Search For Lepton Flavor Violation Research Articles

Searching for lepton flavor violation with the CMS experiment

Searches for Lepton Flavor Violation (LFV) stand at the forefront of experimental particle physics research, offering a sensitive probe to many scenarios of physics beyond the Standard Model. The high proton–proton collision energy and luminosity provided by the CERN Large Hadron Collider (LHC) and the excellent CMS detector performance allow for an extensive program of LFV searches. This paper reviews a broad range of LFV searches conducted at the CMS experiment using data collected in LHC Run 2, including [Formula: see text] decays, Higgs boson decays, and top quark production and decays. In each analysis, the online and offline event selections, signal modeling, background suppression and estimation, and statistical interpretation are elucidated. These searches involve various final state particles in a large transverse momentum range, showcasing the capability of the CMS experiment in exploring fundamental questions in particle physics.

Charged Lepton-Flavour Violation

We review the experimental status of searches for lepton-flavour violation in the charged sector. We give an overview of searches for lepton-flavour violation in purely leptonic decays, hadron decays, and decays of heavy bosons. We focus on the most stringent constraints on lepton-flavour violating processes in these areas and give prospects for ongoing and future experiments.

B meson anomalies and large 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}$$ {B}^{+}\ o {K}^{+}\ u \\overline{\ u} $$\\end{document} in non-universal U(1)′ models

In view of both the latest LHCb measurement of RK∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {R}_{K^{\\left(\\ast \\right)}} $$\\end{document} and the new 2.7σ deviation reported by Belle II on 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}$$ {B}^{+}\ o {K}^{+}\ u \\overline{\ u} $$\\end{document} decays, we present a fit to the B meson anomalies for various one and two dimensional hypothesis including complex Wilson coefficients. We show in a model-independent way that the generic non-universal U(1)′ extensions of the SM, without flavour violation, fail to simultaneously fit those observables and corroborate that they can modify BRB+→K+νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{BR}\\left({B}^{+}\ o {K}^{+}\ u \\overline{\ u}\\right) $$\\end{document} up to only a 10%. In view of this deficit, we propose a new way in which those models can accommodate the data at tree level by introducing lepton flavour violating couplings and non-diagonal elements of the charged lepton mixing matrix, with implications in future charged lepton flavour violation searches.

Search for lepton-flavour violation in high-mass dilepton final states using 139 fb−1 of pp collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV with the ATLAS detector

A search is performed for a heavy particle decaying into different-flavour, dilepton final states, using 139 fb−1 of proton-proton collision data at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV collected in 2015–2018 by the ATLAS detector at the Large Hadron Collider. Final states with electrons, muons and hadronically decaying tau leptons are considered (eμ, eτ or μτ). No significant excess over the Standard Model predictions is observed. Upper limits on the production cross-section are set as a function of the mass of a Z′ boson, a supersymmetric τ-sneutrino, and a quantum black-hole. The observed 95% CL lower mass limits obtained on a typical benchmark model Z′ boson are 5.0 TeV (eμ), 4.0 TeV (eτ), and 3.9 TeV (μτ), respectively.

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.

Searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ in sqrt{s} = 13 TeV pp collisions with the ATLAS detector

This paper presents direct searches for lepton flavour violation in Higgs boson decays, H → eτ and H → μτ, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy sqrt{s} = 13 TeV, corresponding to an integrated luminosity of 138 fb−1. Leptonic (τ → ℓνℓντ) and hadronic (τ → hadrons ντ) decays of the τ-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, mathcal{B} (H → eτ) < 0.20% (0.12%) and mathcal{B} (H → μτ ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H → eτ and H → μτ signals. The best-fit branching ratio difference, mathcal{B} (H → μτ) → mathcal{B} (H → eτ), measured with the Symmetry method in the channel where the τ-lepton decays to leptons, is (0.25 ± 0.10)%, compatible with a value of zero within 2.5σ.

