Flavor Violation Research Articles

Discrete leptonic flavor symmetries: UV mediators and phenomenology

Given the absence of a definitive top-down indication for understanding the peculiar structure of the lepton sector, discrete flavor symmetries offer a profound perspective for examining the intricate patterns of lepton masses and mixings. In this work, drawing upon previous studies on the interplay of flavor symmetries with the potential UV completions from a purely bottom-up perspective, three well-motivated discrete flavor groups, suitable for portraying the leptonic sector as well as the neutrino masses, specifically A4, A5, and S4, are explored within this framework, leading to the comprehensive classification of the new physics (NP) mediators, along with the tree-level matching relations onto dimension-6 Standard Model effective field theory operators. Particular emphasis is placed on the , for which a phenomenological analysis is carried out in order to constrain various NP mediators, where significant focus is directed towards the examination of the charged lepton flavor violation operators, which, for the wide range of applicable cases, offer the leading constraint. Published by the American Physical Society 2024

Lepton flavor violating decays Z → $l^{\pm}_{i}l^{\mp}_{j}$ in the B-L Supersymmetric Standard Model

Abstract Lepton flavor violation (LFV) represents a clear new physics (NP) signal beyond the
standard model (SM). In this paper, we study LFV decay $Z\rightarrow l^{\pm}_{i}l^{\mp}_{j}$ with $l_i^{-}$ representing lepton and $l_i^{+}$ representing antilepton in the B-L Supersymmetric Standard Model(B-LSSM). We calculate these processes separately in the mass eigenstate basis and the electroweak interaction basis, and the latter adopt the mass insertion approximation (MIA) method. The effect of the LFV decays $Z\rightarrow l^{\pm}_{i}l^{\mp}_{j}$ changing with parameters can be showed clearly in the analytic level by MIA, which provides a new way for us to analyze the LFV processes. At the same time, the corresponding constraints from the LFV decays $l^{-}_{j} \rightarrow l^{-}_{i} \gamma$ and $(g-2)_{\mu}$ are considered to analyze the numerical results.

Renormalization-group running of dimension-8 four-fermion operators in the SMEFT

We compute the renormalization-group equations governing the evolution of dimension-8 four-fermion operators in the Standard Model effective field theory (SMEFT). We describe the calculation and present analytic results for both the full flavor structure of the SMEFT and with the assumption of minimal flavor violation. We present numerical results for the renormalization-group evolution of the coefficients, and study their impact on fits of the Large Hadron Collider (LHC) Drell-Yan data. The effects of running on the dimension-8 coefficients can reach 50% or more when evolving from 10 TeV scale down to few-GeV energies relevant for the analysis of fixed-target data. However, the impact of the dimension-8 running on the analysis of Drell-Yan data from the LHC is minimal. Published by the American Physical Society 2024

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.

Implications of B→Kνν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B \\rightarrow K \ u {\\bar{\ u }}$$\\end{document} under rank-one flavor violation hypothesis

We study the implications of the observed excess in B+→K+νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B^+ \\rightarrow K^+ \ u \\bar{\ u }$$\\end{document} under the assumption of Rank-One Flavour Violation, i.e. that New Physics couples to a single specific direction in flavour space. By varying this direction we perform analyses at the level of the low-energy EFT, the SMEFT, and with explicit mediators such as leptoquarks and colorless vectors (Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z^\\prime $$\\end{document} and V′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$V^\\prime $$\\end{document}). We study correlations with other flavour, electroweak and collider observables, finding that the most interesting ones are with K→πνν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K \\rightarrow \\pi \ u \\bar{\ u }$$\\end{document}, Bs→μ+μ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B_s \\rightarrow \\mu ^+ \\mu ^-$$\\end{document}, meson mixing and the LHC searches in τ+τ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au ^+ \ au ^-$$\\end{document} high-energy tails. Among the various mediators, the scalar leptoquarks R~2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ilde{R}_2$$\\end{document} and S1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$S_1$$\\end{document} offer the best fits of the Belle-II excess, while being consistent with the other bounds. On the other hand, colorless vectors are strongly constrained by meson mixing and resonance searches in pp→τ+τ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p p \\rightarrow \ au ^+ \ au ^-$$\\end{document}. In all cases we find that a flavour alignment close to the third generation is generically preferred.

