Lepton Flavor Violation Research Articles

Search for Muon-to-Electron Conversion with the COMET Experiment

Charged Lepton Flavor Violation is expected to be one of the most powerful tools to reveal physics beyond the Standard Model. The COMET experiment aims to search for the neutrinoless coherent transition of a muon into an electron in the field of a nucleus. Muon-to-electron conversion has never been observed, and can be, and would be, clear evidence of new physics if discovered. The experimental sensitivity of this process, defined as the ratio of the muon-to-electron conversion rate to the total muon capture rate, is expected to be significantly improved by a factor of 100 to 10,000 in the coming decade. The COMET experiment will take place at J-PARC with single event sensitivities of the orders of 10−15 and 10−17 in Phase-I and Phase-II, respectively. The ambitious goal of the COMET experiment is achieved by realizing a high-quality pulsed beam and an unprecedentedly powerful muon source together with an excellent detector apparatus that can tolerate a severe radiation environment. The construction of a new beam line, superconducting magnets, detectors and electronics is in progress towards the forthcoming Phase-I experiment. We present the experimental methods, sensitivity and backgrounds along with recent status and prospects.

Studying ΔL = 2 Lepton Flavor Violation with Muons

Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that violate the conservation of lepton (muon) flavor by two units with muonia and muonium–antimuonium oscillations.

Charged Lepton Flavor Violation at the High-Energy Colliders: Neutrino Mass Relevant Particles

We summarize the potential charged lepton flavor violation (LFV) from neutrino mass relevant models, for instance the seesaw mechanisms. In particular, we study, in a model-dependent way, the LFV signals at the high-energy hadron and lepton colliders originating from the beyond standard model (BSM) neutral scalar H, doubly charged scalar H±±, heavy neutrino N, heavy WR boson, and the Z′ boson. For the neutral scalar, doubly charged scalar and Z′ boson, the LFV signals originate from the (effective) LFV couplings of these particles to the charged leptons, while for the heavy neutrino N and WR boson, the LFV effects are from flavor mixing in the neutrino sector. We consider current limits on these BSM particles and estimate their prospects at future high-energy hadron and lepton colliders.

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).

Zee model with quasidegenerate neutrino masses and where to find it

We present a Zee model with a family dependent Z_2 symmetry for radiative neutrino masses. Our motivation is to get a model that correctly describes neutrino oscillation phenomena, while at the same time offers definite predictions. The imposed Z_2 symmetry greatly reduces the number of free parameters in the model. These parameters are then determined from the neutrino data, from which one can study its outcomes. Our setup only admits quasidegenerate neutrino masses with the sum of neutrino masses between 100 and 130 meV, the effective Majorana mass between 20 and 40 meV, and the effective electron neutrino mass between 48 and 53 meV. The ratio of the vacuum expectation values of the Higgs doublets, tan beta , is found to be tan beta lesssim 0.5 and tan beta gtrsim 10. The former is ruled out by lepton flavor violation (LFV) processes, such as mu rightarrow egamma and mu rightarrow e conversion, which are determined up to two loops. For the latter, these LFV processes are within reach of the next generation of experiments. Moreover, for tan beta gtrsim 10, the couplings of heavy neutral scalars to dimuon are significant. If they are sufficiently light, i.e., lesssim 200 GeV, collider search for their decays into muon pair provides a stronger constraint on most parts of the model parameter space than the LFV ones.

Neutrino masses and magnetic moments of electron and muon in the Zee Model

We explore parameter space in the Zee Model to resolve the long-standing tension of the electron and muon anomalous magnetic moment (AMM). The model comprises a second Higgs doublet and a charged singlet at electroweak scale and generates Majorana neutrino masses at one-loop level; the neutral partner of the SU(2)L doublet contributes to the AMM of electron and muon via one loop and two-loop corrections. We propose two minimal flavor structures that can explain these anomalies while fitting the neutrino oscillation data. We find that the neutral Higgs resides in the mass range of roughly 10–300 GeV or 1–30 GeV, depending on the flavor structures. The model is consistent with constraints from colliders, electroweak precision data, and lepton flavor violation. To be comprehensive, we examine the constraints from the electric dipole moment (EDM) and find a region of parameter space that gives a sizable contribution to muon EDM while simultaneously giving corrections to muon AMM. In addition to the light scalar, the two charged scalars with masses as low as 100 GeV can induce nonstandard neutrino interactions εee as large as 8%, potentially hinting at new physics. We also investigate the projected capability of future lepton colliders to probe the currently allowed parameter space consistent with both electron and muon AMMs via direct searches in the ℓ+ℓ− → ℓ+ℓ−(H → ℓ+ℓ−) channel.

