Lepton flavor violating decays in the SSM model within the Mass Insertion Approximation
Abstract
Three singlet new Higgs superfields and right-handed neutrinos are added to MSSM to obtain SSM model. Its local gauge group is . In the framework of SSM, we study muon anomalous magnetic moment and lepton flavor violating decays within the Mass Insertion Approximation(MIA). Through the MIA method, we can find the parameters that directly affect the analytical result of the lepton flavor violating decays , which make our work more convenient. We want to provide a set of simple analytic formulas for the form factors and the associated effective vertices, that may be very useful for future phenomenological studies of the lepton flavor violating decays. According to the accuracy of the numerical results which the influence of different sensitive parameters, we come to the conclusion that the non-diagonal elements which correspond to the generations of the initial lepton and final lepton are main sensitive parameters and lepton flavor violation(LFV) sources. This work can provide a clear signal of new physics(NP).
I introduction
Lepton has unitary matrix similar to Cabbibo-Kobayashi-Maskawa(CKM) mixed matrix. The breaking theory of electric weak symmetry and neutrino oscillation experiment show that lepton flavor violation (LFV) exists both theoretically and experimentally1 . The standard model(SM) is already a mature theory. However, the lepton number is conserved in the SM, so there is no LFV process in the SM2 . Through research, it is necessary to expand the SM. Any sign of LFV can be regarded as evidence of the existence of new physics(NP)02 .
Physicists have extended SM and obtained a large number of extended models, among which the minimum supersymmetric standard model (MSSM) is the most concerned model. However, it is gradually found that MSSM also has problems: problem3 and zero mass neutrinosUU1 . To solve these problems, we pay attention to the U(1) expansions of MSSM. We extend the MSSM with gauge group, whose symmetry group is Sarah1 ; Sarah2 ; Sarah3 . It adds three Higgs singlet superfields and right-handed neutrino superfields beyond MSSMMSSM . There are five neutral CP-even Higgs component fields in the model, which come from two Higgs doublets and three Higgs singlets respectively. Therefore, the mass mixing matrix is , and the 125.1 GeV Higgs particlesm1 corresponds to the lightest mass eigenstate.
To improve the corrections to LFV processes of , people discuss different SM extended models220725 , for example minimal R-symmetric supersymmetric standard model9 , MSSM extension with gauged baryon and lepton numbers 21 , SM extension with a hidden gauged symmetrynew1 and lepton numbers and supersymmetric low-scale seesaw modelsnew2 . It is worth noting that in our previous work, we have studied lepton flavor violating decays in the SSM model20 . The above works and most of the research on LFV are studied with the mass eigenstate method. Using this method to find sensitive parameters is often not intuitive and clear enough, which depends on the mass eigenstates of the particles and rotation matrixes. It will lead us to pay too much attention to many unimportant parameters. Now we use a novel calculation method called as Mass Insertion Approximation(MIA)04 ; 07 ; 05 ; 06 , which uses the electroweak interaction eigenstate and treats perturbatively the mass insertions changing slepton flavor. By means of mass insertions inside the propagators of the electroweak interaction sleptons eigenstates, at the analytical level, we can find many parameters that have direct impact on LFV. It is worth noting that these parameters are considered between all possible flavor blends among SUSY partner of leptons, in which their particular origin has no assumption and is independent of the model07 . In addition, the MIA method has been applied to other works related to LFV, including the decays induced from SUSY loops07 , effective lepton flavor violating vertex from right-handed neutrinos06 , one-loop effective LFV vertex from heavy neutrinos05 and so on. This method provides very simple and intuitive analytical formula, and is also clear about the changes of the main parameters affecting lepton taste destruction, which provides a new idea for other work of LFV in the future.
In the process of LFV, because the mass of lepton is much greater than and , there are more LFV decay channels08 . The decay processes of are the most interesting. This work is to study the LFV of the processes under the SSM model. The effects of different reasonable parameter spaces on the branching ratio Br() are compared. The latest upper limits on the LFV branching ratio of , and at 90% confidence level (C.L.)10 are
(1) |
The paper is organized as follows. In Sec.II, we mainly introduce the SSM including its superpotential and the general soft breaking terms. In Sec.III, we give analytic expressions for muon anomalous magnetic moment and the branching ratios of decays in the SSM. In Sec.IV, we give the numerical analysis, and the summary is given in Sec.V.
II the SSM
SSM is the U(1) extension of MSSM, whose local gauge group is 04 ; UU1 ; UU3 . On the basis of MSSM, SSM has new superfields such as three Higgs singlets and right-handed neutrinos . Through the seesaw mechanism, light neutrinos obtain tiny masses at the tree level. The neutral CP-even parts of and mix together and form a mass squared matrix, whose lightest mass eigenvalue corresponds to the lightest CP-even Higgs. The particle content and charge assignments for SSM can be found in our previous workUU1 . To get 125.1 GeV Higgs massLCTHiggs1 ; LCTHiggs2 , the loop corrections should be taken into account. The sneutrinos are disparted into CP-even sneutrinos and CP-odd sneutrinos, and their mass squared matrixes are both extended to .
In SSM, the concrete form of the superpotential is:
(2) |
We collect the explicit forms of two Higgs doublets and three Higgs singlets here
(7) | |||
(8) |
The vacuum expectation values(VEVs) of the Higgs superfields , , , and are denoted by , and respectively. Two angles are defined as and .
The soft SUSY breaking terms of this model are shown as
(9) |
We have proven that SSM is anomaly free in our previous workUU3 . Two Abelian groups and produce a new effect called as the gauge kinetic mixing in the SSM, which is MSSM never before.
In general, the covariant derivatives of SSM can be written as UMSSM5 ; B-L1 ; B-L2 ; gaugemass
(15) |
with and representing the gauge fields of and respectively.
Under the condition that the two Abelian gauge groups are unbroken, we use the rotation matrix UMSSM5 ; B-L2 ; gaugemass to perform a change of the basis
(21) |
with the redefinitions
(30) |
Then the covariant derivatives of SSM are changed as
(36) |
At the tree level, three neutral gauge bosons and mix together, whose mass matrix is shown in the basis 04
(40) |
with and .
We use two mixing angles and to get mass eigenvalues of the matrix in Eq.(40). is the Weinberg angle and the new mixing angle is defined from the following formula
(41) |
The used mass matrixes can be found in the workUU1 ; 20 . Here, we show some needed couplings in this model. We deduce the vertexes of
(43) |
We deduce the vertex couplings of neutralino-lepton-slepton
(44) |
III formulation
In this section, we study the LFV of the and muon anomalous magnetic moment under the SSM model21 with the MIA. The simplified form is discussed.
III.1 Using MIA to calculate in SSM model
If the external lepton is on shell, the amplitude of is
(45) |
where is the injecting lepton momentum, is the photon momentum, and is the mass of the th generation charged lepton. and are the wave functions for the external leptons. The final Wilson coefficients are obtained from the sum of these diagrams’ amplitudes.