Multi-component dark matter from Minimal Flavor Violation

Minimal Flavor Violation (MFV) offers an appealing framework for exploring physics beyond the Standard Model. Interestingly, within the MFV framework, a new colorless field that transforms non-trivially under a global SU(3)3 quark flavor group can naturally be stable. Such a new field is thus a promising dark matter candidate, provided it is electrically neutral. We extend the MFV framework for dark matter and demonstrate that dark matter can naturally be multi-component across a broad parameter space. For illustration, we consider a gauge singlet, flavor triplet scalar field and identify parameter spaces for multi-component dark matter, where only the lightest flavor component is absolutely stable and heavy flavor components are decaying with lifetimes sufficiently longer than the age of the universe. Phenomenological, cosmological and astrophysical aspects of multi-component flavored dark matter are briefly discussed.

Exploring optimal sensitivity of lepton flavor violating effective couplings at the e+e− colliders

We analyze lepton flavor violation (LFV) using the Standard Model effective field theory (SMEFT) framework at the future lepton colliders. Our focus is on the associated production of tau lepton with electron/muon at the electron-positron (e+e−) colliders, related to four-Fermi SMEFT effective operators. In accordance with the upper limits on effective couplings from lepton flavor violating tau decays, we conduct a cut-based analysis to achieve sufficient signal significance. We utilize the optimal observable technique (OOT) to estimate the optimal sensitivity of the effective couplings. The impact of electron beam polarization and the interplay of signal and background in enhancing the optimal sensitivity of the effective couplings are discussed in detail. We find that the sensitivity of flavor-violating effective couplings is enhanced by order of one for 3 TeV center-of-mass (CM) energy and 1000 fb−1 integrated luminosity at the e+e− colliders. Published by the American Physical Society 2024

Top quark production through flavor violating neutral currents within the 2HDM type-III

In this paper, we explore the parameter space for processes with fermionic flavor violation within the Two Higgs Doublets Model (2HDM) type III, processes of Flavor Changing Neutral Currents (FCNCs) are present at Leading Order (LO) as dynamics beyond the Standard Model (SM) are included. We analyze the possible Yukawa couplings beyond the SM involving the quark top, as the experimental signatures would be a clear signal to conclude about the model. The processes we analyzed are same sign top pair production at the [Formula: see text] collisions, and single top quark production in [Formula: see text] collisions, through Deep Inelastic Scattering (DIS). To examine the scenarios that could exhibit such exotic events, we perform a scan in the parameter space to determine the observation possibilities for FCNC. We show the results considering the model parameters, coming from modified Yukawa interactions, [Formula: see text] of the order one. In addition, we explore the order of magnitude of possible single top production via [Formula: see text], at the proposed muon–proton collider. We found scattering values for these exotic processes of up to [Formula: see text].

Aligned yet large dipoles: a SMEFT study

We study a non-universal flavor scenario at the level of the Standard Model Effective Field Theory, according to which the matrix of Wilson coefficients cuW of an up-type electroweak quark dipole operator is aligned with the up-type Yukawa coupling. Such an alignment usually follows from the assumption of Minimal Flavor Violation (MFV), away from which we step by allowing the entries of cuW to be sizable along the first quark generations. A particular example, which we refer to as “inverse hierarchy MFV”, features Wilson coefficients inversely proportional to quark masses, and arises from BSM models respecting MFV and containing heavy fields that replicate the mass hierarchy of SM quarks. We then analyze the phenomenology driven by cuW at colliders and at lower-energy flavor experiments. We show that precision measurements of the process pp → Wh → γγℓν at FCC-hh could set an upper bound on |cuW| ≲ O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(10−2)(Λ/TeV)2, with Λ the cutoff of the effective field theory. This bound is an order of magnitude stronger than the existing LHC bounds. Moreover, we estimate that Wh → bb¯ℓν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ b\\overline{b}\\ell \ u $$\\end{document} at HL-LHC could also give competitive bounds. In the low-energy regime, we consider bounds arising from rare kaon decays, which turn out to be loose, |cuW11\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {c}_{uW}^{11} $$\\end{document}| <O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(1)(Λ/TeV)2. We finally demonstrate that our flavor and operator assumptions can be derived from a weakly-coupled UV model, which we choose to simultaneously illustrate the UV origin of inverse hierarchy MFV.

