and has large decay rates into and channels. To see the dependence of the decay properties of on its mass, the partial widths of the main decay channels together with the total width as functions of the mass are also plotted in Fig. 4. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV.
In Ref. Song:2015fha , the authors suggested that the natural parity state observed in the final state by the LHCb collaboration LHCb:2013jjb may be explained with according to their mass and strong decay analysis. However, our predicted mass and width of , MeV and MeV, are notably larger than the data MeV and MeV measured by LHCb LHCb:2013jjb . Furthermore, the predicted decay rates into the channel is tiny, which is inconsistent with the fact that was first observed in the final state. To establish the , more observations of the other decay channels, such as and , are needed in future experiments.
In the -meson sector, the mass for the second radial excitation is predicted to be MeV within our potential model calculations, which is consistent with the predictions in Refs. Godfrey:2015dva ; Zeng:1994vj . The mass splitting between and is estimated to be MeV, which is nearly equal to that for the charmed sector. The strong decay properties of have been shown in Table 7, it is seen that may be a relatively narrow state with a width of
(34) |
and mainly decays into and channels. The dependence of the decay properties of on its mass is also shown in Fig. 4. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. Our predicted decay properties are comparable with those of model in Refs. Godfrey:2015dva ; Song:2015nia . It should be mentioned that for the higher excited states, the predicted decay properties have large model dependencies.
IV.4 -wave states
State | Channel | (MeV) | Br (% ) | (MeV) Zyla:2020zbs | |
---|---|---|---|---|---|
as | Total | ||||
as | |||||
Total | |||||
as | Total | ||||
as | Total |
State | Channel | (MeV) | Br (%) | (MeV) Zyla:2020zbs | |
---|---|---|---|---|---|
Total | |||||
as | |||||
Total | |||||
Total | |||||
as | Total |
IV.4.1
The broad resonance listed in RPP Zyla:2020zbs is generally considered to be the state in -meson family. The neutral state with was first observed in the channel by the Belle collaboration in 2003 Belle:2003nsh , and was confirmed by the BaBar collaboration in 2009 BaBar:2009pnd . The charged state was also established in the channel by the FOCUS collaboration in 2003 FOCUS:2003gru , and was confirmed by the LHCb collaboration in 2015 Aaij:2015sqa . In experiments, only the channel is observed since the other OZI-allowed two-body strong channels are forbidden. The average measured mass and width of from RPP are MeV and MeV, respectively Zyla:2020zbs , which are consistent with the quark model expectations Close:2005se ; Tan:2018lao ; Zhong:2008kd ; Godfrey:2005ww ; DiPierro:2001dwf ; Goity:1998jr ; Song:2015nia ; Ebert:2009ua ; Lu:2014zua .
Our predicted mass of is MeV. Taking into account the loop correction to bare mass of the core, from Fig. 5 it is seen that the physical mass is reduced to MeV, there is a mass shift of MeV compared with the bare mass. The physical mass of the dressed state is about 90 MeV lower than the PDG average mass MeV Zyla:2020zbs , however, is close to measured value MeV from Belle and BaBar experiments Belle:2003nsh ; BaBar:2009pnd .
Taking the measured mass MeV and the wave function extracted from the potential model, we predict that the is a broad state with a width of MeV, which is about a factor of 2.3 larger than the average data MeV from RPP Zyla:2020zbs . It should be mentioned that the mass of measured from different collaborations is quite different, while the measured width also bears large uncertainties. In some recent works, the resonance was suggested to be a two-pole structure in chiral dynamics Albaladejo:2016lbb ; Du:2017zvv ; Du:2020pui . The recent Lattice calculations of the scattering amplitudes obtain a complex state resonance pole with a mass MeV and a width MeV Gayer:2021xzv . The mass and width are in contrast to the currently reported experimental results. To better understand the nature of , more accurate measurements are needed to be carried out in future experiments.
In the -meson sector, our predicted mass of is MeV, which is comparable with the predictions in the Refs. Godfrey:2015dva ; Ebert:2009ua ; Lahde:1999ih . The mass of state is about 100 MeV overestimated by the potential model if considering as the state. The mass calculated with lattice QCD also is significantly higher than than of Moir:2013ub . In Ref. Segovia:2015dia , the study indicates that including the one-loop corrections of the OGE potential the mass of will be reduced by about 130 MeV. Then, the mass of , MeV, is close to the that of . The study of scattering in full lattice QCD supports the interpretation of the as a molecule Liu:2012zya . Recently, Zhi Yang et al. studied the positive parity resonant states within the Hamiltonian effective field theory by combining it with the quark model, they found that may consist of the state, but at the same time couple to the channel Yang:2021tvc .
In this work, we also estimate the mass shift of by including the coupled-channel interaction within our chiral quark model. The mass shift is determined in Fig. 5. From the figure, we can see a cusp singularity in the mass shift curve of , this is a typical -wave mass shift, the formation mechanism was discussed in Ref. Isgur:1998kr . Our result shows that the large -wave coupling to channel of induces a mass shift of about MeV. The the physical mass of the dressed state is estimated to be MeV, which is very close to the measured mass of . Our coupled-channel analysis within the chiral quark model is consistent with that in Refs. Segovia:2015dia ; Ortega:2016mms ; Yang:2021tvc . Since the mass of is below the mass threshold of the channel, its extremely narrow width can be understood in theory.
IV.4.2
The resonance is assigned as the state of the -meson family. Our theoretical mass MeV and width MeV are in good agreement with the average measured values MeV and MeV from RPP Zyla:2020zbs . The dominantly decays into the and channels. The partial width ratio between and is predicted to be
(35) |
which is also in good agreement with the data Zyla:2020zbs . The decay properties predicted in this work are consistent with our previous predictions with the SHO wave functions Zhong:2008kd and other predictions in the models Close:2005se ; Godfrey:2005ww , and the PCAC and low energy theorem Zhang:2016dom .
The coupled-channel effects on the mass shift of are also studied. The results are shown in From Fig. 5. One can seen that the mass shift (i.e., MeV) is tiny when including the and loop corrections. There are two main reasons for the negligibly small coupled-channel contribution: (i) the mass of is far from the and thresholds; (ii) couples to and channels via a weak -wave coupling.
In the -meson family, the state has been well established in experiments. The narrow resonance listed in RPP Zyla:2020zbs should belong to the state. Our theoretical mass MeV and width MeV can well reproduce the average measured values MeV and MeV from RPP Zyla:2020zbs . The mainly decays into the channel, while the decay rate into channel is sizeable. Our predicted partial width ratio between and ,
(36) |
is also consistent with the data Zyla:2020zbs . The decay properties predicted with the genuine wave function from our potential model calculations in this work are consistent with our previous predictions with the SHO wave functions Zhong:2008kd , and other predictions by using the models Close:2005se ; Godfrey:2005ww and the PCAC and low energy theorem Zhang:2016dom .
Finally, we also study the coupled-channel effects on the mass of the bare state. From the strong decay analysis we know that mainly decays into and channels. Considering the core coupling to these channels, a tiny mass shift MeV can be seen in Fig. 5. The tiny couple-channel effects on are mainly due to a weak -wave coupling to and channels. Our conclusion is consistent with that of the recent study Yang:2021tvc .
IV.4.3 and
In the -wave states, the two states and should be mixed with each other by the antisymmetric part of the spin-orbit potential. The physical states and states are expressed as
(37) |
In this work, the and correspond to the low-mass and high-mass mixed states, respectively.
In the -meson family, the masses for the two mixed states and are determined to be MeV and MeV, respectively. The mass splitting between and is estimated to be MeV, which is close to the prediction in Ref. Ebert:2009ua . The mixing angle determined in this work is similar to the determinations in Refs. Lu:2014zua ; Godfrey:2015dva , however, is about a factor of smaller than the value extracted in the heavy quark limit. The low mass state should be a broad state with a width of about several hundred MeV, while the high mass state is a narrow state with a width of about several tens MeV. The channel is the only OZI-allowed two-body strong decay channel of and .
The and resonances listed in RPP Zyla:2020zbs can be assigned to the -wave mixed state and , respectively. For the broad resonance , its average measured mass and width are MeV and MeV, respectively Zyla:2020zbs . While for the narrow resonance , its average measured mass and width are MeV and MeV, respectively Zyla:2020zbs . The average mass splitting from the measurements, MeV, is smaller than our potential model prediction MeV.