The Feynman diagrams of under the SSM model are obtained by MIA09 in Fig.1. The sneutrinos are disparted into CP-even sneutrinos and CP-odd sneutrinos . After our analysis, in Fig.1(f) since right-handed sneutrinos are strongly depressed by , the situation of right-handed sneutrinos here is neglected. In other words, there are only two cases of left-handed CP-even sneutrinos and left-handed CP-odd sneutrinos in Fig.1(f). In order to more directly express the influencing factors of LFV of , we use to express the one-loop corrections by MIA.
1. The one-loop contributions from --.
(46) | |||
(47) |
here, is the particle mass, with . The functions and are
(48) | |||
(49) |
2. The one-loop contributions from ---.
(50) | |||
(51) |
here and . The specific forms of and are
(52) | |||
(53) |
3. The one-loop contributions from ---.
(54) | |||
(55) |
4. The one-loop contributions from ---.
(56) |
5. The one-loop contributions from .
(57) |
We show the one-loop functions and in the following form
(58) | |||
(59) |
6. The one-loop contributions from chargino and left-handed CP-even(odd) sneutrino.
(60) | |||
(61) |
The one-loop functions , and read as
(62) | |||
(63) | |||
(64) |
From the above formulas, we can find that are mostly affected by and and there is a positive correlation. have the lepton flavor violating sources. It provides a reference for our subsequent work. Finally, we get the final Wilson coefficient and decay width of ,
(65) |
The beanching ratio of is
(66) |
III.2 Degenerate Result
In order to more intuitively analyze the factors affecting lepton flavor violating processes , we suppose that all the masses of the superparticles are almost degenerate. In other words, we give the one-loop results (chargino-sneutrino, neutralino-slepton) in the extreme case, where the masses for superparticles () are equal to 04 :
The functions and are much simplified as
(67) | |||
(68) |
Then, we obtain the much simplified one-loop results of
(69) |
In the above formula, after simple approximation, we can find that in the formula of the second line, due to the existence of , the result is 2-3 orders of magnitude smaller than other terms. Therefore, we will not consider the term with here. It can be found that and have a certain impact on the correction of . According to , we assume and , and get the larger value of
Due to the different orders of magnitude of branching ratios, we set GeV and discuss in two cases:
1.
We take and as variables to study the effect on . In Fig.2, it can be found that both and have great influence on and they are all increasing trend. The larger the values of and , the easier it is to approach the upper limit of the experiment.