Muon g−2 and lepton flavor violation in supersymmetric GUTs

We present a class of supersymmetric (SUSY) GUT models that can explain the apparent discrepancy between the SM predictions and experimental values of muon g−2 while providing testable signals for lepton flavor violation in charged lepton decays. Moreover, these models predict LSP neutralino abundance that is compatible with the Planck dark matter bounds. We find that scenarios in the framework of SU(4)c×SU(2)L×SU(2)R unification, with additional symmetries to explain fermion masses and neutrino oscillations, provide interesting benchmarks for the search of SUSY by correlating a possible manifestation of it in dark matter, rare lepton decays, and LHC signals. Published by the American Physical Society 2024

Exploring the lepton flavor violating decay modes b → sμ±τ∓ in SMEFT approach

We perform an analysis of the consequences of various new physics operators on the lepton flavor violating (LFV) decay modes mediated through b→sℓ1ℓ2 transitions. We scrutinize the imprints of the (pseudo)scalar and axial (vector) operators on the exclusive LFV decay channels B(s)→(ϕ,K⁎,K2⁎)ℓ1ℓ2 and Λb→Λℓ1ℓ2, where ℓ1,ℓ2 represent μ or τ. The new physics parameters are constrained by using the upper limits of the branching fractions of the Bs→τμ and B→Kτμ processes, assuming the new physics couplings to be real. We then explore the key observables such as the branching fractions, the forward-backward asymmetries, and the longitudinal polarization fractions of the B→(K⁎,ϕ,K2⁎)τ±μ∓ decays. In addition, we also investigate the impact of the new physics couplings on the baryonic Λb→Λτ±μ∓ decay channels mediated by the b→s quark level transition. With the experimental prospects at LHCb upgrade and Belle II, we also predict the upper limits of the above-discussed observables, which could intrigue the new physics search in these channels.

Muon-induced baryon number violation

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for μ→e conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the μ→e signal peak. A concrete example of such light new physics is μ−-nucleon annihilation into a light dark sector, which can produce an energetic e− as well as e+e− by-products. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars. Published by the American Physical Society 2024

(g−2)e,μ anomalies and decays h, Z→ebea in 3-3-1 models with inverse seesaw neutrinos

The lepton flavor violating (LFV) decays h,Z→ebea, and eb→eaγ are discussed in a class of general 3-3-1 models adding heavy neutral leptons and singly charged Higgs bosons to accommodate experimental data of neutrino oscillation and (g−2)ea anomalies of charged leptons through the inverse seesaw mechanism. We show that the models with the minimal number of inverse seesaw neutrinos cannot reach the 1σ range of (g−2)μ data due to the experimental upper bounds on decay rates of (τ→μγ) and (μ→eγ). Features of LFV decays are presented in detail, focusing on the regions of parameter space satisfying the 1σ ranges of (g−2)e,μ data. Published by the American Physical Society 2024

The Higgs boson decay h → bs in the U(1)XSSM

In the U(1)XSSM, we delve into the quark flavor violation of h → bs, where h is identified as the SM-like Higgs boson discovered at the LHC. As the U(1) extension of the minimal supersymmetric standard model (MSSM), the U(1)XSSM has new super fields such as right-handed neutrinos and three Higgs singlets. We conduct a thorough analysis of the underlying mechanisms and parameter dependencies of h → bs in the U(1)XSSM. In the U(1)XSSM, we discover that the Br(h → bs) for the Higgs decay to bs could significantly differ from the expectation in the standard model (SM), depending on the values of the new parameters introduced in the model. Our research not only contributes to a deeper understanding of Higgs physics within the U(1)XSSM, but also provides valuable guidance for new physics (NP).

