in a flavor violating extension of MSSM
Abstract
meson rare decays play a crucial role in exploring new physics beyond the standard model. In this study, we explore the rare decay process in a flavor violating extension of the Minimal Supersymmetric Standard Model (MSSM), namely the -from- SSM (SSM). Combined with the decay , the numerical results indicate that the SSM can successfully accommodate the experimental data for and additionally narrow down the parameter space.
pacs:
12.60.Jv, 14.80.DaI Introduction
While the Standard Model (SM) has achieved great success in describing known phenomena, it is still believed to require improvement or expansion to describe physics at higher energy scales. Beyond the SM, supersymmetry is regarded as one of the most credible candidates. In supersymmetric(SUSY) theory, novel TeV-scale particles can feature in competing diagrams, inducing detectable effects on the rate or other attributes of the decay process. The examinations of rare decays offer a means to identify new physics beyond the SM as they are less susceptible to uncertainties stemming from nonperturbative QCD effects. Recently, the average experimental data on the branching ratios of and are reported as follows HFLAV:2022esi ; ParticleDataGroup:2024cfk ; ATLAS:2018cur ; CMS:2019bbr ; LHCb:2021vsc ; CMS:2022mgd
(1) |
The SM predicts the branching ratios for recently as Misiak:2020vlo
(2) |
The SM prediction for including power-enhanced QED correction is Beneke:2017vpq ; Beneke:2019slt ; Czaja:2024the
(3) |
These SM predictions align well with the experimental results, indicating that the precise measurements of rare decay processes impose stringent constraints on new physics beyond the SM. The primary objective of investigating decays is to seek evidence of new physics and define its parameter space.
Indeed, the analyses of constraints on extensions of the SM are widely discussed in the literature. The calculation of the rate inclusive decay is detailed by the authors of Refs. Ciuchini1 ; Ciafaloni ; Borzumati1 within the framework of the Two-Higgs doublet model (THDM). The impact of supersymmetry on is deliberated in Refs. NPB4 ; NPB5 ; NPB6 ; NPB8 ; NPB7 ; Zhang1 ; Feng1 , while the next-to-leading order (NLO) QCD corrections are provided in Ref. NPB9 . The branching ratio for in the THDM and SUSY extensions of the SM has been calculated in Refs. He:1988tf ; Skiba:1992mg ; Choudhury:1998ze ; Huang:2000sm ; Feng2 ; Feng3 . Additionally, Ref. NPB11 delves into hadronic decays, while CP-violation in these processes is discussed in Ref. NPB12 . Ref. NPB13 explores the potential for observing SUSY effects in rare decays and at the B-factory. The investigation of SUSY effects on these processes is highly intriguing, and research on them could illuminate the fundamental features of the SUSY model. The pertinent reviews can be found in Refs. NPB16 ; NPB17 .
In the context of the Supersymmetric Standard Model with a neutrino Yukawa sector (SSM)ref2 ; ref3 ; ref4 ,the model addresses the problem ref5 that arises in the MSSM ref6 ; ref7 ; ref8 . This resolution is facilitated through the inclusion of lepton number-breaking couplings between the right-handed neutrino superfields and the Higgs fields in the superpotential. The term is spontaneously generated through the vacuum expectation values (VEVs) of the right-handed neutrino superfields, denoted as , upon the breaking of the electroweak symmetry (EWSB).
In our previous work, we have investigated the decay in the SSM Zhang1 . In this paper, we investigate the flavor changing neutral current (FCNC) processes within the framework of the SSM using a minimal flavor-violating scenario for the soft breaking terms, combined with the decay . Our presentation is structured as follows: Section II provides a brief summary of the key components of the SSM, encompassing the superpotential and general soft breaking terms. Section III presents the effective Hamiltonian for . The numerical analyses are detailed in Section IV, with Section V offering a summary. Appendix contains the detailed formulas.
II the SSM
Compared to the MSSM, the includes three right-handed sneutrino superfields (where ) with non-zero VEVs. The superpotential of the can be expressed as ref2 :
(4) |
where represents the antisymmetric tensor. , , , and represent doublet superfields. The symbols , , and denote the singlet superfields corresponding to the down-type quark, up-type quark, and lepton, respectively. Furthermore, , , and are dimensionless matrices, a vector, and a totally symmetric tensor. The indices , are indices, while , , represent generation indices.