Dark linear seesaw mechanism

We propose a minimal model where a dark sector, odd under a Z2 discrete symmetry, is the seed of lepton number violation in the neutrino sector at the loop level, in the context of the linear seesaw mechanism. Neutrino mass suppression stems from a naturally small scalar potential coupling which breaks the lepton number symmetry softly. The fact that we consider (dark) Dirac vector-like neutral leptons, prevents the appearance of other mass terms that could contribute to neutrino masses via alternative mechanisms. We study the dark-matter phenomenology of the model, focusing on the case in which the stable particle is the lightest neutral scalar arising from the dark scalar sector. Prospects for testing our framework with the results of current and future lepton flavour violation searches are also discussed.

Riding the Seesaw: what Higgsstrahlung may reveal about massive neutrinos

We investigate if the projected high-precision measurements of the cross section of the Higgsstrahlung process e^+ e^- rightarrow Zh at a future electron–positron collider can be utilised to indirectly probe the fermionic Seesaw models. We consider the two centre-of-mass energies sqrt{s}=240 GeV and 365 GeV, and compare the collider reaches to constraints from electroweak observables, probes of lepton flavour universality and the existing and prospective bounds from searches for lepton flavour violation. For the analysis we assume the limit of an exactly conserved lepton-number symmetry. We find that while any appreciable correction to the Higgsstrahlung cross section is already strictly constrained in the Type-I Seesaw model, effects of up to {mathcal {O}}(10%) are possible within Type-III Seesaw.

Axion dark matter from lepton flavor-violating decays

We propose simple scenarios where lepton flavor-violating couplings generate the observed dark matter abundance through freeze-in of an axion-like particle with mass in the few keV range. Compared to flavor-diagonal freeze-in, this mechanism enhances dark matter stability, softens stellar cooling constraints and improves the experimental sensitivity of accelerator-based searches. These scenarios can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by new searches for lepton flavor violation at high-intensity experiments such as Mu3e and MEG II.

Rare decays at LHCb

Flavour-changing neutral-current processes, such as b → sℓ + ℓ −, are forbidden at tree level in the Standard Model and hence might receive comparatively large corrections from new particles. This proceeding paper highlights recent measurements from LHCb on b → sℓ + ℓ − and purely-leptonic decays, including tests of lepton flavour universality and searches for lepton flavour violation.

Relic challenges for vector-like fermions as connectors to a dark sector

New dark sectors consisting of exotic fields that couple only very feebly to the Standard Model (SM) have strong theoretical motivation and may be relevant to explaining the abundance of dark matter (DM). An important question for such sectors is how they connect to the SM. For a dark sector with a new gauge interaction, a natural connection arises from heavy vector-like fermions charged under both the visible and dark gauge groups. The gauge charges of such fermions imply that one or more of them is stable in the absence of additional sources of dark symmetry breaking. A generic challenge for such connectors is that they can produce too much dark matter or interact too strongly with nuclei if they were ever thermalized in the early universe. In this paper we study this challenge in a simple connector theory consisting of new vector-like electroweak doublet and singlet fermions that also transform under the fundamental representation of a new Abelian gauge force, and we show that these connectors in their minimal form are almost always ruled out by existing direct DM searches. To address this challenge, we investigate two solutions. First, we study mitigating scattering on nuclei by introducing a Majorana mass term for the singlet. And second, we investigate a mixing with SM leptons that allows the connectors to decay while remaining consistent with cosmological tests and searches for charged lepton flavour violation. Both solutions rely on the presence of a dark Higgs field with a specific charge.

Charged lepton flavor violation searches in the charmonium system

An invited research highlight article on Charged lepton flavor violation searches in the charmonium system.

Searches for Lepton Flavour and Lepton Number violation in K+ decays

The latest results on searches for Lepton Flavour and Lepton Number violation in K+→π∓μ±e+ and K+→π−l+l+ with l=(μ,e) decays at the NA62 experiment at CERN SPS are presented, improving on previous limits.