Study lepton flavor violation $B^0\rightarrow{{l_i}^{\pm}{l_j}^{\mp}}$ within the Mass Insertion Approximation

Abstract We study lepton flavor violating (LFV) decays $B^0\rightarrow{{l_i}^{\pm}{l_j}^{\mp}}$ ($B^0\rightarrow e{\mu}$, $B^0\rightarrow e{\tau}$ and $B^0\rightarrow {\mu}{\tau}$) in the $U(1)_X$SSM (SSM is acronym for the supersymmetric standard model), which is the $U(1)_X$ extension of the minimal supersymmetric standard model (MSSM). The local gauge group of $U(1)_X$SSM is $SU(3)_C\times SU(2)_L \times U(1)_Y \times U(1)_X$. These processes ($B^0\rightarrow e{\mu}$, $B^0\rightarrow e{\tau}$ and $B^0\rightarrow {\mu}{\tau}$) are strictly forbidden in the standard model (SM), but these LFV decays are a signal of new physics (NP).&amp;#xD;We use the Mass Insertion Approximation (MIA) to find sensitive parameters that directly influence the result of the branching ratio of LFV decay $B_d\rightarrow{{l_i}^{\pm}{l_j}^{\mp}}$. Combined with the latest experimental results, we analyze the relationship between different sensitive parameters and the branching ratios of the three processes. According to the numerical analysis, these elements are very restricted from the non observation of CLFV.

Lepton flavor violation in exclusive b→dℓiℓj and b→sℓiℓj decay modes

We discuss the exclusive lepton flavor violating (LFV) decays modes based on b→dℓiℓj and b→sℓiℓj by considering the ground state mesons and baryons. After spelling out the expressions for such decay rates in a low energy effective theory which includes generic contributions arising from physics beyond the Standard Model, we show that the experimental bounds on meson decays can be used to bound the corresponding modes involving baryons. We find, for example, B(Λb→Λμτ)≲4×10−5. We also consider two specific models and constrain the relevant LFV couplings by using the low energy observables. In the first model, we assume the Higgs mediated LFV and find the resulting decay rates to be too small to be experimentally detectable. We also emphasize that the regions favored by the bounds B(h→μτ)Atlas and B(h→eτ)Atlas are not compatible with B(μ→eγ)MEG to 1σ. In the second model, we assume LFV mediated by a heavy Z′ boson and find that the corresponding b-hadron branching fractions can be O(10−6), thus possibly within experimental reach at LHCb and Belle II. 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.

Flavor violating di-Higgs couplings

Di-Higgs couplings to fermions of the form h2f‾f are absent in the Standard Model, however, they are present in several physics Beyond Standard Model (BSM) extensions, including those with vector-like fermions. In Effective Field Theories (EFTs), such as the Standard Model Effective Field Theory (SMEFT) and the Higgs Effective Field Theory (HEFT), these couplings appear at dimension 6 and can in general, be flavour-violating (FV). In the present work, we employ a bottom-up approach to investigate the FV in the lepton and quarks sectors through the di-Higgs effective couplings. We assume that all FV arises from this type of couplings and assume that the Yukawa couplings Yij are given by their SM values, i.e. Yij=2miδij/v. In the lepton sector, we set upper limits on the Wilson coefficients Cll′ from l→3l′ decays, l→lγ decays, muonium oscillations, the (g−2)μ anomaly, LEP searches, muon conversion in nuclei, FV Higgs decays, and Z decays. We also make projections on some of these coefficients from Belle II, the Mu2e experiment and the LHC's High Luminosity (HL) run. In the quark sector, we set upper limits on the Wilson coefficients Cqq′ from meson oscillations and from B-physics searches. A key takeaway from this study is that current and future experiments should set out to measure the effective di-Higgs couplings Cff′, whether these couplings are FV or flavour-conserving. We also present a matching between our formalism and the SMEFT operators and show the bounds in both bases.

Neutrino mass model and dark matter with Y = 0 inert triplet scalar

We study a one-loop induced neutrino mass model with an inert isospin triplet scalar field of Y=0 and heavier isospin doublet vector-like leptons and singlet Majorana right-handed fermions. In addition to the neutrino mass matrix, we explain sizable scale of muon anomalous magnetic dipole moment 10−9 by introducing a singly-charged boson S±. We show numerical analysis of neutrino oscillation, lepton flavor violations, Z boson decays, and demonstrate our allowed regions in cases of normal and inverted hierarchies. We find the sizable scale of muon anomalous magnetic dipole moment for both cases. Then, we move on to the discussion of dark matter candidates to satisfy the relic density where we have two candidates; fermionic dark matter and bosonic one. And, we classify four cases fermionic dark matter with normal and inverted hierarchies, bosonic one with normal and inverted hierarchies and search for each of the allowed points in the model.

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].