In Eq. (4), the initial three terms mirror those found in the MSSM. Following the EWSB, the subsequent terms can generate effective bilinear expressions such as and , where and . The final two terms explicitly break lepton number and R-parity, with the last term capable of generating effective Majorana masses for neutrinos at the electroweak scale. Throughout this paper, the summation convention is assumed for repeated indices.
In the SSM, the general soft SUSY-breaking terms are as follows:
(5) |
Here, the first two lines feature mass-squared terms of squarks, sleptons, and Higgs bosons. The following two lines encompass the trilinear scalar couplings. The final line specifies the Majorana masses for gauginos , , and denoted as , , and respectively. Besides the terms from , the tree-level scalar potential also receives the typical contributions from and terms ref3 .
After spontaneous breaking of the electroweak symmetry, the neutral scalars typically acquire VEVs:
(6) |
One has the option to characterize the neutral scalars as
(7) |
and
(8) |
Then, given that , we can define the value of as usual, where .
In the charged scalar mass matrix , there exist the massless unphysical Goldstone bosons which can be expressed as ref-zhang ; ref15 ; ref-zhang-LFV ; ref-zhang1 :
(9) |
In the unitary gauge, these Goldstone bosons are absorbed by the -boson and are no longer present in the Lagrangian. Consequently, the mass squared of the -boson is given by:
(10) |
where represents the electromagnetic coupling constant, with being the Weinberg angle.
III Theoretical calculation on
The effective Hamiltonian for the transition at the hadronic scale can be expressed as bobeth ; bobeth02 :
(11) |
with the CKM combination and
(12) |


Since the contribution of is doubly Cabibbo-suppressed compared to the contribution of , we can ignore it in the subsequent calculations. The new effective Hamiltonian can be written as:
(13) |
where and are defined as follows:
(14) |
Here, represents the strong coupling, refers to the electromagnetic field strength tensors, denotes the gluon field strength tensors, and are the generators of .
The primary Feynman diagrams that contribute to the process in the are depicted in Fig. 1, where () denote neutral scalars, () denote neutral fermions, () denote charged scalars, () denote up-type squarks, () denote three generation of up-type quarks and () denote charged fermions. At the electroweak energy scale , the corresponding Wilson coefficients can be denoted as
(15) |
Here, the superscripts (1, …, 11) correspond to the new physics corrections in Fig. 1, and the specific expressions for these Wilson coefficients are detailed in Appendix A. The Wilson coefficients at the hadronic energy scale, ranging from the SM to next-to-next-to-logarithmic (NNLL) accuracy, are presented in Table I Altmannshofer:2008dz .
Furthermore, the Wilson coefficients in Eqs.(15) are computed at the matching scale and subsequently evolved down to the hadronic scale through the renormalization group equations:
(16) |
where
(17) |
According to Ref. Gambino1 , the definitions of are as follows:
(18) |
where and . Note that the definition of in this work differs slightly from that in Ref. Altmannshofer:2008dz which includes factorisable contributions from the quark loops: . Correspondingly, the evolving matrices are approximated as
(19) |
By utilizing the Eq. (30) from Ref. Gambino1 , we can calculate the corresponding anomalous dimension matrices
(30) |
(35) |
Then, the squared amplitude can be denoted as
(36) |
and
(37) | |||
(38) | |||
(39) |
Here, the decay constant is denoted by FlavourLatticeAveragingGroupFLAG:2021npn ; Bazavov:2017lyh ; ETM:2016nbo ; Dowdall:2013tga ; Hughes:2017spc , and the mass of the neutral meson is represented by ParticleDataGroup:2024cfk .
Ultimately, the branching ratio of can be expressed as:
(40) |
where ps HFLAV:2022esi denotes the lifetime of .
IV Numerical analysis
In this section, we provide the numerical results of the branching ratios for rare meson decays and . We analyzed how individual parameters affect the branching ratios of these two processes. To better understand the impact of these parameters on the branching ratios, we need to first make a reasonable selection of other parameters.
The relevant SM input parameters are presented in Table 2. All other parameters in SM remain unchanged compared to those listed in Table I of Ref. Bobeth:2013uxa , as their modification would have negligible impact on either because they are already measured with high precision or because their influence on is minimal.
