Lepton Flavor Violation In Decays Research Articles

Leptoquark-induced CLFV decays with a light SM-singlet scalar

The Standard Model (SM), if augmented with a light SM-singlet scalar ϕ and a TeV-scale scalar leptoquark (LQ) S1, 2-body charged lepton flavor violating (CLFV) decay channels can be accessed with ϕ as one of the final states, where the leading order effective interactions between ϕ and the SM fields arise at one-loop level. Further, in the presence of S1, ϕ can mediate 3-body CLFV processes with either two photons or two gluons in the final states. Thus, the model predicts an exotic 3-body CLFV channel: ℓA→ℓBgg, which can be tested/constrained only through some future high-energy experiments looking for di-gluon signals from a leptonic decay.

Electron EDM and LFV decays in the light of Muon (g-2)μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(g-2)_\\mu $$\\end{document} with U(2) flavor symmetry

We study the interplay of New Physics (NP) among the lepton magnetic moment, the lepton flavor violation (LFV) and the electron electric dipole moment (EDM) in light of recent data of the muon (g-2)μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(g-2)_\\mu $$\\end{document}. The NP is discussed in the leptonic dipole operator with the U(2) flavor symmetry of the charged leptons, where possible CP violating phases of the three family space are taken into account. It is remarked that the third-family contributes significantly to the LFV decay, μ→eγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\rightarrow e\\gamma $$\\end{document}, and the electron EDM. The experimental upper-bound on μ→eγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\rightarrow e\\gamma $$\\end{document} decay gives a severe constraint on the parameters of the flavor model. The predicted electron EDM is rather large due to the CP violating phases in the three family space. In addition, we also study (g-2)e,τ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(g-2)_{e,\ au }$$\\end{document} of the electron and tauon, and EDMs of the muon and tauon as well as the τ→eγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au \\rightarrow e \\gamma $$\\end{document} and τ→μγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au \\rightarrow \\mu \\gamma $$\\end{document} decays. The τR→μLγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au _R \\rightarrow \\mu _L \\gamma $$\\end{document} decay is predicted to be close to the experimental upper-bound.

Rare decay t→cγγ via scalar leptoquark doublets

A calculation of the one-loop contribution to the rare three-body flavor changing neutral current top quark decay $t\to c\gamma\gamma$ is presented in the framework of models with one or more scalar leptoquark $SU(2)$ doublets with hypercharge $7/6$. Analytical expressions for the invariant amplitude of the generic decay $f_i\to f_j\gamma\gamma$, with $f_{i,j}$ a lepton or quark, are presented in terms of Passarino-Veltman integral coefficients, from which the amplitudes for the processes $t\to c\gamma\gamma$ and $\ell_i\to \ell_j\gamma\gamma$ follow easily. An analysis of the current constraints on the parameter space is presented in the scenario with only one scalar LQ doublet and bounds on the LQ couplings are obtained from the muon $g-2$ anomaly, the lepton flavor violating (LFV) decay $\tau\to \mu\gamma$ and extra constraints meant to avoid tension between theory predictions and experimental data. For a LQ with a mass in the range of $1$--$1.5$ TeVs, the estimate ${\rm Br}(t\to c\gamma\gamma)\sim 10^{-11}$--$10^{-12}$ is obtained for the largest allowed values of the LQ coupling constants, which means that this decay would be below the reach of future experimental measurements. We also consider an scenario with three scalar doublets, which was recently proposed to explain the lepton flavor universality violation anomalies in $B$ decays as well as the muon $g-2$ anomaly. Although this scenario allows large LQ couplings to the tau lepton and the $c$ and $t$ quarks, the branching ratio of the $t\to c\gamma\gamma$ decay is also of the order of $10^{-11}$--$10^{-12}$ for LQ masses of 1.7 TeV.