Taking the physical masses and predicted mixing angle for and , we calculate the strong decay properties, our results are listed in Table 8. It is seen that our predicted decay width of are in good agreement with the data, however, the predicted width of is slightly smaller than the lower limit of the measured width MeV. With the mixing angle extracted in the heavy quark limit, the theoretical width of , MeV, is more close to the data. The predicted width of , MeV, is slightly smaller than the PDG average value MeV, however, is in good agreement with the measurements, MeV, from Belle, BESIII, CDF, CLEO listed by PDG Zyla:2020zbs . In Fig. 6, we show the dependence of the decay widths of the and resonances on the mixing angle. It is found that the decay widths are sensitive to the mixing angle. Taking the mixing angle around value extracted in the heavy quark limit, i.e. , the predicted decay properties of the and resonances are consistent with the measurements.
The coupled-channel effects on the masses of the two states and are further studied. Our results are shown in Fig 5. It is seen that including the loop correction, the mass shifts of and are predicted to be and MeV, respectively. There are only small corrections of the coupled-channel effects to the masses of and , because their masses are far from the threshold.
In the -meson family, the masses for the two mixed states and are estimated to be MeV and MeV, respectively. Their masses is very close to the mass threshold of . The mass splitting between and is estimated to be MeV, which is close to the predictions in Refs. Zeng:1994vj ; Lahde:1999ih . The mixing angle is almost the same as that of the charmed sector, and is consistent with the determinations in Ref. Godfrey:2015dva . Adopting this mixing angle, masses, and wave functions determined from our potential model calculations, we study the two-body OZI-allowed strong decays, our results are listed in Table 9. It is found that the low mass state may be a broad state with a width of MeV, while the high mass state may be a narrow state with a width of MeV. The channel is the only OZI-allowed two-body strong decay channel of and .
The resonance can be assigned to the high mass state . When the mixing angle predicted by our potential model is taken into account, the width of is MeV, which is significantly larger than MeV. From Fig. 6, one can find that the decay width of is very sensitive to the mixing angle. Taking the mixing angle around value extracted in the heavy quark limit, i.e. , we find that the theoretical width MeV is consistent with the measured width of MeV. Our recent analysis of and also shows that as the -wave mixed states their mixing angle more favors li:2021hss . Thus, for the heavy-light mesons, the mixing angle of the -wave states seems to be close to the value extracted in the heavy quark limit. It should be mentioned that it is still a puzzle for the mixing between the and states, which cannot be well understood within the various potential models.
The simple potential model overestimates the mass of the low-mass state if considering as a candidate of it. The mass calculated with lattice QCD also is significantly higher than that of Moir:2013ub . The coupled-channel effects may play an important role because of the closeness behavior for the bare and threshold. Recently, Zhi Yang et al. studied these effects within the Hamiltonian effective field theory by combining it with the quark model Yang:2021tvc . They found that the resonance may consist of the state, but at the same time couples to the channel Yang:2021tvc . In the present work, including the loop we also study the coupled-channel effects on the mass shifts of the two states. Our results are shown in Fig. 5. It is found that there is a MeV correction to the bare mass of due to a strong -wave interaction. The physical mass of the dressed is reduced to MeV, which is close to the measured mass of . It should be mentioned that coupled-channel effects on are negligibly small due to its weak coupling with the channel. Our coupled-channel analysis within the chiral quark model is consistent with that in Ref. Yang:2021tvc . Since the mass of is below the threshold, it becomes an extremely narrow state.
IV.5 -wave states
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
609.8 | 56.6 | 52.0 | 26.9 | – | – | – | – | ||
190.8 | 17.7 | 6.0 | 3.1 | – | – | – | – | ||
82.8 | 7.7 | 2.9 | 1.5 | – | – | – | – | ||
17.8 | 1.7 | 0.03 | 0.02 | – | – | – | – | ||
– | – | 23.7 | 12.3 | 199.5 | 31.4 | 77.2 | 32.0 | ||
– | – | 1.0 | 0.5 | 53.6 | 8.4 | 21.3 | 8.8 | ||
– | – | 0.5 | 0.3 | 27.6 | 4.3 | 10.6 | 4.4 | ||
151.0 | 14.0 | 7.5 | 3.9 | – | – | – | – | ||
– | – | 5.9 | 3.1 | 88.9 | 14.0 | 45.6 | 18.9 | ||
– | – | – | – | 22.8 | 3.6 | 2.0 | 0.8 | ||
– | – | – | – | – | – | ||||
– | – | – | – | 0.7 | 0.1 | ||||
– | – | 9.4 | 4.9 | 21.5 | 3.4 | 23.6 | 9.8 | ||
2.5 | 0.2 | 4.1 | 2.1 | 3.2 | 0.5 | 11.3 | 4.7 | ||
14.7 | 1.4 | 6.4 | 3.3 | 2.2 | 0.3 | 1.8 | 0.7 | ||
– | – | 14.6 | 7.5 | 138.4 | 21.8 | 4.7 | 1.9 | ||
– | – | 4.3 | 2.2 | 44.4 | 7.0 | 1.7 | 0.7 | ||
– | – | 0.4 | 0.2 | 25.6 | 4.0 | 5.1 | 2.1 | ||
6.5 | 0.6 | 41.4 | 21.4 | 5.0 | 0.8 | 28.7 | 11.9 | ||
2.3 | 0.2 | 13.3 | 6.9 | 1.4 | 0.2 | 7.7 | 3.2 | ||
Total |
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
549.8 | 78.3 | 30.9 | 29.8 | – | – | – | – | ||
117.4 | 16.7 | 3.0 | 2.9 | – | – | – | – | ||
18.0 | 2.6 | 0.03 | 0.03 | – | – | – | – | ||
– | – | 9.1 | 8.8 | 176.5 | 44.4 | 77.4 | 56.7 | ||
– | – | 0.4 | 0.4 | 36.7 | 9.2 | 16.6 | 12.2 | ||
– | – | – | – | 18.7 | 4.7 | 0.2 | 0.1 | ||
– | – | – | – | 0.6 | 0.2 | ||||
– | – | 2.0 | 1.9 | 11.6 | 2.9 | 4.0 | 2.9 | ||
0.5 | 0.07 | 6.7 | 6.5 | 0.6 | 0.2 | 4.4 | 3.2 | ||
3.9 | 0.6 | 2.1 | 2.0 | 1.1 | 0.3 | 7.8 | 5.7 | ||
– | – | 10.2 | 9.8 | 137.9 | 34.7 | 5.2 | 3.8 | ||
– | – | 0.09 | 0.09 | 11.2 | 2.8 | 2.5 | 1.8 | ||
13.0 | 1.9 | 39.4 | 38.0 | 2.5 | 0.6 | 18.5 | 13.5 | ||
Total |
Total | |||||
---|---|---|---|---|---|
IV.5.1
In the -meson family, our predicted mass for the state, MeV, is comparable with the predictions in Refs. Liu:2015uya ; Liu:2016efm ; Ebert:2009ua ; Song:2015fha , however, is about 100 MeV larger than the predictions in Refs. Zeng:1994vj ; Li:2010vx ; Lahde:1999ih . The mass gap between and is estimated to be MeV, which is consistent with those predicted in Refs. Godfrey:2015dva ; Ebert:2009ua ; Zeng:1994vj . The may be a very broad state with a width of
(38) |
and dominantly decays into the , and final states. More details of the decay properties can be seen in Table 10. In Refs. Xiao:2014ura ; Lu:2014zua , the is also predicted to be a very broad state with a width of MeV, although the predicted partial widths show some model dependencies.
In Ref. Sun:2013qca , the authors suggested that the natural parity state observed in the final state by the LHCb collaboration LHCb:2013jjb may be explained with according to their mass and strong decay analysis. However, both our predicted mass and width of , MeV and MeV, are inconsistent with the data MeV and MeV LHCb:2013jjb . According to our prediction, the may be difficult to be established in experiments due to its rather broad width.