Similar to the above, in Fig.3, with the increase of , the value of gradually increases, and when increases, also increases. When and , reaches the experimental upper limit. But in numerical terms, the effect of is greater than .

2.
Since the numerical results of and are close and have similar characteristic, we take as an example. In Fig.4, when the values of and enlarger, the value of also increases, which can well reach the experimental measured value.

In Fig.5, we analyze and on the . The value of also increases with the increasing and , but the effect from is greater than . So the correction of to is greater than that of .

All in all, we can find that and all have direct impact on the correction to and .
III.3 Muon anomalous magnetic moment
The one-loop corrections to muon anomalous magnetic moment are obtained with MIA. Here, we show the one-loop contributions from chargino and CP-even(odd) sneutrino as04
(71) | |||
(72) |
The concrete forms of the one-loop functions and are
(73) | |||
(74) |
The other one-loop contributions are obtained from --, --, -- and . To save space in the text, we do not show their concrete forms here, which can be found in Ref.04 . In our previous work04 , the one-loop contributions of muon anomalous magnetic moment in the degenerate form are get with the supposition
(75) |
IV numerical results
In this section, we study the numerical results and consider the constraints from lepton flavor violating processes . In addition, we have considered the following conditions: 1. the lightest CP-even Higgs mass =125.1 GeVsu1 ; su2 . 2. The latest experimental results of the mass of the heavy vector boson is TeVxin1 . 3. The limits for the masses of other particles beyond SM. 4. The bound on the ratio between and its gauge coupling is TeV at 99% C.L.ZPG1 ; ZPG2 . 5. The constraint from LHC data, TanBP . 6. The scalar lepton masses larger than 700 GeV and chargino masses larger than 1100 GeVA2021 .
Considering the above constraints in the front paragraph, we use the following parameters
(76) |
To simplify the numerical research, we use the relations for the parameters and they vary in the following numerical analysis
(77) |
Without special statement, the non-diagonal elements of the parameters are supposed as zero.
IV.1 Muon anomalous magnetic moment
In this subsection, we study the one-loop in SSM model by MIA and expect to get some inspiration about using MIA to find LFV. The new experiment data of muon is reported by the workers at Fermilab National Accelerator Laboratory (FNAL)ZZZ1 ; ZZZ2 ; ZZZ3 ; ZZZ4 . Combined with the previous Brookhaven National Laboratory (BNL) E821 resultZZZ5 , we get the new averaged experiment value of muon anomaly is (0.35ppm). The departure from the SM prediction is , which is about 4.2. We set the particle masses as: , , , , , and to get Fig.6. When in Fig.6(a), we can see that from bottom to top are solid line (), dashed line () and dotted line () and the overall trend of the three lines is downward. That is to say, is a sensitive parameter and larger leads to larger .
We set in Fig.6(b). It is obvious that three lines with the same tendency to decrease and then increase. The dotted line () is below the dashed line (), and the dashed line is below the solid line (). When increases, decreases. The influence of on mainly depends on its own value: when is less than 0.3, increases and decreases, but when , the situation is just the opposite. The value of can reach at most, it can reach about of the departure(), which better meets the experimental limitations. The above conclusion is the same as Eq.(75), so we can find other sensitive parameters more intuitively through formula Eq.(75).


IV.2 The processes of
In order to study the parameters affecting LFV, we need to study some sensitive parameters. To show the numerical results clearly, we draw the relation diagrams and scatter diagrams of with different parameters.
The gray area is current limit on LFV decay in Fig.7. With the parameters , and , we plot versus in the Fig.7(a). The dashed curve corresponds to and the solid line corresponds to . On the whole, both lines show a downward trend. and are inversely proportional. The smaller is, the greater the value of is. Separately, the dashed line is larger than the solid line, and the ranges consistent with the experimental value are and respectively. and are positively correlated. If the value of gets smaller, the value of can be less than 1000 GeV.
We show varying with by the solid curve ( GeV) and dashed curve ( GeV) in the Fig.7(b). We can see that the overall values meet the limit, and the trend is a subtractive function, and the solid line is greater than the dotted line. So we can conclude that as increases, also increases. When increases, decreases. The numerical results are tiny and at the order of .
Finally, we analyze the effects of the parameter on branching ratio of . The numerical results are shown in the Fig.7(c) by the dashed curve () and solid curve (). The value of the solid line is greater than that of the dashed line, and both show an upward trend. Therefore, The relationship between and , and is the similar, and they are all positively correlated.