Exploring the feasibility of the charged lepton flavor violating decay μ → e + γ in inverse and linear seesaw mechanisms with A4 flavor symmetry

One of the possible ways to explain the observed flavor structure of fundamental particles is to include flavor symmetries in the theories. In this work, we investigate the rare charged lepton flavor violating (cLFV) decay process [Formula: see text] in two of the low-scale ([Formula: see text][Formula: see text]TeV) seesaw models: (i) the Inverse seesaw (ISS) and (ii) Linear seesaw (LSS) models within the framework of [Formula: see text] flavor symmetry. Apart from the [Formula: see text] flavor symmetry, some other symmetries like [Formula: see text], [Formula: see text] and [Formula: see text] are included to construct the Lagrangian. We use results from our previous work [M. R. Devi and K. Bora, A comparative study of type-II, inverse and linear seesaw mechanisms with [Formula: see text] flavor symmetry, presented at DAE HEP Symp., 2020, Jatni, Odisha; M. R. Devi and K. Bora, Mod. Phys. Lett. A 37, 2250073] where we computed unknown neutrino oscillation parameters within [Formula: see text] limits of their global best fit values, and apply those results to compute the branching ratio (BR) of the muon decay for both the seesaw models. Next we compare our results with the current experimental bounds and sensitivity limits of BR[Formula: see text] as projected by various experiments, and present a comparative analysis that which of the two models is more likely to be tested by which current/future experiment. This is done for various values of currently allowed non-unitarity parameter. This comparative study will help us to pinpoint that which of the low-scale seesaw models and triplet flavon VEV alignments will be more viable and favorable for testing under a common flavor symmetry ([Formula: see text] here), and hence can help discriminate between the two models.

Commissioning and preliminary performance of the MEG II drift chamber

In the quest for Lepton Flavor Violation (LFV) the MEG experiment at the Paul Scherrer Institut (PSI) represents the state of the art in the search for the charged LFV decay μ+→e+γ, setting the most stringent upper limit on the BR(μ+→e+γ)≤4.2×10−13 (90% C.L.). An upgrade of MEG, MEG II, was designed and it recently started the physics data taking, with the aim to reach a sensitivity level of 6×10−14. The Cylindrical Drift CHamber (CDCH) is a key detector in order to improve the e+ angular and momentum resolutions at the 6.5 mrad and 100 keV/c level. The CDCH is a low-mass single volume detector with high granularity: 9 layers of 192 drift cells each, few mm wide, defined by 12000 wires in a stereo configuration for longitudinal hit localization. After the assembly, the CDCH was transported to PSI for the commissioning phase and it has been integrated into the MEG II experimental apparatus since 2018. The operational stability was reached in 2020 and the complete readout electronics was tested for the first time in 2021. A preliminary analysis of 2020–2021 data is presented.

Lepton flavor violation, lepton (g − 2)μ, e and electron EDM in the modular symmetry

We study the lepton flavor violation (LFV), the leptonic magnetic moments (g − 2)μ, e and the electric dipole moment (EDM) of the electron in the Standard-Model Effective Field Theory with the ΓN modular flavor symmetry. We employ the stringy Ansatz on coupling structure that 4-point couplings of matter fields are written by a product of 3-point couplings of matter fields. We take the level 3 finite modular group, Γ3 for the flavor symmetry, and discuss the dipole operators at nearby fixed point τ = i, where observed lepton masses and mixing angles are well reproduced. Suppose the anomaly of the anomalous magnetic moment of the muon to be evidence of the new physics (NP), we have related it with (g − 2)e, LFV decays, and the electron EDM. It is found that the NP contribution to (g − 2)e is proportional to the lepton masses squared likewise the naive scaling. We also discuss the correlations among the LFV processes μ → eγ, τ → μγ and τ → eγ, which are testable in the future. The electron EDM requires the tiny imaginary part of the relevant Wilson coefficient in the basis of real positive charged lepton masses, which is related to the μ → eγ transition in our framework.

Loop-corrected Higgs masses in the NMSSM with inverse seesaw mechanism

In this study, we work in the framework of the Next-to-Minimal extension of the Standard Model (NMSSM) extended by six singlet leptonic superfields. Through the mixing with the three doublet leptonic superfields, the non-zero tiny neutrino masses can be generated through the inverse seesaw mechanism. While R-parity is conserved in this model lepton number is explicitly violated. We quantify the impact of the extended neutrino sector on the NMSSM Higgs sector by computing the complete one-loop corrections with full momentum dependence to the Higgs boson masses in a mixed on-shell-overline{text{ DR }} renormalization scheme, with and without the inclusion of CP violation. The results are consistently combined with the dominant two-loop corrections at mathcal{O}(alpha _t(alpha _s+alpha _t)) to improve the predictions for the Higgs mixing and the loop-corrected masses. In our numerical study we include the constraints from the Higgs data, the neutrino oscillation data, the charged lepton flavor-violating decays l_i rightarrow l_j + gamma , and the new physics constraints from the oblique parameters S, T, U. We present in this context the one-loop decay width for l_i rightarrow l_j + gamma . The loop-corrected Higgs boson masses are included in the Fortran code NMSSMCALC-nuSS.