Search for Lepton Number and Flavour Violation in K+ and π0 Decays

The NA62 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in Run 1 from 2016 to 2018. This sample provides sensitivities to rare decays with branching fractions as low as 10−11. Results of searches for the lepton number and/or flavour violating decays K+→π±μ∓e+ and π0→μ−e+ are reported. No evidence of these decays is found, and upper limits on the branching fractions are obtained at 90% confidence level: B(K+→π−μ+e+)<4.2×10−11, B(K+→π+μ−e+)<6.6×10−11, and B(π0→μ−e+)<3.2×10−10. These results improve by approximately one order of magnitude over previous results for these decay modes.

Searches for Lepton Flavor Violation in Tau Decays at Belle II

Searches for lepton flavor violation in tau decays are unambiguous signatures of new physics. The branching ratios of tau leptons at the level of 10−10–10−9 can be probed using 50 ab−1 of electron-positron annihilation data being collected by the Belle II experiment at the world’s highest luminosity accelerator, the SuperKEKB, located at the High Energy Accelerator Research Organization, KEK, in Tsukuba, Japan. Searches with such expected sensitivity will either discover new physics or strongly constrain several new physics models.

Constraints on leptoquarks from lepton-flavour-violating tau-lepton processes

Leptoquarks are ubiquitous in several extensions of the Standard Model and seem to be able to accommodate the universality-violation-driven B-meson-decay anomalies and the (g −2)μ discrepancy interpreted as deviations from the Standard Model predictions. In addition, the search for lepton-flavour violation in the charged sector is, at present, a major research program that could also be facilitated by the dynamics generated by leptoquarks. In this article, we consider a rather wide framework of both scalar and vector leptoquarks as the generators of lepton-flavour violation in processes involving the tau lepton. We single out its couplings to leptoquarks, thus breaking universality in the lepton sector, and we integrate out leptoquarks at tree level, generating the corresponding dimension-6 operators of the Standard Model Effective Field Theory. In ref. [1] we obtained model-independent bounds on the Wilson coefficients of those operators contributing to lepton-flavour-violating hadron tau decays and ℓ–τ conversion in nuclei, with ℓ = e, μ. Hence, we use those results to translate the bounds into the couplings of leptoquarks to the Standard Model fermions.

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).

Lepton Flavor Violating μ- - e- Conversion: An Overview

The search for lepton flavor violation in charged lepton decays is a highly sensitive tool to look for physics beyond the Standard Model. Among the possible processes, µ-decays are considered to have the largest discovery potential in most of the standard model extensions. Many searches have been performed in the past, but no evidence has been found so far. In this paper, we have reviewed the current theoretical and experimental status of the field of muon to electron decay and its potential to search for new physics beyond the Standard Model. Future prospects of experiments for further progress in this field are also discussed.

Lepton flavour violation in rare Lambda _b decays

Lepton flavour violation (LFV) naturally occurs in many new physics models, specifically in those explaining the B anomalies. While LFV has already been studied for mesonic decays, it is important to consider also baryonic decays mediated by the same quark transition. In this paper, we study LFV in the baryonic Lambda _b rightarrow Lambda ell _1 ell _2 using for the first time a full basis of New Physics operators. We present expected bounds on the branching ratio in a model-independent framework and using two specific new physics models. Finally, we point out the interplay and orthogonality between the baryonic and mesonic LFV searches.

Phenomenology of the Zee model for Dirac neutrinos and general neutrino interactions

The Zee model for Dirac neutrinos is one of the simplest models featuring one-loop Dirac neutrino masses. The interactions between the new scalars (two singly-charged fields) and neutrinos induce general neutrino interactions (GNI) which, as a generalisation of the non standard neutrino interactions, constitute an additional tool to probe models beyond the SM like this. In this work, we consider a $U(1)_{{B-L}}$ gauge symmetry as the responsible for the Diracness of the neutrinos and the radiative character of the neutrino masses. We determine the viable parameter space consistent with neutrino oscillation data, leptonic rare decays and collider constraints, and establish the most relevant experimental prospects regarding lepton flavor violation searches and GNI in future solar neutrino experiments.