Lepton Flavor Violation Processes Research Articles

Production of the massless dark photon $\widetilde{\gamma}$ associated with the photon $\gamma$ from charged lepton flavor violating decay processes

Abstract The massless dark photon $\widetilde{\gamma}$ can only interact with Standard Model (SM) sector via higher-dimensional operators. In this letter, we investigate its production associated with the ordinary photon $\gamma$ from the lepton flavor violation (LFV) process $\ell_i\rightarrow\ell_j\widetilde{\gamma}\gamma$ and di-production from the LFV process $\ell_i\rightarrow\ell_j \widetilde{\gamma} \widetilde{\gamma}$ induced by dipole operators. Comparing the obtained numerical results with the corresponding experimental measurements, we obtain the constraints on the effective couplings of $\widetilde{\gamma}$ with the SM charged leptons. The upper limit of the effective coupling ${\left|D_L^{\mu e}\right|}^2+{\left|D_R^{\mu e}\right|}^2$ coming from the process $\mu \rightarrow e\widetilde{\gamma}\gamma$ is looser than the process $\mu \rightarrow e\widetilde{\gamma}$ by about one order of magnitude.

Probing ALP lepton flavor violation at μTRISTAN

Axionlike particles (ALPs) with lepton flavor-violating (LFV) interactions are predicted within a wide range of flavored ALP models. The proposed μTRISTAN high-energy e−μ+ and μ+μ+ collider will provide a good opportunity to explore flavor physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of μTRISTAN to probe ALP LFV couplings. We analyze the testability of selected ALP production channels with potential sensitivity at μTRISTAN, considering different beams and collision energies, including e−μ+→aγ, e−μ+→e−τ+a, μ+μ+→μ+τ+a, and e−μ+→τ−μ+a. The produced ALP a is either long-lived or can promptly decay to flavor-violating or flavor-conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of μTRISTAN to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that μTRISTAN can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the O(1)–O(100) GeV range. In the light ALP regime, however, the parameter space that μTRISTAN is sensitive to has been already excluded by low-energy searches for LFV decays. Published by the American Physical Society 2024

Lepton-flavor violation in the left-right supersymmetric standard model

In the standard model, the charged lepton-flavor violation (CLFV) processes are forbidden, so the observation of CLFV represents a clear signal of new physics that goes beyond the standard model. In this work, we focus on the CLFV processes lj−→li−γ and lj−→li−li−li+ in the left-right supersymmetric standard model (LRSSM). Considering the constraints of the updated experimental data, the numerical results show that the new contributions of SU(2)R gauge interaction and a large number of other new particles, such as the Z′ boson and double-charged Higgs bosons in LRSSM can make significant contributions to the CLFV processes lj−→li−γ and lj−→li−li−li+, which is much different from the ones predicted in other supersymmetry models. This work provides a theoretical base for finding the lepton-flavor violation phenomena in new physics. Published by the American Physical Society 2024

Rare B and K decays in a scotogenic model

A scotogenic model can radiatively generate the observed neutrino mass, provide a dark matter candidate, and lead to rare lepton flavor-violating processes. We aim to extend the model to establish a potential connection to the quark flavor-related processes within the framework of scotogenesis, enhancing the unexpectedly large branching ratio (BR) of B+→K+νν¯ observed by the Belle II Collaboration. Meanwhile, the model can address tensions between some experimental measurements and standard model (SM) predictions in flavor physics, such as the muon g−2 excess and the higher BR of Bs→μ−μ+. In the model, we introduce the following dark particles: a neutral singlet Dirac-type lepton (N); two inert Higgs doublets (η1,2), with one carrying a lepton number; a charged singlet dark scalar (χ+); and a singlet vectorlike up-type dark quark (T). The first two entities are responsible for the radiative neutrino mass, and χ+ couples to right-handed quarks and leptons and can resolve the tensions existing in muon g−2 and Bs→μ−μ+. Furthermore, the BR of B+→K+νν¯ can be enhanced up to a factor of 2 compared to the SM prediction through the mediations of the dark T and the charged scalars. In addition, we also study the impacts on the K→πνν¯ decays. Published by the American Physical Society 2024