In the -meson sector, our predicted mass for the , MeV, is comparable with the predictions in Refs. Liu:2016efm ; Zeng:1994vj ; Lahde:1999ih . Our predicted strong decay properties have been shown in Table 11. It is found that the may be a broad state with a width of
(39) |
and dominantly decays into the and final states. The width predicted in the present work is notably than our previous prediction MeV with a SHO wave function Xiao:2014ura , which indicates that the decay properties of are very sensitive to the details of its wave function adopted in the calculations.
It should be mentioned that there are strong model dependencies in the strong decay predictions. For example, within the model the and states may be relatively narrow states with a width of MeV Godfrey:2015dva .
IV.5.2
In the -meson family, our predicted mass of is MeV, which is consistent with the predictions in Refs. Li:2010vx ; Liu:2015uya ; Liu:2016efm ; Godfrey:2015dva ; Zeng:1994vj . Our predicted strong decay properties have been shown in Table 10. We find that the state is a relatively narrow state with a width of
(40) |
and dominantly decays into the and final states with branching fractions and , respectively. There are some differences between the predictions in this work and those obtained with the SHO wave function in our previous work Xiao:2014ura , where the decay rate into is tiny. It indicates that the decay properties are sensitive to the details of the wave function of . Furthermore, to see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 7. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range of MeV. The decay properties predicted within various models show large model dependencies. Within the model the width of is predicted to be in the range of MeV Godfrey:2015dva ; Sun:2013qca ; Song:2015fha .
From the point of view of mass, the resonance with a natural parity observed by the LHCb collaboration in 2013 LHCb:2013jjb might be a candidate of the state. Our predicted mass MeV of is close to the measured value MeV of . Taking as the state, we further study its strong decay properties, our results are listed in Table 12. It is found that should dominantly decay into the channel with a branching fraction of , which is consistent with the observations. However, our predicted width,
(41) |
is notably larger than the data MeV measured by LHCb LHCb:2013jjb . The as the assignment is suggested in Refs. Gandhi:2019lta ; Liu:2013maa ; Liu:2015uya ; Liu:2016efm . If corresponds to indeed, it may have a large decay rate into the channel as well, the partial width ratio between and is predicted to be
(42) |
which may be useful to test the nature of .
In the -meson sector, the mass of is predicted to be MeV, which is comparable with the predictions in Refs. Godfrey:2015dva ; Ebert:2009ua ; Li:2010vx . Our predicted strong decay properties have been shown in Table 11. It is found that the may be a relatively narrow state with a width of
(43) |
and dominantly decays into the and final states with branching fractions and respectively. There may be a sizeable decay rate () into the channel as well. The , and are also predicted to be the main decay channels in the other works Godfrey:2015dva ; Ferretti:2015rsa , although the predicted partial width ratios are very different with each other. There are some differences between the predictions in this work and those obtained with the SHO wave function in our previous work Xiao:2014ura , where the decay rate into is tiny. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 7. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range of MeV. It should be mentioned that the typical mass gap between and is around 100 MeV. If corresponds to indeed, the mass of is most likely to be MeV. Searching for the missing may be helpful to understand the nature of . To establish the missing state, the main decay channels, such as and , are worth to observing in future experiments.
IV.5.3 and
The physical states and are mixed states between states and via the following mixing scheme:
(44) |
In this work, the and correspond to the low-mass and high-mass mixed states, respectively.
In the -meson family, the masses for the two mixed states and are determined to be MeV and MeV, respectively. The mass splitting between and is estimated to be MeV, which is close to the predictions in Refs. Li:2010vx ; Godfrey:2015dva . The mixing angle determined in this work is similar to the determinations in Refs. Lu:2014zua ; Godfrey:2015dva . Our predicted strong decay properties have been shown in Table 10. The low mass state should be a broad state with a width of about MeV, while the high mass state is a relatively narrow state with a width of
(45) |
Both and have large decay rates into the , , , and channels. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 7. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. It should be mentioned that the predicted decay properties strongly depend on the approaches adopted in the literature Xiao:2014ura ; Lu:2014zua ; Godfrey:2015dva ; Song:2015fha .
It is interesting to find that the resonance observed in the channel at LHCb LHCb:2013jjb might be a good candidate of the high mass mixed state . With this assignment, both our predicted mass and width are consistent with the data MeV and MeV LHCb:2013jjb . The mainly decays into and channels, which can naturedly explain why is first observed in the channel. It should be mentioned that in our previous work the study with the SHO wave function shows that may favor the low mass mixed state Xiao:2014ura . However, in the present work, with this assignment both our predicted mass and width are inconsistent with the observations. To further clarify the nature of , the other decay modes, such as , , and , are worth to observing in future experiments.
In the -meson sector, the masses of the two mixed states and are predicted to be MeV and MeV, respectively. The mass splitting between and is estimated to be MeV, which is consistent with that of Ref. Godfrey:2015dva . The mixing angle is similar to that for the charmed sector. Our predicted strong decay properties have been shown in Table 11. It is seen that the low mass state should be a broad state with a width of
(46) |
and dominantly decays into , , and channels. While the high mass state is a relatively narrow state with a width of
(47) |
and dominantly decays into and channels. Finally, it should mentioned that if the corresponds to the high mass mixed state , by combining the typical mass splitting MeV between the charmed and charmed-strange mesons, as its flavor partner, the mass of is estimated to be MeV, while the low mass state may have a mass around MeV. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 7. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV.
In 2009, the resonance with mass MeV and width MeV was observed in the channel by the BaBar collaboration BaBar:2009rro . The may be favor the mixed states in the -meson family Godfrey:2015dva ; Sun:2009tg ; Chen:2009zt ; Xiao:2014ura ; Song:2015nia ; Colangelo:2010te ; Li:2017zng . Comparing our predicted mass and decay properties with the data, we find that the seems to more favor the low mass state , however, the assignment of the cannot be excluded due to the large uncertainties of the data. The partial width ratio between and can be used to test the nature of . For the assignment, the partial width ratio is predicted to be
(48) |
which is different from the value for the assignment. It should be emphasized that the observed in the channel may be contributed by both and , since these two states have similar masses and dominantly decay into channel with a large branching fraction of . To establish the and states and uncover the nature of , more accurate observations in these dominant channels, such as , and , are needed in future experiments.
IV.6 -wave states
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
138.9 | 29.9 | 18.5 | 39.3 | – | – | – | – | ||
33.3 | 7.2 | 1.0 | 2.1 | – | – | – | – | ||
18.3 | 3.9 | 0.8 | 1.7 | – | – | – | – | ||
45.1 | 9.7 | 17.5 | 37.2 | 91.6 | 35.4 | 30.4 | 53.0 | ||
6.8 | 1.5 | 0.3 | 0.6 | 17.5 | 6.8 | 0.9 | 1.6 | ||
4.7 | 1.0 | 0.2 | 0.4 | 12.0 | 4.6 | 0.8 | 1.4 | ||
0.01 | – | – | – | – | |||||
– | – | – | – | – | – | ||||
– | – | – | – | 0.02 | 8.1 | 14.1 | |||
1.4 | 0.3 | 3.3 | 7.0 | 117.1 | 45.3 | 4.4 | 7.7 | ||
26.4 | 5.7 | 5.0 | 10.6 | 0.5 | 0.2 | 0.1 | 0.2 | ||
189.6 | 40.6 | 0.03 | 0.06 | 0.2 | 0.08 | 5.2 | 9.1 | ||
0.2 | 0.04 | 0.4 | 0.8 | 15.2 | 5.9 | 5.8 | 10.1 | ||
0.04 | 0.09 | 0.2 | 4.3 | 1.7 | 1.7 | 3.0 | |||
Total |
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
132.4 | 63.7 | 11.8 | 54.4 | – | – | – | – | ||
25.3 | 12.2 | 0.8 | 3.7 | – | – | – | – | ||
43.7 | 21.0 | 8.6 | 39.6 | 107.1 | 81.5 | 28.4 | 64.5 | ||
6.4 | 3.1 | 0.3 | 1.4 | 10.5 | 8.0 | 1.8 | 4.1 | ||
– | – | – | – | 3.2 | 7.3 | ||||
0.1 | 0.05 | 0.2 | 0.9 | 13.8 | 10.5 | 10.6 | 24.1 | ||
Total |
IV.6.1
In the -meson family, our predicted mass of , MeV, is consistent with that predicted in Refs. Ebert:2009ua ; Li:2010vx ; Lahde:1999ih ; Song:2015fha . From Table 13, it is found that the may be a broad state with a width of
(49) |
and dominantly decays into the and channels with branching fractions and , respectively. Furthermore, the decay rates into , and are also notable, their branching fractions can reach up to . In the literature, the state is also predicted to be a broad state with a width of MeV Zhong:2010vq ; Lu:2014zua ; Chen:2015lpa ; Close:2005se ; Song:2015fha .