Leptonic scalars and collider signatures in a UV-complete model

We study the non-standard interactions of neutrinos with light leptonic scalars (ϕ) in a global (B − L)-conserved ultraviolet (UV)-complete model. The model utilizes Type-II seesaw motivated neutrino interactions with an SU(2)L-triplet scalar, along with an additional singlet in the scalar sector. This UV-completion leads to an enriched spectrum and consequently new observable signatures. We examine the low-energy lepton flavor violation constraints, as well as the perturbativity and unitarity constraints on the model parameters. Then we lay out a search strategy for the unique signature of the model resulting from the leptonic scalars at the hadron colliders via the processes H±±→ W±W±ϕ and H±→ W±ϕ for both small and large leptonic Yukawa coupling cases. We find that via these associated production processes at the HL-LHC, the prospects of doubly-charged scalar H±± can reach up to 800 (500) GeV and 1.1 (0.8) TeV at the 2σ (5σ) significance for small and large Yukawa couplings, respectively. A future 100 TeV hadron collider will further increase the mass reaches up to 3.8 (2.6) TeV and 4 (2.7) TeV, at the 2σ (5σ) significance, respectively. We also demonstrate that the mass of ϕ can be determined at about 10% accuracy at the LHC for the large Yukawa coupling case even though it escapes as missing energy from the detectors.

Neutrino mass and lepton flavor violation in A4-based left–right symmetric model with linear seesaw

We explore an [Formula: see text] flavor-based left–right symmetric model where neutrino masses and mixing are explained through linear seesaw mechanism. Taking [Formula: see text] flavor symmetry and left–right gauge symmetry together and adding a low-scale seesaw mechanism gives the advantage of (a) getting diagonal structures for both charged lepton mass matrix and Dirac neutrino mass matrix, (b) simpler relations among neutrino observables in the model and (c) large light-heavy neutrino mixing which gives dominant contributions to various lepton flavor violating decays like [Formula: see text], [Formula: see text], [Formula: see text]. By saturating the experimental bounds on these decays we derive constraints on input model parameters and study analytically as well as numerically the correlation among model parameters and their dependence on experimentally determined neutrino parameters like [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. We also study CP-violation via Jarlskog invariants and show extra contributions to CP-violating effects that the model generates due to unitarity violation in the neutrino sector.

Constraining low-scale flavor models with (g−2)μ and lepton flavor violation

We present here two concrete examples of models where a sub-TeV scale breaking of their respective $\mathcal{T}_{13}$ and $A_5$ flavor symmetries is able to account for the recently observed discrepancy in the muon anomalous magnetic moment, $(g-2)_\mu$. Similarities in the flavor structures of the charged-lepton Yukawa matrix and dipole matrix yielding $(g-2)_\mu$ give rise to strong constraints on low-scale flavor models when bounds from lepton flavor violation (LFV) are imposed. These constraints place stringent limits on the off-diagonal Yukawa structure, suggesting a mostly (quasi-)diagonal texture for models with a low flavor breaking scale $\Lambda_f$. We argue that many of the popular flavor models in the literature designed to explain the fermion masses and mixings are not suitable for reproducing the observed discrepancy in $(g-2)_\mu$, which requires a delicate balance of maintaining a low flavor scale while simultaneously satisfying strong LFV constraints.

Neutrinoless double-beta decay and lepton flavor violation in discrete flavor symmetric left–right symmetric model

In this work, we have studied neutrinoless double beta decay (NDBD) and charged lepton flavor violation (CLFV) in a generic left–right symmetric model (LRSM). In this framework, type-I and type-II seesaw terms arise naturally. We have used [Formula: see text] discrete flavor symmetry to realize the LRSM. Within the model, we have considered type-I and type-II dominant cases and analyzed the new physics contributions to the NDBD process coming from different particles of LRSM. We tried to find the leading-order contributions to NDBD in type-I and type-II seesaw along with the decay rate of the process in our work. We have also studied different charged lepton flavor violating processes such as [Formula: see text] and [Formula: see text] and correlated with neutrino mass within the model.