Lepton-flavor-violating ALP signals with TeV-scale muon beams

We explore the feasibility of using TeV-energy muons to probe lepton-flavor-violating (LFV) processes mediated by an axionlike particle (ALP) a with mass O(10 GeV). We focus on μτ LFV interactions and assume that the ALP is coupled to a dark state χ, which can be either less or more massive than a. Such a setup is demonstrated to be consistent with χ being a candidate for dark matter, in the experimentally relevant regime of parameters. We consider the currently operating NA64-μ experiment and proposed FASERν2 detector as both the target and the detector for the process μA→τAa, where A is the target nucleus. We also show that a possible future active muon fixed-target experiment operating at a 3 TeV muon collider or in its preparatory phase can provide an impressive reach for the LFV process considered, with future FASERν2 data providing a pilot study towards that goal. The implications of the muon anomalous magnetic moment (g−2)μ measurements for the underlying model, in case of a positive signal, are also examined, and a sample UV completion is outlined. Published by the American Physical Society 2024

Study of spectroscopic properties of some nuclei participating in (μ-, e-) lepton flavor violation process

Lepton flavor violation (LFV) is a clear sign of new physics beyond the standard model. A prominent process concerning LFV is μ- ⟶ e- conversion in a muonic atom. In the present work, we have investigated the spectroscopic properties of three nuclei namely 24Mg, 32S, and 44Ca which participate in this μ- ⟶ e- lepton flavor violating process. We have used USD interaction for sd shell nuclei namely 24Mg and 32S and Z20 Bonn interaction for pf shell nucleus 44Ca, to calculate these properties.

Fractionary charged particles confronting lepton flavor violation and the muon’s anomalous magnetic moment

In light of the recent result published by the Fermilab Muon [Formula: see text] experiment, we investigate a simple model that includes particles of fractional electric charges: a color-singlet fermion and a scalar with charges [Formula: see text] and [Formula: see text], respectively. The impact of these particles on the muon anomalous magnetic moment is examined, particularly the restrictions on their Yukawa couplings with the light leptons. Given that lepton flavor violation processes impose stringent constraints on certain scenarios beyond the Standard Model, we evaluate the one-loop contribution of the new particles to [Formula: see text] in order to identify regions in the parameter space consistent with the Fermilab results and compatible with the current and projected limits on the branching ratio [Formula: see text]. Taking into account the current lower bound for the masses of fractionary charged particles, which is around 634[Formula: see text]GeV, we show that the mass of the scalar particle with fractional charge must exceed 1[Formula: see text]TeV. In particular, we present some estimatives for double production of the color-singlet fermion at the 14[Formula: see text]TeV LHC. Finally, we also study the validity of our model in light of the QCD lattice results on the muon [Formula: see text].

Lepton flavor violation in semileptonic observables

In this paper, we perform a comprehensive study of lepton flavor violation (LFV) in semileptonic transitions in the framework of an effective field theory with general flavor structure. We account for the renormalization group equations, which induce nontrivial correlations between the different types of LFV processes. In particular, we show that these loop effects are needed to improve the bounds on several coefficients that are not efficiently constrained at tree level. For illustration, we consider a few concrete scenarios, with predominant couplings to third-generation quarks, and we explore the correlations between the various tree- and loop-level constraints on these models. As a by-product, we also provide expressions for several semileptonic LFV meson decays by using the latest determinations of the relevant hadronic form factors on the lattice. Published by the American Physical Society 2024

Top-quark FCNC decays, LFVs, lepton g − 2, and W mass anomaly with inert charged Higgses