Impact of non-universal {{Z}}^{{'}} in the lepton flavour violating {{B}}left( {{B}}_{{{s}}} right) {rightarrow }{{K}}^{{*}}left( {varphi } right) {{l}}_{{{1}}}^{{-}}{{l}}_{{{2}}}^{{+}} decays

In recent years, lepton flavour violating (LFV) decays are one of the most trending topics to probe new physics (NP). The latest results of LHCb have motivated us to study the LFV decays through brightarrow s transition. The branching ratios, forward backward asymmetries and longitudinal polarization fractions of B(B_{s})rightarrow K^{*}(varphi )l_{1}l_{2} decays are studied in non-universal Z^{'} model. Here, we have structured the four-fold angular distribution of the decays in terms of transversity amplitudes. The variation of the observables in whole kinematic region shows the sensitivity of NP. The observables estimated in this work are very intriguing and might lead a new track towards NP in near future.

Lepton flavor violating Z and Higgs decays in the scotogenic model

In this work, we have studied lepton flavor violating (LFV) decays of Z gauge boson and Higgs boson (H) in the scotogenic model. We have computed branching ratios for the decays Zrightarrow ell _alpha ell _beta and Hrightarrow ell _alpha ell _beta in this model. Here, ell _alpha and ell _beta are different charged lepton fields. After fitting to the neutrino oscillation observables in the scotogenic model, we have found that the branching ratios for the LFV decays of Z and H can be as large as sim 10^{-8} and sim 10^{-3} respectively. However, after satisfying the constraints due to non-observation of ell _alpha rightarrow ell _beta gamma decays, the above mentioned branching ratio results are found to be suppressed by a factor of sim 10^{-7}.

Search for lepton-flavor-violating ALPs at a future muon collider and utilization of polarization-induced effects

Axion-Like Particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global U(1) symmetries. Such particles can have lepton-flavor-violating (LFV) couplings to the SM charged leptons. LFV ALPs provide the possibility to address some of the SM long-lasting problems. We investigate the sensitivity of a future muon collider suggested by the Muon Accelerator Program (MAP) to the production of LFV ALPs in the ALP mass range ma≤1 MeV. ALPs are assumed to be produced through the LFV decay τ→ℓa (ℓ=e,μ) of one of the tau leptons produced in the muon-anti muon annihilation. Performing a realistic detector simulation and deploying a multivariate technique, we constrain the LFV couplings cτe and cτμ for both the cases of unpolarized and polarized muon beams. Three different chiral structures are considered for the LFV ALP coupling and the muon collider is assumed to operate at the center-of-mass energies of 126, 350 and 1500 GeV. We present a procedure to search for LFV ALPs at colliders which takes advantage of tau polarization-induced effects. Polarized tau leptons which produce such effects can be produced when the initial muon beams are polarized. Utilizing the properties of polarized tau decays, the main SM background which overwhelms the ALP production in the case of unpolarized muon beams can be significantly suppressed. We present 95% CL expected limits on the LFV couplings and show that the present analysis can improve current experimental limits on the ALP LFV couplings by roughly one order of magnitude.

Search for Lepton Flavor Violation in ϒ(3S)→e^{±}μ^{∓}.

We report on the first search for electron-muon lepton flavor violation (LFV) in the decay of a b quark and b antiquark bound state. We look for the LFV decay ϒ(3S)→e^{±}μ^{∓} in a sample of 118million ϒ(3S) mesons from 27 fb^{-1} of data collected with the BABAR detector at the SLAC PEP-II e^{+}e^{-} collider operating with a 10.36GeV center-of-mass energy. No evidence for a signal is found, and we set a limit on the branching fraction B[ϒ(3S)→e^{±}μ^{∓}]<3.6×10^{-7} at 90% C. L. This result can be interpreted as a limit Λ_{NP}/g_{NP}^{2}>80 TeV on the energy scale Λ_{NP} divided by the coupling-squared g_{NP}^{2} of relevant new physics (NP).