In 2015, the LHCb collaboration observed a resonance in the channel by using the process LHCb:2015eqv . The resonance mass and width are determined to be MeV and MeV, respectively. From the point of view of the mass, numbers and decay modes, the favors the assignment, however, our predicted width is about two times larger than the measured one. The mixing may overcome this problem, which will be further discussed later.
In the -meson sector, the mass of is predicted to be MeV, which is comparable with the predictions in Refs. Song:2015nia ; Godfrey:2015dva ; Zeng:1994vj ; Lahde:1999ih . Our predicted strong decay properties have been shown in Table 14. It is found that the may have a medium width of
(50) |
and mainly decays into , and channels with branching fractions , and , respectively. Our predicted strong decay properties are in good agreement with the predictions with the model in Refs. Song:2015nia ; Song:2014mha ; Godfrey:2015dva .
In 2014, the LHCb collaboration observed a new resonance in the final state by using the process LHCb:2014ott . Its measured mass and width are MeV and MeV, respectively. From the point of view of the mass, numbers, decay modes and width, the favors the assignment. It should be mentioned that the possibility of the resonance as a mixed state between and cannot be excluded, which will be further discussed later.
IV.6.2
In the -meson family, the mass of is predicted to be MeV, which is consistent with that predicted in Refs. Godfrey:2015dva ; Zeng:1994vj ; Song:2015fha . From Table 13, it is found that the state is narrow state with a width of MeV, and dominantly decays into the and final states. The partial width ratio between and channels is predicted to be
(51) |
Our predicted width and dominant decay modes for are consistent with the predictions in the literature Wang:2016enc ; Godfrey:2015dva ; Chen:2015lpa ; Yu:2016mez ; Yu:2014dda ; Li:2010vx , while our predicted partial ratio is similar to the predictions in Refs. Godfrey:2015dva ; Chen:2015lpa ; Li:2010vx ; Lu:2014zua ; Song:2015fha .
The resonance listed in RPP Zyla:2020zbs favors the assignment of . This resonance was first observed in the and/or channels by the BaBar collaboration in 2010 BaBar:2010zpy , and confirmed by the LHCb collaboration by using the collision processes LHCb:2013jjb and decay processes Aaij:2015sqa ; Aaij:2016fma ; Aaij:2019sqk . The spin-parity numbers are determined to be by the LHCb collaboration Aaij:2015sqa . The average measured mass and width of are MeV and MeV Zyla:2020zbs . As the assignment of , the mass and width of are in good agreement with the theoretical predictions. However, our predicted ratio is notably larger than the measured value at BaBar BaBar:2010zpy . To confirm the nature of , the partial width ratio is expected to be further measured in future experiments.
In the -meson sector, the mass of is predicted to be MeV, while the mass gap between and is estimated to be MeV. Our predictions are consistent with those predicted in Refs. Wang:2016enc ; Song:2015nia ; Zeng:1994vj ; Lahde:1999ih . The state may be a very narrow state with a width of MeV, and mainly decays into and final states. The strong decay properties predicted in this work are consistent with our previous predictions with SHO wave functions Zhong:2009sk and other predictions in the literature Chen:2015lpa ; Wang:2016enc ; Godfrey:2015dva ; Godfrey:2014fga ; Zhang:2006yj ; Li:2009qu . It should be mention that there are obvious model dependencies in the predictions of the partial width ratio between and , which scatters in the range of .
In 2006, the BaBar collaboration observed a new charmed-strange meson structure in the channel with mass of MeV and a width of MeV BaBar:2006gme , which is consistent with the resonance observed in the channel in 2009 BaBar:2009rro . In 2014, the LHCb collaboration further studied the structure around GeV in the decay LHCb:2014ott ; LHCb:2014ioa . They found two overlapping spin-1 resonance and spin-3 resonance in the final state. The resonance parameters of , MeV and MeV extracted by LHCb LHCb:2014ott ; LHCb:2014ioa , are consistent with those of extracted by BaBar. The spin-3 resonance can be assigned to the charmed-strange state . As this assignment, both the mass and decay properties of can be reasonably understood within the quark model. It should be mentioned that the channel is the optimal channel for establishing spin-1 state and spin-3 state due to no contributions from the other two -wave states with .
IV.6.3 and
The physical states and are mixed states between states and via the following mixing scheme:
(52) |
In this work, the and correspond to the low-mass and high-mass mixed states, respectively.
In the -meson family, the masses of the two mixed states and are predicted to be MeV and MeV, respectively. The mass splitting between and is estimated to be MeV, which is slightly smaller that of MeV predicted in Li:2010vx , however, is about a factor of 2 larger than MeV predicted in Refs. Ebert:2009ua ; Godfrey:2015dva ; Zeng:1994vj ; Lahde:1999ih ; Lu:2014zua . Our predicted mixing angle between and , , is similar to the angle determined within the relativized quark model Ebert:2009ua ; Lu:2014zua . The predicted strong decay properties of both and are listed in Table 13. It is found that the low mass state may be a broad state with a width of
(53) |
and dominantly decays into the and channels with branching fractions and , respectively. While the high mass state may have a narrow width of
(54) |
and dominantly decays into the channel.
Some evidence of the mixed states and may have been observed in experiments. In 2010, the BaBar collaboration observed a new resonance with a mass of MeV and a width of MeV in the channel BaBar:2010zpy . In 2013, the LHCb collaboration observed an unnatural parity state in the channel. The measured mass and width MeV and MeV at LHCb LHCb:2013jjb are consistent with the observations of at BaBar. The spin-parity numbers are identified as . In 2019, the LHCb collaboration carried out a determination of quantum numbers for several excited charmed mesons by using the decays Aaij:2019sqk . In this experiment, the spin-parity numbers of [denoted by ] was confirmed to be , while the measured mass MeV and width MeV are slightly different from their previous measurements LHCb:2013jjb .
In our previous work Zhong:2010vq ; Xiao:2014ura , we predicted that / is most likely to be the high-mass mixed state , which is consistent with the prediction in Ref. Song:2015fha . Considering as , our predicted decay width MeV is in agreement with the PDG average data MeV Zyla:2020zbs , however, our predicted mass MeV is about MeV larger than the data. On the other hand, considering as the low mass state , we find that although the predicted mass MeV is consistent with the observations, our predicted width MeV is too broad to comparable with the data. It should be mentioned that two LHCb experiments Aaij:2019sqk ; LHCb:2013jjb do not give very stable resonance parameters for . This indicates that the structure around observed in the invariant mass spectrum may be contributed by both the broad state and the relatively narrow state at the same time. To distinguish and and establish them finally, more observations of the , and are suggested to be carried out in future experiments.
In the -meson sector, our predicted masses of and are MeV and MeV, respectively, which are close the predictions in the literature Godfrey:2015dva ; Zeng:1994vj ; Lahde:1999ih . Our determined mixing angle is similar to that for the -meson sector. The splitting between and , MeV, is similar to that predicted in Ref. Li:2010vx , however, is a factor of larger than that predicted in Refs. Ebert:2009ua ; Godfrey:2015dva ; Lahde:1999ih . The predicted strong decay properties of both and are listed in Table 14. It is found that the low mass state has a width of
(55) |
and dominantly decays into the and channels with branching fractions and , respectively. While the high mass state may have a relatively narrow width of
(56) |
and dominantly decays into the and channels with branching fractions and , respectively. The decay properties predicted with genuine wave functions extracted from potential model in this work are consistent with those predicted with the SHO wave functions in our previous works Zhong:2009sk ; Xiao:2014ura . The as the main decay channel of the state has also been predicted within the model Song:2015nia ; Godfrey:2015dva , however, for the high mass state , their predicted decay rates into is tiny.