The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the W-gauge boson. Nevertheless, the top-quark FCNCs and lepton flavor-violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as a top quark decaying into a light quark with a Higgs or gauge boson in the final state, i.e. t → q(h, V) with V = γ, Z, g, h→ℓℓ′ , and ℓ→ℓ′γ . To achieve the assumption that the induced FCNCs are all from quantum loops, we consider the scotogenic mechanism, where a Z 2 symmetry is introduced and only new particles carry an odd Z 2 parity. With the extension of the SM to include an inert Higgs doublet, an inert charged Higgs singlet, a vector-like singlet quark, and two neutral leptons, it is found that, with relevant constraints taken into account, the t → c(h, Z), h → μ τ, and τ → ℓ γ decays can be enhanced up to the expected sensitivities in experiments. The branching ratios of h → μ + μ −/τ + τ − from only new physics effects can reach up to O(10−3) . Intriguingly, the resulting muon g − 2 can fit the combined data within 2 standard deviations, whereas the electron g − 2 can have either sign with a magnitude of O(10−13−10−12) . In addition, we examine the oblique parameters in the model and find that the resulting W-mass anomaly observed by CDF II can be accommodated.

High efficiency muon registration system based on scintillator strips

Experiments such as mu2e (FNAL, USA) and COMET (KEK, Japan) seek the direct muon-to-electron conversion to study Charged Lepton Flavor Violation processes. A highly efficient (up to 99.99% efficiency) cosmic muon detection system is needed to achieve a single-event sensitivity below 10−17. In this article, the possibility of attaining such efficiency in the short and long term is discussed for modules based on 7-mm- or 10-mm-thick and 40-mm-wide plastic scintillation strips with a single 1.2 mm WLS fiber glued into the groove along the strip and using MPPC/SiPM for light detection.A Simplified Light Yield Distribution (SLYD) method to estimate the efficiency of the module is proposed, and the simulation results obtained with GEANT 4 for a system based on a 4-by-4 array of 7x40x3000 mm strips are compared with the experimental data. It is found that for the systems requiring efficiency of 99.99% and higher, it is important to improve the light yield as much as possible and to achieve the smallest possible gap between neighbor scintillation volumes is important.

FlexibleSUSY extended to automatically compute physical quantities in any beyond the standard model theory: Charged lepton flavor violation processes, Higgs decays, and user-defined observables

FlexibleSUSY is a framework for the automated computation of physical quantities (observables) in models beyond the Standard Model (BSM). This paper describes an extension of FlexibleSUSY which allows to define and add new observables that can be enabled and computed in applicable user-defined BSM models. The extension has already been used to include Charged Lepton Flavor Violation (CLFV) observables, but further observables can now be added straightforwardly. The paper is split into two parts. The first part is non-technical and describes from the user's perspective how to enable the calculation of predefined observables, in particular CLFV observables. The second part of the paper explains how to define new observables such that their automatic computation in any applicable BSM model becomes possible. A key ingredient is the new NPointFunctions extension which allows to use tree-level and loop calculations in the model-independent setup of observables. Three examples of increasing complexity are fully worked out. This illustrates the features and provides code snippets that may be used as a starting point for implementation of further observables. Program summaryProgram title:NPointFunctionsCPC Library link to program files:https://doi.org/10.17632/kf7m8gn8vp.2Developer's repository link:https://github.com/FlexibleSUSY/FlexibleSUSYLicensing provisions: GPLv3Programming language:C++, Wolfram Language, Fortran, Bourne shellJournal reference of previous version:: Comput. Phys. Commun. 230 (2018) 145–217; PoS CompTools2021 (2022) 036Does the new version supersede the previous version?: YesReasons for the new version: Program extension including new observables and file structuresNature of problem: Determining observables for an arbitrary extension of the Standard Model supported by FlexibleSUSY, input by the user.Solution method: Generation of the code from automated algebraic manipulations. Automatic filling and compiling of predefined template files.Additional comments including restrictions and unusual features: Vertices with a direct product of Lorentz and color structures are supported. Settings of the advanced NPointFunctions mode rely on explicit specification of topologies.