Parameter | Value | Unit | Ref. |
---|---|---|---|
ps | HFLAV:2022esi | ||
– | ParticleDataGroup:2024cfk | ||
– | ParticleDataGroup:2024cfk | ||
MeV | ParticleDataGroup:2024cfk | ||
MeV | FlavourLatticeAveragingGroupFLAG:2021npn ; Bazavov:2017lyh ; ETM:2016nbo ; Dowdall:2013tga ; Hughes:2017spc | ||
– | Finauri:2023kte | ||
– | Charles:2004jd |
In the SUSY extensions of the SM, there exist numerous free parameters. Given the structure of the soft SUSY-breaking terms, the free parameters in the are:
(41) |
To streamline numerical results, we apply the minimal flavor violation (MFV) assumption to certain parameters in the . This assumption includes:
(42) |
For the relevant parameters for quarks, one can have
(43) |
where denotes the CKM matrix CKM1 ; CKM2 . Constrained by the masses of quarks and leptons, approximate relations can be defined: , , , where , , and denote the masses of up-quarks, down-quarks, and charged leptons, respectively. The specific values are adopted from PDG. ParticleDataGroup:2024cfk . Additionally, the aforementioned soft masses and can be substituted with the VEVs in Eq. (6). By utilizing and the SM Higgs VEV, with the electroweak gauge couplings estimated at the scale by , one can derive the SUSY Higgs VEVs, and . Given that , it follows that .
In our prior investigation neu-mass6 , we extensively examined how the neutrino oscillation data restrict neutrino Yukawa couplings and left-handed sneutrino VEVs within the SSM through the TeV-scale seesaw mechanism. As the tiny neutrino masses have minimal impact on and , we can approximate and . For the Majorana masses of the gauginos, we will imply the approximate GUT relation and . The gluino mass, , is larger than about TeV from the ATLAS and CMS experimental data ATLAS-sg1 ; ATLAS-sg2 ; CMS-sg1 ; CMS-sg2 . So, we conservatively choose . Finally, the free parameters in Eq. (41) have been replaced with the following:
(44) |
Among the parameters in Eq. (44), , and have a significant impact on the results. The remaining parameters of the model are fixed as shown in Table 3.
TeV |
---|
, |
TeV |
, TeV |
TeV |
, , TeV |
GeV |
For squarks, the first two generations of squarks are strongly constrained by direct searches at the Large Hadron Collider (LHC) ATLAS.PRD ; CMS.JHEP . Therefore, we take . The third generation squark masses are not as strictly constrained by the LHC as the first two generations. Additionally, the smaller the mass of the third-generation squarks, the more pronounced the effect of the parameter on . To clearly investigate the impact of on , we have set the mass of the third-generation squarks slightly lower, specifically .
The variations in , , , and have a negligible impact on the calculation results for and . Based on the parameter space analysis outlined in Ref. ref3 , we can select reasonable values for certain parameters: , , and for simplicity in subsequent numerical computations. The parameter is a key factor influencing the subsequent numerical calculations.
In the , the sneutrino sector may exhibit tachyonic behavior. The squared masses of these tachyons are negative. Therefore, it is necessary to analyze the masses of the sneutrinos. The masses of the left-handed sneutrinos are primarily determined by , while the three right-handed sneutrinos are essentially degenerate. The mass squared for the CP-even and CP-odd right-handed sneutrinos can be approximated as follows ref-zhang1 :
(45) | |||
(46) |
In these expressions, the primary contribution to the mass squared comes from the first term when is large, in the limit of . Hence, we can use the approximate relation
(47) |
to prevent the occurrence of tachyons. While has minimal influence on the computation results, both and are crucial parameters affecting the calculations. In the subsequent analysis, varies between 0.1 and 1, while ranges from 1.5TeV to 2.5TeV. According to Eq. (47), we can conservatively set to avoid the emergence of tachyons.
In the limit of ref-limit-MH , the squared mass of the charged Higgs in the SSM can be expressed as:
(48) |
with the squared mass of the neutral pseudoscalar:
(49) |
In the scenarios of the MSSM and NMSSM Domingo:2007dx , the additional physics contributions to and primarily depend on , , and the charged Higgs mass . To demonstrate the impacts of and on the theoretical evaluations of and in the , we temporarily fix the parameters , , and . Specifically, we set , , and . The plots in Fig. 2 show and as functions of for (solid line) and (dashed line).
Additionally, since our calculation method essentially combines the new contributions from SUSY particles with the existing SM contributions to obtain the final result, we can roughly consider the theoretical uncertainty in the SM as the minimum uncertainty for the . We treat the theoretical uncertainty in the SM from Eq. (2, 3) as the minimum uncertainty in the , which is then combined with the experimental uncertainty, represented by the gray band in the figure. The central value of the gray band is the experimental central value from Eq. (1). The range of the gray band in Fig. 2(a) is , while the range of the gray band in Fig. 2(b) is .