For more multidimensional analysis of sensitive parameters, we scatter points according to Table 1 (Part of ) to get Fig.8. We set the range of (0), () and ( ) to represent the results in different parameter spaces for the process of .
The relationship between and is shown in Fig.8(a). are mainly concentrated in the upper left corner, then the outer layer are and finally . When is near 0 and is near , gets the minimum value. Fig.8(b) is plotted in the plane of versus . We can clearly find that the points are mainly concentrated in the right, and the color gradually deepens from lower left to upper right. The effects of and on are shown in the Fig.8(c). All points are mainly concentrated in the upper left corner and on both sides of axis and axis . From the inside to the outside, they are , and . When becomes larger and becomes smaller, the value of increases to reach the experimental measurement. Fig.8(d) shows the effect of and on . All points are mainly concentrated near the x-axis, and the value increases from bottom to top.




IV.3 The processes of
To study the influence of parameters and on in Fig.9, we suppose the parameters , and and plot the solid line () and dashed line (). In Fig.9(a), we can see that corresponds to . We plot varying with in the Fig.9(b). Both figures show an upward trend within the experimental limit, and the dashed line is larger than the solid line, so we can draw a conclusion: when or increases, also increases. In the whole, the numerical results in Fig.9 are very tiny.


We scatter points according to the parameters given in Table 1 (part of ) to obtain Fig.10. Where , and represent (0), ( ) and () respectively.
corresponds to in Fig.10(a). Horizontally, are mainly concentrated in , are in , and are distributed in . Vertically, and are concentrated near , and there are obvious stratification, from bottom to top are , , . So we can know that as increases, increases, and when increases, decreases. We plot varying with in the Fig.10(b). The three types of points are almost symmetrical about . The smaller the is, the greater the value of . The farther away the value of from the 0 axis, the greater the value of . Fig.10(c) is shown in the plane of versus , where the centralized distributions of the three types of points are distributed in a ”U” shape on both sides of the axis. distribute on the innermost side, followed by and on the outermost side. So as increases, decreases. Finally, we analyze the effects from parameters and in Fig.10(d). All points are , and from top to bottom. The smaller the value of , the larger the value of .




IV.4 The processes of
Based on the Table 1 (part of ), we analyze to study the possibility of LFV in Fig.11. The branching ratio of process is denoted by: (0), ( ) and ( from to ).
The Fig.11(a) shows the effects from and . Most points are concentrated in lower right quarter. are on the innermost side of the whole region. are in the middle and are on the outermost side. The numerical performance is that the larger the and the smaller the , the larger the . In Fig.11(b), we analyze the effects of and on . In the whole figure, are mainly close to both sides of the x-axis and y-axis, and then with the same trend, and the rest is . decreases with the increase of and . Fig.11(c) has two axes versus . All three points show ”” shaped distribution, from left to right are , , . So increases with the increase of .



V discussion and conclusion
The SSM has new superfields including righ-handed neutrinos and three Higgs superfields , and its local gauge group is . We use MIA to study muon anomalous magnetic moment. Combined with the latest experimental data, our numerical results can reach about , which can better fit the measurement result, and play a certain role in promoting the study of LFV. We use the method of MIA to study lepton flavor violating decays in the SSM model. From the order of magnitude of branching ratio and data analysis, we can find that the restriction on lepton flavor violation in the process of is stronger. This provides a reference for other lepton flavor violation work in the future.
We take into account the constraints from the upper limits on LFV branching ratios of . In the numerical calculation, we take many parameters as variables including and . Through the analysis of the numerical results, we find that and are sensitive parameters. is an increasing function of , and decreasing function of and . can also give influence on the numerical results but not very large. That is to say they give mild influences on the numerical results. Finally, we come to the conclusion that the non-diagonal elements which correspond to the generations of the initial lepton and final lepton are main sensitive parameters and LFV sources.
Acknowledgments
This work is supported by National Natural Science Foundation of China (NNSFC) (No. 11535002, No. 11705045), Natural Science Foundation of Hebei Province (A2020201002). Post-graduate’s Innovation Fund Project of Hebei University (HBU2022ss028).
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