A multi-charged particle model with local U(1) μ-τ to explain muon g–2, flavor physics, and possible collider signature * *This research was supported by an appointment to the JRG Program at the APCTP through the Science and Technology Promotion Fund and Lottery Fund of the Korean Government

We consider a model with multi-charged particles, including vector-like fermions, and a charged scalar under a local symmetry. We search for an allowed parameter region explaining muon anomalous magnetic moment (muon ) and anomalies, satisfying constraints from the lepton flavor violations, Z boson decays, meson anti-meson mixing, and collider experiments. Via numerical analysis, we explore the typical size of the muon and Wilson coefficients to explain the anomalies in our model when all other experimental constraints are satisfied. Subsequently, we discuss the collider physics of the multicharged vectorlike fermions, considering a number of benchmark points in the allowed parameter space.

Primordial black holes and lepton flavor violation with scotogenic dark matter

Abstract We show that if the lepton flavor-violating μ → eγ process is observed in the MEG II experiment, the initial density of primordial black holes (PBHs) can be constrained with scotogenic dark matter. As a benchmark case, if PBH evaporation occurs in the radiation-dominated era, the initial density may be 2 × 10−17 ≲ β ≲ 3 × 10−16 for the $\mathcal {O}$(TeV)-scale dark sector in the scotogenic model, where β is the ratio of the PBH density ρPBH to the radiation density ρrad at the time of PBH formation. As another benchmark case, if PBHs evaporate in the PBH-dominated era, the initial density may be 1 × 10−8 ≲ β ≲ 3 × 10−7 for $\mathcal {O}$(GeV)-scale dark matter, with other $\mathcal {O}$(TeV)-scale particles in the scotogenic model.

A renormalizable left-right symmetric model with low scale seesaw mechanisms

We propose a low scale renormalizable left-right symmetric theory that successfully explains the observed SM fermion mass hierarchy, the tiny values for the light active neutrino masses and is consistent with the lepton and baryon asymmetries of the Universe, the muon and electron anomalous magnetic moments as well as with the constraints arising from the meson oscillations. In the proposed model the top and exotic quarks obtain masses at tree level, whereas the masses of the bottom, charm and strange quarks, tau and muon leptons are generated from a tree level Universal Seesaw mechanism, thanks to their mixings with the charged exotic vector like fermions. The masses for the first generation SM charged fermions arise from a radiative seesaw mechanism at one loop level, mediated by charged vector like fermions and electrically neutral scalars. The light active neutrino masses are produced from a one-loop level inverse seesaw mechanism mediated by electrically neutral scalar singlets and right handed Majorana neutrinos. Our model is also consistent with the experimental constraints arising from the Higgs diphoton decay rate as well as with the constraints arising from charged lepton flavor violation. We also discuss the Z′ and heavy scalar production at a proton-proton collider.

Lepton flavor violation in the Littlest Higgs Model with T parity realizing an inverse seesaw

We study lepton flavor violation (LFV) within the Littlest Higgs Model with T parity (LHT) realizing an inverse seesaw (ISS) mechanism of type I. With respect to the traditional LHT, there appear new \U0001d4aa(10 TeV) Majorana neutrinos, driving LFV. For τ → mathrm{ell ell}^{prime}overline{mathrm{ell}}^{{primeprime} } (including wrong-sign, ℓ = e, μ) decays and μ → e conversion in Ti, we get typical rates only one order of magnitude below present bounds (ℓ → ℓ′γ can reach the current upper limit) and for Z → overline{tau}mathrm{ell } , μ → eeoverline{e} and conversion in Au, results are within two orders of magnitude from present limits. Correlations among modes are drastically different to the traditional LHT and other models, which would ease the confrontation of this scenario to eventual measurements of LFV processes involving charged leptons.

Sensitivity of lepton flavor violation process B+ → K+μ−τ+ in future experiments

The upcoming upgrades of Large Hadron Collider (LHC) to High Luminosity LHC (HL-LHC) and the proposed Future Circular Collider (FCC-hh) offer an opportunity of observing the Lepton Flavor Violating (LFV) process beyond the Standard Model. Monte Carlo simulation of generating and detecting an LFV process [Formula: see text] on CMS at LHC (Run-3), HL-LHC, and FCC-hh reference detector (FRD) is performed. This process is favored by the leptoquark models, particularly the three-site Pati–Salam model but disfavored by the gauged [Formula: see text] model, both of which are introduced to address the [Formula: see text] anomalies. We study possible backgrounds with similar final states and use Multi-Variable Analysis (MVA) to discriminate signal events from possible backgrounds. The MVA output is used to estimate Single Event Sensitivity (SES) of this process, yielding encouraging results on CMS at HL-LHC and FRD at FCC-hh with considerable SES values of about [Formula: see text] (CMS, LHC Run-3), [Formula: see text] (CMS, HL-LHC) and [Formula: see text] (FRD, FCC-hh), suggesting it’s promising to probe such process in the future high energy physics experiments and constrain the new physics models more rigorously.