A mechanism relating the fermionic mass hierarchy to the flavor mixing

Considering the hierarchical structure of fermionic masses and the fermionic flavor mixing puzzles in the Standard Model, we propose to relate them by the see-saw mechanism, i.e. only the third generation of quarks and charged leptons achieves the masses at the tree level, the first two generations achieve masses through the mixings with the third generation, and the neutrinos achieve tiny Majorana masses by the so-called Type-I see-saw mechanism. This new picture at the fermion sector can explain simultaneously the flavor mixing puzzle and mass hierarchy puzzle in the SM. In addition, a flavor-dependent model (FDM) is proposed to realize the new mechanism, and observing the top quark rare decay processes t→ch, t→uh and the lepton flavor violation processes μ→3e,τ→3e,μ→3μ is effective to test the proposed FDM.

SMEFT analysis of charged lepton flavor violating B -meson decays

Charged lepton flavor violation (cLFV) processes, potentially important for various beyond the Standard Model physics scenarios are analyzed in the Standard Model effective field theory framework. We consider the most relevant two-quark–two-lepton (2q2ℓ) operators for the leptonic and semileptonic LFV B-decay (LFVBD) processes Bs→μ+e−,B+→K+μ+e−,B0→K*0μ+e−, and Bs→ϕμ−e+. We analyze the interplay among the Wilson coefficients responsible for these LFVBDs and other cLFV processes, like CR(μ→e), ℓi→ℓjγ, ℓi→ℓjℓkℓm, and Z→ℓiℓj, to find the maximal possible LFV effects in B-meson decays. We probe the scale of new physics in relation to the constraints imposed by both classes of the LFV decays while considering both the present bounds and future expectations. In view of proposed experiments at LHCb-II and Belle II to study charged LFV processes, we have also provided the upper limits on the indirect constraints on such LFVBDs. For the processes where the B meson is decaying to μ± and e∓, we show that new physics can be constrained by an enhancement of 2–4 orders of magnitude on the current sensitivities of the branching ratios of B+→K+μ+e−,B0→K*0μ+e−, and Bs→ϕμ±e∓. Published by the American Physical Society 2024

Distinguishing models with μ → e observables

Upcoming experiments will improve the reach for the lepton flavour violating (LFV) processes μ → eγ, μ → ee¯e\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ e\\overline{e}e $$\\end{document} and μA → eA by orders of magnitude. We investigate whether this upcoming data could rule out some popular TeV-scale LFV models (the type II seesaw, the inverse seesaw and a scalar leptoquark) using a bottom-up EFT approach. We take the data to be the twelve Wilson coefficients that experiments can constrain and in principle determine independently. In this 12-dimensional coefficient space, each model can only predict points in a specific subspace; for instance, flavour change involving singlet electrons is suppressed in the seesaw models, and the leptoquark induces negligible coefficients for 4-lepton scalar operators. Using the fact that none of these models can populate the whole region accessible to upcoming experiments, we show that μ → e experiments have the ability to rule them out.

MEG II physics and detector performance

In the panorama of the state-of-the-art searches for extremely rare Charged Lepton Flavor Violating (CLFV) processes, the Mu-E-Gamma (MEG) experiment is definitely a reference point in the intensity frontier of modern physics research, setting the best upper limit on the μ + → e + γ decay. The upgrade of MEG, MEG II, wants to give further impetus to the CLFV searches with muons. MEG II relies on a series of upgrades: on the photon side we point up improvements of the γ detector resolutions and acceptance; on the positron side we rely on completely brand new detectors with better acceptance, efficiency and performances; on the Trigger and Data Acquisition (DAQ) side we are able to exploit a higher muon beam intensity despite the increased number of read out channels thanks to a new and optimized electronics. After three years of commissioning, in 2021 the MEG II experiment finally entered the physics data taking phase. An overview of the MEG II physics and experimental contexts is presented, together with the current detector performances based on data. Thanks to the new experimental apparatus the final sensitivity goal is expected to be one order of magnitude better than the first phase of MEG.