No-lose theorem for discovering the new physics of (g−2)μ at muon colliders

We perform a model-exhaustive analysis of all possible beyond Standard Model (BSM) solutions to the $(g-2)_\mu$ anomaly to study production of the associated new states at future muon colliders, and formulate a no-lose theorem for the discovery of new physics if the anomaly is confirmed and weakly coupled solutions below the GeV scale are excluded. Our goal is to find the highest possible mass scale of new physics subject only to perturbative unitarity, and optionally the requirements of minimum flavour violation (MFV) and/or naturalness. We prove that a 3 TeV muon collider is guaranteed to discover all BSM scenarios in which $\Delta a_\mu$ is generated by SM singlets with masses above $\sim $ GeV; lighter singlets will be discovered by upcoming low-energy experiments. If new states with electroweak quantum numbers contribute to $(g-2)_\mu$, the minimal requirements of perturbative unitarity guarantee new charged states below $\mathcal{O}(100 {\rm TeV})$, but this is strongly disfavoured by stringent constraints on charged lepton flavour violating (CLFV) decays. Reasonable BSM theories that satisfy CLFV bounds by obeying Minimal Flavour Violation (MFV) and avoid generating two new hierarchy problems require the existence of at least one new charged state below $\sim 10$ TeV. This strongly motivates the construction of high-energy muon colliders, which are guaranteed to discover new physics: either by producing these new charged states directly, or by setting a strong lower bound on their mass, which would empirically prove that the universe is fine-tuned and violates the assumptions of MFV while somehow not generating large CLFVs. The former case is obviously the desired outcome, but the latter scenario would perhaps teach us even more about the universe by profoundly revising our understanding of naturalness, cosmological vacuum selection, and the SM flavour puzzle.

On the role of leptonic CPV phases in cLFV observables

In extensions of the standard model by Majorana fermions, the presence of additional CP violating phases has been shown to play a crucial role in lepton number violating processes. In this work we show that (Dirac and Majorana) CP violating phases can also lead to important effects in charged lepton flavour violating (cLFV) transitions and decays, in some cases with a significant impact for the predicted rates of cLFV observables. We conduct a thorough exploration of these effects in several cLFV observables, and discuss the implications for future observation. We emphasise how the presence of leptonic CP violating phases might lead to modified cLFV rates, and to a possible loss of correlation between cLFV observables.

Minimal inverse-seesaw mechanism with Abelian flavour symmetries

We study the phenomenology of the minimal (2, 2) inverse-seesaw model supplemented with Abelian flavour symmetries. To ensure maximal predictability, we establish the most restrictive flavour patterns which can be realised by those symmetries. This setup requires adding an extra scalar doublet and two complex scalar singlets to the Standard Model, paving the way to implement spontaneous CP violation. It is shown that such CP-violating effects can be successfully communicated to the lepton sector through couplings of the scalar singlets to the new sterile fermions. The Majorana and Dirac CP phases turn out to be related, and the active-sterile neutrino mixing is determined by the active neutrino masses, mixing angles and CP phases. We investigate the constraints imposed on the model by the current experimental limits on lepton flavour-violating decays, especially those on the branching ratio BR(μ → eγ) and the capture rate CR(μ − e, Au). The prospects to further test the framework put forward in this work are also discussed in view of the projected sensitivities of future experimental searches sensitive to the presence of heavy sterile neutrinos. Namely, we investigate at which extent upcoming searches for μ → eγ, μ → 3e and μ − e conversion in nuclei will be able to test our model, and how complementary will future high-energy collider and beam-dump experiments be in that task.

Search for a cosmologically relevant boson in muon decay

Experiments looking for a lepton flavor-violating decay $\mu^{+}\!\!\rightarrow \!e^{+} X^{0}$ are reviewed in light of present-day germanium detector technology, with an eye on scenarios where a long-lived, slow-moving massive boson $X^{0}$ might have a cosmological impact. A broad swath of interesting, unexplored parameter space very close to the kinematic limit of the decay is found to be within the reach of a new proposed search. A number of possible roles for $X^{0}$ in past and present epochs can be investigated.

Constraints on the charged-current non-standard neutrino interactions induced by the gauge boson W′

Many new physics scenarios predict the existence of the extra charged gauge boson W′, which can induce the charged-current (CC) non-standard neutrino interactions (NSI). We investigate the constraints on the CC NSI in model-independent fashion via considering the W′ contributions to the lepton flavor violating (LFV) decays ℓi→ℓjγ, the pure leptonic flavor conservation (FC) decays ℓi→ℓjνiνj¯, leptonic decays of charged pion meson, semileptonic τ decays, and superallowed β decays. We find that the constraints on the pure leptonic CC NSI are generally stronger than the ones for the CC NSI with first generation quarks. The most stringent constraints on the CC NSI arise from the LFV decay μ→eγ. The constraints on the CC NSI from direct searches at the LHC are also discussed.