Lepton anomalous magnetic moment with singlet-doublet fermion dark matter in a scotogenic U(1)Lμ−Lτ model

We study an extension of the minimal gauged ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ model including three right-handed singlet fermions and a scalar doublet to explain the anomalous magnetic moments of muon and electron simultaneously. The presence of an in-built ${Z}_{2}$ symmetry under which the right-handed singlet fermions and $\ensuremath{\eta}$ are odd, gives rise to a stable dark matter candidate along with light neutrino mass in a scotogenic fashion. In spite of the possibility of having positive and negative contributions to muon and electron ($g\ensuremath{-}2$) respectively from vector boson and charged scalar loops, the minimal scotogenic ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ model cannot explain both muon and electron ($g\ensuremath{-}2$) simultaneously while being consistent with other experimental bounds. We then extend the model with a vectorlike lepton doublet which not only leads to a chirally enhanced negative contribution to electron ($g\ensuremath{-}2$) but also leads to the popular singlet-doublet fermion dark matter scenario. With this extension, the model can explain both electron and muon ($g\ensuremath{-}2$) while being consistent with neutrino mass, dark matter and other direct search bounds. The model remains predictive at high energy experiments like collider as well as low energy experiments looking for charged lepton flavor violation, dark photon searches, in addition to future ($g\ensuremath{-}2$) measurements.

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.

Models of the muonium to antimuonium transition

Muonium is a bound state composed of an antimuon and an electron, and it constitutes a hydrogen-like atom. Because of the absence of the hadronic matter in the bound state, the muonium is a useful probe to explore new physics being free from the hadronic uncertainties. The process of the muonium-to-antimuonium transition is considered to be effective to identify fundamental interactions which relate to the lepton flavor and lepton number violation. New experiments are being planned at J-PARC in Japan and CSNS in China, and it is expected to attract more attention in the near future. In this paper, we will study what kind of model can be verified in the next generation of the muonium-to-antimuonium transition search experiments while escaping the limitations from other experiments. Though the transition probability is strongly suppressed by the lepton flavor conservation in the standard model, it can be much larger by the exchanges of neutral and doubly charged bosons, and by box loop diagrams in new physics beyond the standard model. We study the neutrino models with heavy Majorana neutrinos at TeV scale, a type-II seesaw model, left--right models, and models for radiative neutrino masses such as the Zee-Babu model in particular, in addition to other possible models to induce the sizable transition probability, which can be tested in the forthcoming experiments.

Z mediated flavor changing neutral currents with a fourth vectorlike family

We discuss $Z$ mediated flavor changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vectorlike family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1{)}^{\ensuremath{'}}$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vectorlike fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violating (CLFV) $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$, $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $Z\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ decays and find that they cannot significantly constrain the masses of charged vectorlike leptons. However, the 790 GeV mass bound arising from collider searches on vectorlike lepton doublets can set further constraints on the model parameter space. We also consider rare $t\ensuremath{\rightarrow}cZ$ decays as well as unitarity violation in the CKM mixing in order to constrain the quark sector of the model under consideration.

Singlet fermion dark matter and Dirac neutrinos from Peccei-Quinn symmetry

The Peccei-Quinn (PQ) mechanism not only acts as an explanation for the absence of strong CP violation but also can play a main role in the solution to other open questions in particle physics and cosmology. Here we consider a model that identifies the PQ symmetry as a common thread in the solution to the strong CP problem, the generation of radiative Dirac neutrino masses and the origin of a multicomponent dark sector. Specifically, scotogenic neutrino masses arise at one loop level with the lightest fermionic mediator field acting as the second dark matter (DM) candidate thanks to the residual $Z_2$ symmetry resulting from the PQ symmetry breaking. We perform a phenomenological analysis addressing the constraints coming from the direct searches of DM, neutrino oscillation data and charged lepton flavor violating (LFV) processes. We find that the model can be partially probed in future facilities searching for WIMPs and axions, and accommodates rates for rare leptonic decays that are within the expected sensitivity of upcoming LFV experiments.