Search for charged lepton flavor violation in J/ψ decays at BESIII

The observation of any charged lepton flavor violation (CLFV) process would be a clear signal of new physics beyond the Standard Model. Various decay modes, including lepton (μ, τ) decays, pseudoscalar meson (K,π) decays, vector meson (ϕ, J/ψ, Υ) decays, and Higgs decays, have been explored to detect CLFV. Focusing on the search for CLFV at the τ-charm region, the results of the search for J/ψ → eτ/eμ using the 10 billion J/ψ events collected by the BESIII experiment are presented. The upper limits (ULs) at the 90% confidence level are ℬ(J/ψ → e ± τ ∓) < 7.5 × 10-8 and ℬ(J/ψ → e ± μ ∓) < 4.5 × 10-9, respectively. Improving the previously published limits by two orders of magnitudes, the results are the most stringent CLFV searches in heavy quarkonium system.

Box-enhanced charged lepton flavor violation in the Grimus-Neufeld model

In the Grimus-Neufeld model (GNM) the neutrino mass generation from an extended Higgs sector leads to bounds for charged lepton flavor violating (cLFV) processes. Here we update bounds from a previous study by extending the parameter space to a nonvanishing Majorana phase of the Pontecorvo-Maki-Nakagawa-Sakata matrix and to heavier charged Higgs boson masses. Three-body cLFV decays are shown to contribute significantly in the large mass regions, as the boxes are enhanced relatively to photonic diagrams. This is in contrast to the smaller mass region studied before, in which the two-body decays tightly restrict the parameter space. The Majorana phase is shown to change limits within 1 order of magnitude. The tiny seesaw scale is assumed, which makes the cLFV decays in the GNM similar to the cLFV decays in the scotogenic model and the scoto-seesaw models.

Phenomenological and cosmological implications of a scotogenic three-loop neutrino mass model

We propose a scotogenic model for generating neutrino masses through a three-loop seesaw. It is a minimally extended inert doublet model with a spontaneously broken global symmetry U(1)′ and a preserved ℤ2 symmetry. The three-loop suppression allows the new particles to have masses at the TeV scale without fine-tuning the Yukawa couplings. The model leads to a rich phenomenology while satisfying all the current constraints imposed by neutrinoless double-beta decay, charged-lepton flavor violation, and electroweak precision observables. The relatively large Yukawa couplings lead to sizable rates for charged lepton flavor violation processes, well within future experimental reach. The model could also successfully explain the W mass anomaly and provides viable fermionic or scalar dark matter candidates.

A minimal modular invariant neutrino model

We present a neutrino mass model based on modular symmetry with the fewest input parameters to date, which successfully accounts for the 12 lepton masses and mixing parameters through 6 real free parameters including the modulus. The neutrino masses are predicted to be normal ordering, the atmospheric angle θ23 is quite close to maximal value and the Dirac CP phase δCP is about 1.34π. We also study the soft supersymmetry breaking terms due to the modulus F-term in this minimal model, which are constrained to be the non-holomorphic modular forms. The radiative lepton flavor violation process μ → eγ is discussed.

Measuring lepton flavor violation at the LHC

A new process with the lepton flavor violation (LFV) is presented in the setup for the LHC. The LFV is induced by the one-loop effect through Higgs bosons in the framework of the type-III two Higgs doublet model. At the decoupling limit of the parameter space of type-III two Higgs doublet model, where the masses of heavier Higgs bosons are at TeV scale while the mass of the lighter Higgs boson remains as the SM Higgs boson, the LFV process is strongly suppressed at the tree level. However, one-loop induced LFV process is instead realized to be a sizeable production cross section in vector-boson fusion production process at LHC. Since the LFV process may not be produced by the $s$-channel $H/A$ or ${H}^{\ifmmode\pm\else\textpm\fi{}}$ bosons decays only, rather mixture with the $t$-channel production mode allows us to access to the higher Higgs boson mass region than current $s$-channel based searches at the LHC. This gives complementary path to cover the parameter spaces in LFV couplings.