Our previous studies Zhong:2009sk ; Xiao:2014ura indicates that the - mixing might be crucial to uncover the longstanding puzzle about the narrow structure in the charmed-strange meson family, which was first observed in the channel, then confirmed in the channels by the BaBar collaboration BaBar:2006gme ; BaBar:2009rro . Many people believe that the might be the state due to its narrow width. However, considering the as the state only, one cannot well understand the partial width ratio of measured by BaBar BaBar:2009rro . To overcome the puzzle about the measured radio, in Refs. Zhong:2009sk ; Xiao:2014ura we proposed that the mixed state mainly decaying into might highly overlap with the state around the mass region GeV, which is compatible with the theoretical analyses in Refs. Godfrey:2013aaa ; Gandhi:2020vap . In 2014, the LHCb collaboration observed two overlapping spin-1 resonance and spin-3 resonance in the final state by analyzing the decay LHCb:2014ott ; LHCb:2014ioa . The measured partial width ratio is considered to belong to the resonance by PDG Zyla:2020zbs . In fact, for our predicted partial width ratio is still inconsistent with the measured value at BaBar BaBar:2009rro . Since the structure around GeV in the invariant mass spectrum can be contributed by both and , we may expect that the structure around GeV in the invariant mass spectrum observed at BaBar BaBar:2009rro can be contributed by all of the -wave states with , due to their large decay rates. This is also proposed in Refs. Segovia:2015dia ; Gandhi:2020vap ; Zhong:2009sk .
To uncover the longstanding puzzle about , searches for the missing and are urgently needed to be carried out in experiments. The channel may be the optimal channel for future observations.
IV.7 -wave states
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
192.1 | 23.2 | 34.4 | 20.6 | – | – | – | – | ||
55.7 | 6.7 | 5.0 | 3.0 | – | – | – | – | ||
24.9 | 3.0 | 2.5 | 1.5 | – | – | – | – | ||
12.7 | 1.5 | 0.2 | 0.1 | – | – | – | – | ||
44.9 | 5.4 | 26.6 | 15.9 | 152.3 | 24.3 | 40.4 | 24.8 | ||
10.6 | 1.3 | 2.5 | 1.5 | 45.2 | 7.2 | 4.9 | 3.0 | ||
5.0 | 0.6 | 1.3 | 0.8 | 20.9 | 3.3 | 2.3 | 1.4 | ||
0.8 | 0.1 | 0.01 | 7.5 | 1.2 | 0.4 | 0.2 | |||
50.6 | 6.1 | 5.5 | 3.3 | – | – | – | – | ||
0.02 | 0.01 | – | – | – | – | ||||
1.8 | 0.2 | 0.04 | 0.02 | – | – | – | – | ||
20.1 | 2.4 | 7.8 | 4.7 | 61.2 | 9.8 | 22.6 | 13.9 | ||
– | – | – | – | 0.1 | 0.06 | ||||
– | – | – | – | 0.1 | 0.06 | ||||
– | – | – | – | – | – | ||||
– | – | – | – | 6.7 | 1.1 | 7.3 | 4.5 | ||
– | – | – | – | 0.5 | 0.08 | 0.6 | 0.4 | ||
– | – | – | – | 0.3 | 0.05 | 0.3 | 0.2 | ||
– | – | – | – | 0.1 | 0.02 | 5.5 | 3.4 | ||
0.5 | 0.06 | 8.6 | 5.1 | 105.3 | 16.8 | 14.3 | 8.8 | ||
0.1 | 0.01 | 0.03 | 0.02 | 14.0 | 2.2 | 0.6 | 0.4 | ||
0.2 | 0.02 | 0.03 | 0.02 | 9.5 | 1.5 | 0.8 | 0.5 | ||
0.1 | 0.01 | 1.3 | 0.8 | 63.6 | 10.2 | 3.9 | 2.4 | ||
38.2 | 4.6 | 4.3 | 2.6 | 6.2 | 1.0 | 1.4 | 0.9 | ||
3.9 | 0.5 | 0.01 | 0.2 | 0.03 | 0.2 | 0.1 | |||
10.4 | 1.3 | 0.02 | 0.01 | 0.1 | 0.02 | 0.2 | 0.1 | ||
173.7 | 21.0 | 3.5 | 2.1 | 0.5 | 0.08 | 8.8 | 5.4 | ||
17.3 | 2.1 | 0.06 | 0.04 | 0.1 | 0.02 | ||||
50.6 | 6.1 | 0.1 | 0.06 | 0.1 | 0.02 | ||||
29.8 | 3.6 | 4.1 | 2.5 | 0.2 | 0.03 | 0.03 | 0.02 | ||
44.6 | 5.4 | 0.01 | 0.02 | 4.0 | 2.5 | ||||
3.0 | 0.4 | 0.1 | 0.06 | 1.7 | 0.3 | 4.7 | 2.9 | ||
0.5 | 0.06 | 2.5 | 1.5 | 18.6 | 3.0 | 1.6 | 1.0 | ||
2.1 | 0.3 | 10.2 | 6.1 | 4.6 | 0.7 | 3.1 | 1.9 | ||
20.5 | 2.5 | 5.3 | 3.2 | 0.8 | 0.1 | 2.5 | 1.5 | ||
1.2 | 0.1 | 10.8 | 6.5 | 48.8 | 7.8 | 3.3 | 2.0 | ||
0.4 | 3.4 | 2.0 | 15.6 | 2.5 | 1.1 | 0.7 | |||
0.1 | 1.4 | 0.8 | 10.1 | 1.6 | 2.4 | 1.5 | |||
8.4 | 1.0 | 16.5 | 9.8 | 22.9 | 3.7 | 17.7 | 10.9 | ||
2.5 | 5.3 | 3.2 | 6.9 | 1.1 | 5.5 | 3.4 | |||
0.5 | 3.9 | 2.3 | 1.7 | 0.3 | 2.4 | 1.5 | |||
Total |
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
157.0 | 30.9 | 19.0 | 26.6 | – | – | – | – | ||
33.2 | 6.5 | 2.0 | 2.8 | – | – | – | – | ||
15.6 | 3.1 | 0.2 | 0.3 | – | – | – | – | ||
34.7 | 6.8 | 11.6 | 16.2 | 130.7 | 31.0 | 19.8 | 28.8 | ||
6.3 | 1.2 | 4.3 | 6.0 | 28.7 | 6.8 | 2.0 | 2.9 | ||
0.8 | 0.2 | 1.7 | 2.4 | 5.6 | 1.3 | 0.3 | 0.4 | ||
47.5 | 9.4 | 2.2 | 3.1 | – | – | – | – | ||
1.6 | 0.3 | 0.03 | 0.04 | – | – | – | – | ||
14.7 | 2.9 | 1.4 | 2.0 | 53.7 | 12.7 | 7.5 | 10.9 | ||
– | – | 0.1 | 0.1 | 0.1 | 0.02 | 0.1 | 0.1 | ||
– | – | – | – | 3.1 | 0.7 | 4.9 | 7.1 | ||
– | – | – | – | 0.4 | 0.1 | 0.2 | 0.3 | ||
2.4 | 0.5 | 1.1 | 1.5 | 99.2 | 23.5 | 7.3 | 10.6 | ||
0.2 | 0.04 | 0.02 | 0.03 | 26.0 | 6.2 | 1.3 | 1.9 | ||
25.2 | 5.0 | 0.2 | 0.3 | 3.2 | 0.8 | 1.2 | 1.7 | ||
2.4 | 0.5 | 0.2 | 0.3 | 0.2 | 0.02 | 0.2 | 0.3 | ||
146.2 | 28.8 | 1.7 | 2.4 | 1.1 | 0.3 | 0.1 | 0.1 | ||
13.8 | 2.7 | 0.1 | 0.1 | 0.1 | 0.02 | 0.01 | 0.01 | ||
– | – | 0.02 | 2.4 | 3.5 | |||||
– | – | 0.03 | 0.04 | 1.6 | 0.4 | 0.2 | 0.3 | ||
– | – | 2.8 | 3.9 | 0.1 | 0.02 | 0.3 | 0.4 | ||
0.7 | 0.1 | 6.4 | 9.0 | 43.9 | 10.4 | 5.5 | 8.0 | ||
0.02 | 0.4 | 0.6 | 6.4 | 1.5 | 2.2 | 3.2 | |||
5.1 | 1.0 | 14.3 | 20.0 | 17.5 | 4.1 | 12.2 | 17.7 | ||
0.2 | 0.04 | 1.7 | 2.4 | 0.7 | 0.2 | 1.1 | 1.6 | ||
Total |
IV.7.1
In the -meson family, the mass of the state is predicted to be MeV, which is comparable with the predictions in Refs. Godfrey:2015dva ; Zeng:1994vj ; Li:2010vx ; Song:2015fha . Our predicted strong decay properties for these high -wave states are listed in Table 15. It is found that the state is a very broad state with a width of MeV. This state may be difficult to be observed in experiments due to its broad width. It should be mentioned that the width of predicted within our chiral quark model is about a factor of larger than the predictions within the models in Refs. Godfrey:2015dva ; Song:2015fha .