From Fig. 2, we can see that both and constrain the parameter space of the . From Fig. 2(a), it is evident that as increases, will first decrease before increasing. The experimental data on favors within specific ranges: for and for . From Fig. 2(b), it is evident that decreases with the escalation of . As the value of increases, the curve becomes steeper, consequently narrowing the allowed range of . When is set to 15, the allowable range for is , and when is set to 30, the allowable range for is . The influence of on the calculation results mainly comes from its ability to adjust the coupling strengths between the stop quark and other particles in the Feynman diagrams.
Considering the constraint from the SM-like Higgs mass, in the upcoming parameter analysis, we set = TeV with to ensure the SM-like Higgs mass around . In addition, is a unique parameter in the SSM. To investigate the impact of on the results, we will plot curves for two cases: =1.5 TeV represented by a solid line and =2 TeV represented by a dashed line.


In the MSSM Domingo:2007dx , the theoretical predictions of and will decrease as increases. From Eqs.(48, 49), it can be seen that can influence in the SSM. Moreover, an increase in leads to an increase in . To explore the influences of on and , we plot curves in Fig. 3 showing how and vary with . From Fig. 3(a), it is evident that gradually decreases as increases, eventually stabilizing within the range that is consistent with the experimental data. When is set to 2 TeV, the experimental data concerning favor lying within the range . When is set to 1.5 TeV, is required to be approximately greater than 0.55. From Fig. 3(b), it can be observed that the theoretical predictions for are consistent with the experimental results, and as increases, there is a slight decrease in . From Fig. 3, we can also observe that both and decrease as increases. From Eqs.(48, 49), it can be seen that, similar to , an increase in will also lead to an increase in . Therefore, both and can influence the theoretical predictions for and by affecting . The parameter also have effects similar to those of the parameter .
From Eqs.(48, 49), it can be seen that an increase in the parameter will also lead to an increase in . Thus, we speculate that the parameter , like and , can also influence the theoretical predictions for and . To investigate this influence, we set =0.5, and then plot curves in Fig. 4 showing how and vary with .


From Fig. 4(a), it is evident that gradually decreases as increases, eventually stabilizing within the range consistent with the experimental data. The experimental data on limits that for =1.5 TeV and for =2 TeV. From Fig. 4(b), the theoretical predictions for remain consistent with the experimental results. As increases, will slightly decrease. Similar to the situation in Fig. 3, an increase of will result in a decrease in both and , with this effect becoming more pronounced as decreases.
V Summary
In this study, we examine the branching ratio of the rare decay within the framework of the SSM combined with . Similar to the MSSM and NMSSM, the additional physics contributions to and in the SSM primarily rely on , , and . This is due to the suppressed mixings between charginos and charged leptons in the mass matrix of the SSM, as well as the mixings between charged Higgs bosons and charged sleptons. Additionally, , , and in the SSM can also influence the theoretical predictions of and by affecting . Subject to the constraint of a SM-like Higgs boson with a mass approximately at 125 GeV, the numerical findings indicate that the new physics can align with the experimental data for meson rare decays and consequently narrow down the parameter space. In addition to and , the SSM also contributes to . This contribution may lead to other observables, such as the ”-anomalies”, which we will investigate in detail elsewhere.
Acknowledgements.
The work has been supported by the National Natural Science Foundation of China (NNSFC) with Grants No. 12075074, No. 12235008, Hebei Natural Science Foundation with Grants No. A2022201017, No. A2023201041, and the youth top-notch talent support program of the Hebei Province.Appendix A The Wilson coefficients of the process in the SSM.
In our previous work Bsmumu:2018 , we have calculated the Wilson coefficients of the process in the B-LSSM. Because the SUSY particles contained in the B-LSSM and the SSM are different, the Feynman diagrams involving SUSY particles will vary when calculating the amplitude for the same process . Through the previous work, the Wilson coefficients corresponding to in the SSM can be written as
(50) |
(51) |
(52) |
(54) |
(60) |
(61) |
(62) |
where denotes the constant parts of the interaction vertex about , and denote the interactional particles, denotes photon and boson. and in superscript denote the left-hand part and right-hand part.
Denoting , the concrete expressions for can be given as:
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