An explanation for the muon and electron g \u2212 2 anomalies and dark matter

We propose simple models with a flavor-dependent global U(1)ℓ and a discrete ℤ2 symmetries to explain the anomalies in the measured anomalous magnetic dipole moments of muon and electron, (g − 2)μ,e, while simultaneously accommodating a dark matter candidate. These new symmetries are introduced not only to avoid the dangerous lepton flavor-violating decays of charged leptons, but also to ensure the stability of the dark matter. Our models can realize the opposite-sign contributions to the muon and electron g − 2 via one-loop diagrams involving new vector-like leptons. Under the vacuum stability and perturbative unitarity bounds as well as the constraints from the dark matter direct searches and related LHC data, we find suitable parameter space to simultaneously explain (g − 2)μ,e and the relic density. In this parameter space, the coupling of the Higgs boson with muons can be enhanced by up to ∼ 38% from its Standard Model value, which can be tested in future collider experiments.

Lepton-flavor-violating decays of the SM-like Higgs boson $h\rightarrow e_ie_j$, and $e_i \rightarrow e_j\, \gamma $ in a flipped 3-3-1 model

Abstract In the framework of the flipped 3-3-1 model introduced recently [R. M. Fonseca and M. Hirsch, J. High Energy Phys. 1608, 003 (2016)], the lepton-flavor-violating (LFV) decay $\mu \rightarrow 3e$ was predicted to have a large branching ratio (Br) close to the recent experimental limit. We will show that the Br of LFV decays of the standard-model-like (SM-like) Higgs boson decays (LFVHD) Br$(h\rightarrow e_ae_b)$ may also be large. Namely, Br$(h\rightarrow \mu\tau,e\tau)$ can reach values of $\mathcal{O}(10^{-4}){-}\mathcal{O}(10^{-5})$, which will reach the upcoming experimental sensitivities. On the other hand, for LFV decays of charged leptons (cLFV) $(e_b\rightarrow e_a\gamma)$, the branching ratios are well below experimental bounds.

Radiative neutrino mass model from a mass dimension-11 \u2206L = 2 effective operator

We present the first detailed phenomenological analysis of a radiative Majorana neutrino mass model constructed from opening up a ∆L = 2 mass-dimension-11 effective operator constructed out of standard model fields. While three such operators are generated, only one dominates neutrino mass generation, namely {O}_{47}=overline{L^C}Loverline{Q^C}Loverline{Q}{Q}^C HH, where L denotes lepton doublet, Q quark doublet and H Higgs doublet. The under- lying renormalisable theory contains the scalars {S}_1sim left(overline{3},1,1/3right) coupling as a diquark, {S}_3sim left(overline{3},3,1/3right) coupling as a leptoquark, and Φ3 ∼ (3, 3, 2/3), which has no Yukawa couplings but does couple to S1 and S3 in addition to the gauge fields. Neutrino masses and mixings are generated at two-loop order. A feature of this model that is different from many other radiative models is the lack of proportionality to any quark and charged- lepton masses of the neutrino mass matrix. One consequence is that the scale of new physics can be as high as 107 TeV, despite the operator having a high mass dimension. This raises the prospect that ∆L = 2 effective operators at even higher mass dimensions may, when opened up, produce phenomenologically-viable radiative neutrino mass models. The parameter space of the model is explored through benchmark slices that are subject to experimental constraints from charged lepton flavour-violating decays, rare meson de- cays and neutral-meson mixing. The acceptable parameter space can accommodate the anomalies in RK (∗) and the anomalous magnetic moment of the muon.

Hunting for ALPs with lepton flavor violation

We examine the low-energy signatures of axion-like particles (ALPs) in lep- ton flavor violating (LFV) processes. By using a dimension-5 effective Lagrangian, we compute the most general ALP contributions to LFV decays of leptons and mesons. The provided expressions are valid for any choice of ALP mass and couplings. We explore the complementarity of different processes, identifying specific patterns to be experimentally tested. Constraints on LFV couplings are derived from existing data and prospects for forthcoming experiments are also discussed. As a by-product, we revisit the possibility of a simultaneous explanation of the observed discrepancies in the muon and electron g − 2 through ALP interactions.