In the -meson family, the mass of is predicted to be MeV, which is comparable with those in Refs. Godfrey:2015dva ; Zeng:1994vj . Our predicted strong decay properties for these high -wave states are listed in Table 16. It is found that the state is a very broad state with a width of MeV, and may be difficult to be observed in experiments due to its broad width. It should be mentioned that the width of predicted within our chiral quark model are a factor of larger than the predictions within the models in Refs. Godfrey:2015dva ; Song:2015nia .
IV.7.2
In the -meson family, the mass of the state is predicted to be MeV, which is comparable with those predicted in Refs. Godfrey:2015dva ; Zeng:1994vj ; Li:2010vx ; Song:2015fha . The state is a relatively narrow state with a width of
(57) |
and have relatively large decay rates into and channels with branching fractions and , respectively. More details of the decay properties can be seen in Table 15. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 8. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. A relatively narrow width of is also predicted in Ref. Song:2015fha , although their predicted width MeV is about a factor of 5 smaller than ours. To establish the missing state, the and channels are worth to observing in future experiments. However, it should be pointed out that in Refs. Godfrey:2015dva ; Song:2015fha , the and are not predicted to be the dominant decay modes of .
In the -meson family, the mass of the state is predicted to be MeV, which is comparable with those of Refs. Godfrey:2015dva ; Zeng:1994vj ; Li:2010vx ; Song:2015fha . The state has a narrow width of
(58) |
which is in agreement with the prediction of Ref. Song:2015nia . The state dominantly decays into , and channels with branching fractions , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 8. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. To establish the missing state, the and channels are worth to observing in future experiments.
IV.7.3 and
The physical states and are mixed states between states and via the following mixing scheme:
(59) |
In this work, the and correspond to the low-mass and high-mass mixed states, respectively.
In the -meson family, the masses of the two mixed states and are predicted to be MeV and MeV, respectively. Our predicted masses are comparable with those in Refs. Li:2010vx ; Godfrey:2015dva ; Zeng:1994vj . The mass splitting between and is estimated to be MeV, which is in agreement with that of MeV predicted in Ref. Ebert:2009ua . Our predicted mixing angle, , is similar to that determined within the relativized quark model Godfrey:2015dva ; Lu:2014zua . The predicted strong decay properties of both and are listed in Table 15. It is found that the low mass state may be a broad state with a width of MeV, and have large decay rates into the and channels with branching fractions and , respectively. The state may be difficult to be observed in experiments due to its too broad width. While the high mass state may have a relatively narrow width of
(60) |
and dominantly decays into , and channels with branching fractions , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 8. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The narrow width nature of is also predicted by Song et al. in Ref. Song:2015fha , although their predicted width MeV is a factor of smaller than ours.
In the -meson family, the masses of the two mixed states and are predicted to be MeV and MeV, respectively. Our predicted masses are comparable with those in Refs. Li:2010vx ; Godfrey:2015dva ; Zeng:1994vj . The mass splitting between and is estimated to be MeV, which is comparable with that predicted in Li:2010vx ; Godfrey:2015dva ; Zeng:1994vj . Our predicted mixing angle, , is similar to that determined within the relativized quark model Godfrey:2015dva ; Lu:2014zua . The predicted strong decay properties of both and are listed in Table 16. It is found that the low mass state may be a broad state with a width of MeV, and have large decay rates into the and channels with branching fractions and , respectively. The state may be difficult to be observed in experiments due to its broad width. It should be mentioned that the width of predicted within our chiral quark model is a factor of larger than the predictions within the models in Refs. Godfrey:2015dva ; Song:2015nia . While the high mass state may have a narrow width of
(61) |
which is consistent with the prediction of Ref. Song:2015nia . The dominantly decays into , , and channels with branching fractions , , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 8. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The high mass state may have a large potential to be established in forthcoming experiments. The channel may be the optimal channel for future observations.
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
39.4 | 5.5 | 11.2 | 17.3 | – | – | – | – | ||
11.5 | 1.6 | 1.2 | 1.8 | – | – | – | – | ||
5.7 | 0.8 | 0.8 | 1.2 | – | – | – | – | ||
3.1 | 0.4 | 0.03 | 0.05 | – | – | – | – | ||
25.3 | 3.5 | 13.3 | 20.5 | 42.6 | 12.4 | 41.8 | 24.9 | ||
6.7 | 0.9 | 0.9 | 1.4 | 10.4 | 3.0 | 4.3 | 2.6 | ||
3.5 | 0.5 | 0.6 | 0.9 | 5.8 | 1.7 | 0.2 | 0.1 | ||
0.7 | 0.1 | 0.2 | 0.06 | 0.1 | 0.06 | ||||
2.2 | 0.3 | 0.4 | 0.6 | – | – | – | – | ||
0.4 | 0.06 | 0.3 | 0.5 | 0.8 | 0.2 | 1.4 | 0.8 | ||
– | – | – | – | 1.1 | 0.3 | 9.5 | 5.7 | ||
– | – | – | – | 0.7 | 0.4 | ||||
– | – | – | – | 0.01 | 0.5 | 0.3 | |||
– | – | – | – | ||||||
22.5 | 3.1 | 8.7 | 13.4 | 170.4 | 49.7 | 35.7 | 21.3 | ||
0.6 | 0.1 | – | – | – | – | 0.2 | 0.1 | ||
1.3 | 0.2 | 0.01 | 0.5 | 0.3 | |||||
– | – | – | – | ||||||
44.9 | 6.2 | 13.4 | 20.6 | 1.3 | 0.4 | 0.1 | 0.06 | ||
0.6 | 0.1 | – | – | 0.04 | 0.02 | ||||
3.4 | 0.5 | 0.01 | 0.02 | – | – | ||||
203.1 | 28.1 | 0.5 | 0.8 | 0.04 | 0.01 | 19.7 | 11.7 | ||
26.3 | 3.6 | – | – | 0.1 | 0.06 | ||||
18.5 | 2.6 | – | – | 0.2 | 0.1 | ||||
0.03 | 0.004 | – | – | – | – | ||||
– | – | – | – | ||||||
0.9 | 0.1 | 1.2 | 0.4 | 18.9 | 11.3 | ||||
1.9 | 0.3 | 2.4 | 3.7 | 87.9 | 25.6 | 6.6 | 3.9 | ||
91.8 | 12.7 | 2.7 | 4.2 | 0.1 | 0.03 | 3.4 | 2.0 | ||
88.5 | 12.2 | 0.4 | 0.6 | 0.06 | 0.02 | 1.5 | 0.9 | ||
2.0 | 0.3 | 1.4 | 2.2 | 10.6 | 3.1 | 1.3 | 0.8 | ||
0.6 | 0.1 | 0.4 | 0.6 | 3.3 | 1.0 | 0.4 | 0.2 | ||
0.1 | 0.01 | 0.04 | 0.06 | 0.8 | 0.2 | 0.6 | 0.4 | ||
87.5 | 12.1 | 4.7 | 7.2 | 4.8 | 1.4 | 15.1 | 9.0 | ||
26.8 | 3.7 | 1.5 | 2.3 | 1.4 | 0.4 | 4.7 | 2.8 | ||
2.7 | 0.4 | 0.04 | 0.06 | 0.8 | 0.5 | ||||
Total |
Channel | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | (MeV) | Br (%) | |
---|---|---|---|---|---|---|---|---|---|
33.3 | 8.1 | 8.1 | 26.3 | – | – | – | – | ||
7.4 | 1.8 | 0.7 | 2.3 | – | – | – | – | ||
3.2 | 0.8 | 0.03 | 0.1 | – | – | – | – | ||
21.7 | 5.3 | 8.8 | 28.6 | 40.3 | 19.3 | 26.8 | 37.4 | ||
4.4 | 1.1 | 0.6 | 1.9 | 7.6 | 3.6 | 2.3 | 3.2 | ||
0.5 | 0.1 | 0.2 | 0.1 | 0.1 | 0.1 | ||||
1.0 | 0.2 | 0.01 | 0.03 | – | – | – | – | ||
0.1 | 0.02 | 0.02 | 0.03 | ||||||
– | – | – | – | 0.5 | 0.2 | 6.5 | 9.1 | ||
– | – | – | – | 0.4 | 0.6 | ||||
17.8 | 4.3 | 2.0 | 6.5 | 142.4 | 68.3 | 12.7 | 17.7 | ||
0.9 | 0.2 | 1.8 | 0.9 | 0.3 | 0.4 | ||||
39.5 | 9.6 | 3.9 | 12.7 | 0.3 | 0.1 | 0.1 | 0.1 | ||
3.5 | 0.9 | 0.01 | 0.03 | ||||||
190.2 | 46.3 | 0.04 | 0.1 | 0.02 | 0.01 | 7.5 | 10.5 | ||
15.6 | 3.8 | 0.1 | 0.1 | ||||||
1.4 | 0.3 | 1.2 | 3.9 | 10.5 | 5.0 | 2.0 | 2.8 | ||
0.03 | 0.01 | 0.03 | 0.5 | 0.2 | 0.4 | 0.6 | |||
69.9 | 17.0 | 5.4 | 17.5 | 4.4 | 2.1 | 12.0 | 16.8 | ||
0.4 | 0.1 | – | – | 0.4 | 0.6 | ||||
Total |
IV.8 -wave states
IV.8.1
In the -meson family, our predicted mass for , MeV, is comparable with the predictions in Refs. Zeng:1994vj ; Song:2015fha . From Table 17, it is found that the state might be a very broad state with a width of
(62) |
and dominantly decays into , , and . The decay properties predicted in this work roughly agree with those predicted with the SHO wave functions in our previous work Xiao:2014ura , however, is notably (a factor of ) broader than those predicted within the models in Refs. Godfrey:2015dva ; Song:2015fha . The may be too broad to be observed in experiments according to our present predictions.
In the -meson sector, the predicted masses for is MeV, which is comparable with the predictions in Refs. Godfrey:2015dva ; Zeng:1994vj ; Song:2015nia . From table Table 18, it is found that the state might have a very broad width of
(63) |
and dominantly decays into and with branching fractions and , respectively. The state is also predicted to be a broad state with a width of MeV in Refs. Godfrey:2015dva ; Song:2015fha ; Xiao:2014ura . The state may be difficult to be established in experiments due to its too broad width.
IV.8.2
In the -meson family, our predicted mass for , MeV, is comparable with the predictions in Refs. Zeng:1994vj ; Song:2015fha . From Table 17, it is found that the is a fairly narrow state with a width of
(64) |
and dominantly decays into the , and channels with a comparable branching fraction . To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 9. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The narrow width of and its relatively large decay rates into and are also predicted within the models in Refs. Godfrey:2015dva ; Song:2015fha . The might have a large potential to be observed in the and final states due to its narrow width.
It is interesting to find that the resonance with a natural parity observed in the channel by the LHCb collaboration in 2013 LHCb:2013jjb might be a good candidate of . Our predicted mass MeV is consistent with the measured value MeV of . Taking as the state, we find that has a large decay rate into the channel with a branching fraction of , which is consistent with the observations. The predicted width MeV is also comparable with the data MeV. Considering as the state, it also has a large decay rate into the channel, the partial width ratio between and is predicted to be
(65) |
which may be useful to test the nature of .
In the -meson sector, the predicted mass for , is MeV, which is comparable with the predictions in Refs. Godfrey:2015dva ; Zeng:1994vj ; Song:2015nia . Our predicted strong decay properties are listed in Table 18. It is found that the may be a narrow state with a width of
(66) |
and dominantly decays into the , and channels with branching fractions , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 9. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The , and as the main decay channels are also predicted in Refs. Godfrey:2015dva ; Song:2015fha ; Xiao:2014ura , however, the predicted width in these works are much broader than ours. The might have large potentials to be observed in the dominant and channels.
IV.8.3 and
The physical states and are mixed states between and via the following mixing scheme:
(67) |
In this work, the and correspond to the low-mass and high-mass mixed states, respectively.
In the -meson family, the predicted masses for the two mixed -wave states and are and MeV, respectively, which are comparable with the predictions in Refs. Zeng:1994vj ; Song:2015fha . The mixing angle is determined to be , which is similar to that determined within the relativized quark model Godfrey:2015dva ; Lu:2014zua . Our predicted strong decay properties are listed in Table 17. It is found that the low mass mixed state is a fairly broad state with a width of
(68) |
and dominantly decays into , and with branching fractions , and , respectively. The decay properties of predicted in this work are roughly consistent with those predicted with the SHO wave functions in our previous work Xiao:2014ura , however, is notably (a factor of ) broader than those predictions within the models in Refs. Godfrey:2015dva ; Song:2015fha . While the high mass mixed state is a relatively narrow state with a width of
(69) |
and dominantly decays into , , and with branching fractions , , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 9. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The decay width of predicted in this work is comparable with our previous prediction with the SHO wave functions in Ref. Xiao:2014ura , however, is about a factor of larger than that predicted in Ref. Song:2015fha . To look for the missing and states, the and final states are worth to observing in future experiments.
In the -meson family, the predicted masses for the two mixed -wave states and are and MeV, respectively, which are comparable with the predictions in Refs. Zeng:1994vj ; Song:2015fha . The mixing angle is determined to be , which is similar to that determined within the relativized quark model Godfrey:2015dva ; Lu:2014zua . Our predicted strong decay properties are listed in Table 18. It is found that the low mass mixed state may be a fairly broad state with a width of
(70) |
and dominantly decays into and with branching fractions and , respectively. The dominant decay channels of and predicted in this work are consistent with the predictions in Refs. Song:2015fha ; Xiao:2014ura , although there are large uncertainties in the predictions of the total width. The high mass mixed state is a narrow state with a width of
(71) |
and dominantly decays into , and with branching fractions , and , respectively. To see the dependence of the decay properties of on its mass, we also plot the main partial widths and the total width as functions of the mass in Fig. 9. It is found that the partial and total decay widths increase smoothly with the mass. With a mass uncertainty of MeV, the total width of varies in the range MeV. The decay width predicted in this work is notably narrower than those predicted in Refs. Godfrey:2015dva ; Song:2015fha ; Xiao:2014ura . To look for the missing and states, the and final states are worth to observing in future experiments.
IV.9 The mixing
In Sec. IV.2.2, we have considered the possibility of the and as the candidates of the pure states in the - and -meson families, respectively. It is found that with these assignments, our predicted widths are too narrow to be comparable with the data. In our previous works Zhong:2010vq ; Zhong:2009sk , we have studied the strong decay properties of the and with an SHO approximation. According to our analysis, both and could be explained as the mixed state via the - mixing:
(72) |
where and are assigned to the low-mass and high-mass mixed states, respectively. The mixing angle for the charmed sector is , while that for charmed-strange sector is . To explain the strong decay properties of the and/or , configuration mixing between and is also suggested in the literature Close:2006gr ; Chen:2011rr ; Li:2009qu ; Yu:2020khh ; Sun:2010pg ; Li:2010vx ; Chen:2015lpa ; Song:2015nia . In this work we restudy the and resonances as the mixed states via the - mixing by using the genuine wave functions calculated from our potential model.
Considering as the low-mass mixed state , we plot the strong decay properties as functions of the mixing angle in Figure 10. It is found that if we take a mixing angle , the theoretical width can be consistent with the data MeV measured by the BaBar collaboration BaBar:2010zpy . The mixing angle determined in this work is similar to determined in our previous work Zhong:2010vq . The and are the two dominant decay channels of , which can explain why has been first observed in these two channels. However, the ratio between and
(73) |
is too large to be comparable with the data measured by the BaBar collaboration BaBar:2010zpy . The ratio predicted with the genuine wave functions determined from the potential model in this work is about a factor of larger than that predicted with the SHO wave functions in our previous work Zhong:2010vq . The ratio is very sensitive to the details of the wave function of due to the nodal effects. Thus, the partial width ratio is hard to be accurately predicted in theory.
If the is the low-mass mixed state indeed, the high-mass mixed state might be observed in experiments as well. It is interesting to find that the resonance observed in the channel by the LHCb collaboration LHCb:2015eqv might be a candidate of the high-mass mixed state in the -meson family. Considering the as the assignment, the strong decay properties as functions of the mixing angle are plotted in Figure 11. It is found that within the range of the mixing angle determined by the , the width of is predicted to be MeV, which is close to the upper limit of the measured width MeV. As the high-mass mixed state, should dominantly decay into the , and channels. To confirm the nature of , both and channels are worth observing in future experiments.
In the -meson family, considering as the low-mass mixed state , we plot the strong decay properties as functions of the mixing angle in Figure 12. One sees that if we take the mixing angle with , the decay width MeV and partial width ratio of measured by the BaBar collaboration Lees:2014abp ; BaBar:2009rro can be well described within the uncertainties.
If the is the low-mass mixed state indeed, the high-mass mixed state might be observed in experiments as well. The resonance observed in the final state by the LHCb collaboration LHCb:2014ott might be a candidate of the high-mass mixed state in the -meson family. Considering as the assignment, the strong decay properties as functions of the mixing angle are plotted in Figure 13. It is found that if we take mixing angle , the predicted width of , MeV, is consistent with the measured width of MeV LHCb:2014ott . The partial width ratio between and channels is predicted to be
(74) |
which can be used to test the nature of .
As a whole, our underestimation of the decay widths of and as a pure configuration can be overcome by mixing with some -wave components. Meanwhile, the widths of the resonances and observed by the LHCb collaboration can be reasonably explained with the high-mass mixed states and , respectively. However, the measured ratio for is inconsistent with our predictions. To clarify the natures of these charmed and charmed-strange meson resonances and test various model predictions, (i) both and are waiting to be confirmed by other experiments; (ii) the partial width ratio for and for are waiting to be confirmed by other experiments; (iii) the resonance parameters of and are waiting to be accurately measured in future experiments.
V Summary
In this work we systematically calculate the mass spectra of charmed and charmed-strange meson states up to the excitations with a semi-relativistic potential model. Our results are in good agreement with other quark model predictions, although there are some model dependencies in the predicted masses for the higher - and -wave states. The strong decay properties are further analyzed with a chiral quark model by using the numerical wave functions obtained from the potential model. To well understand the -wave states, we also systematically consider the coupled-channel effects on the masses of the -wave states by using the strong decay amplitudes obtained within the chiral quark model. Based on our good descriptions of the mass and decay properties for the low-lying well-established states , , , and , we give a quark model classification for the high mass resonances observed in recent years. Our main conclusions are summarized as follows.
There are notable couple-channel corrections to the bare masses for the , and states. The and can be explained with the dressed states and by the and loops, respectively. The physical mass for the dressed state is predicted to be MeV, which is about 50 MeV lower than the PDG average mass of .
The resonance can be classified as the state. Considering the newly observed as the flavor partner of , the physical mass of , MeV, is close to the observed mass by including the coupled-channel effects, however, our predicted width is much smaller than the observed one.
and can be classified as the -wave states with the assignments and , respectively. The resonance should be the flavor partner of , and correspond to the state. The state, as the flavor partner of , is most likely to be observed in the channel due to its narrow width nature.
is more favor a candidate of or . As the assignment the predicted width is about a factor of 2 smaller than the observation, while as the assignment the predicted width is about a factor of 2.5 larger than the observation.
may favor the -wave high-mass mixed state . The resonance also favor the -wave mixed state or . Considering as , the predicted width is close to the upper limit of the data, while as the assignment the predicted width is close to the lower limit of the data. The may be contributed by both and .
and may favor the mixed states and via the - mixing, respectively.
There still exist puzzles for understanding the natures of and . Considering and as and , respectively, the predicted widths are inconsistent with the data. While considering them as the mixed states and , their widths are reasonably consistent with the data, however, the ratio for is inconsistent with the observation.
Many missing excited - and -meson states, such as , , , /, /, / and /, have a relatively narrow width, they are most likely to be observed in their dominant decay channels in future experiments.
Acknowledgement
The authors thank Prof. Xiang Liu and Dr. Zhi Yang for very helpful discussions. This work is supported by the National Natural Science Foundation of China (Grants Nos. U1832173, 11775078, 12175065).
Appendix A Coupled-channel model
In this appendix, we give the details of the model including the coupled-channel effects on the charmed and charmed-strange meson mass spectra. This simple coupled-channel model has been widely adopted in the literature Lu:2017hma ; Luo:2019qkm ; Luo:2021dvj ; Yang:2021tvc ; Xie:2021dwe ; Ortega:2021fem ; Ortega:2016mms ; Liu:2011yp ; Lu:2016mbb ; Kalashnikova:2005ui ; Eichten:1978tg .
A bare meson state in the quark model can couple to the two-hadron continuum by hadronic loops as shown in Fig. 14. The experimentally observed state may be an admixture between the bare state and continuum components, thus, the wave function of the physical state is given by
(75) |
where is final two-hadron relative momentum in the initial hadron static system, and denote the probability amplitudes of the bare valence state and continuum components, respectively.
The full Hamiltonian of this mixed system can be written as
(76) |
In the above equation, is the Hamiltonian of the bare meson state in the potential model, while is the Hamiltonian for the continuum state . Neglecting the interaction between the hadrons and , one has
(77) |
where represents the energy of continuum components. The mixing between and is caused by the Hamiltonian , which can be borrowed from our chiral quark model.
The Schrödinger equation of a mixed system can be written as
(78) | ||||
From Eq.( LABEL:coupled-channel_equation), we have
(79) |
(80) |
Deriving from Eq.(80), and substituting it into Eq.(79), we get a coupled-channel equation
(81) |
where the mass shift is given by
(82) |
and is the bare mass of the meson state obtained from the potential model. From Eq.(81) and Eq.(82), the physical mass and the bare state mass shift can be determined simultaneously.
It should be mentioned that when we calculate the mass shift by using the Eq.(82), the nonphysical contributions from higher region may be involved. To know the whole momentum region contributions, as an example, considering the loop, in Fig. 15 we plot the mass shift of (i.e., the integral function in Eq.(82)) as a function of the momentum . It is found that two regions contribute to the mass shift. The main contribution region is the low region dominated by the pole. In the higher region of GeV, a small bump structure exists. This bump contribution may be nonphysical, because the quark pair production rates via the non-perturbative interaction should be strongly suppressed in the high momentum region Morel:2002vk ; Tan:2021bvl . It should be mentioned that in the chiral quark model the chiral interaction is only applicable to the low region.
To soften the hard vertices in the higher momentum region, and reasonably describe the mass shifts, an additional factor is suggested to be introduced into the two-body decay amplitude Morel:2002vk . Adopting suppressed factor with GeV as that used in Ref. Ortega:2016mms , we also plot the mass shift of as a function of the momentum in Fig. 15. It is found that the factor indeed eliminates the contributions from the high momentum region. To eliminate the nonphysical contributions, in our calculations we cut off the momentum at the inflection point in function as shown in Fig. 15. It should be pointed out that the cut-off momentum for each meson states is different due to the different position of the inflection point. With this momentum cut-off approach, our predicted mass shifts due to coupled-channel effects for the and/or meson states are consistent with the predictions in the literature Yang:2021tvc ; Ortega:2016mms ; Xie:2021dwe ; Ortega:2021fem .
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