Narrow Results

We study the lepton flavor violating and the lepton flavor conserving decays in the framework of the general two-Higgs doublet model, the so-called model III. We estimate the decay width of LFV (LFC) process at the order of magnitude of 10^-5 GeV (10^-4 GeV), for m_H⁰ = 150 GeV, and the intermediate values of the coupling . The experimental result of the process under consideration can ensure comprehensive information about the standard model Higgs boson H⁰ and the model III free parameters.

We study the branching ratios of the lepton flavor violating processes μ→eγ, τ→eγ and τ→μγ in the split fermion scenario, in the framework of the two-Higgs doublet model. We observe that the branching ratios are relatively more sensitive to the compactification scale and the Gaussian widths of the leptons in the extra dimensions, for two extra dimensions and especially for the τ→μγ decay.

We demonstrate the existence of model-independent one-to-one correspondence relations between different lepton flavor violating processes (LFV). Applying the criterion of naturalness, based on the idea of "custodial symmetry", we show that all the LFV processes, independently of specific mechanisms behind them, but with the same external leptons, have a priori comparable amplitudes modulo their kinematics and involved hadronic form factors.

We estimate the charged lepton electric dipole moments and the branching ratios of radiative lepton flavor violating decays in the framework of the two Higgs doublet model with the inclusion of two extra dimensions. Here, we consider that the new Higgs doublet is accessible to one of the extra dimensions with a Gaussian profile and the fermions are accessible to the other extra dimension with uniform zero-mode profile. We observe that the numerical values of the physical quantities studied enhance with the additional effects due to the extra dimensions and they are sensitive to the new Higgs localization.

We study the radiative lepton flavor violating l_i→l_jγγ decays in the case that the lepton flavor violation is induced by the scalar unparticle mediation. We restrict the scaling dimension d_u and the scalar unparticle–photon–photon coupling by using the experimental upper limit of the branching ratio of the decay μ→eγγ. Furthermore, we predict the BRs of the other radiative decays by using the restrictions we get. We observe that the measurements of upper limits of BRs of these decays ensure considerable information for testing the possible signals coming from unparticle physics.

We predict the branching ratios of the lepton flavor violating Higgs decays H⁰ → e^± μ^±, H⁰ → e^± τ^± and H⁰ → μ^± τ^± in the case that the lepton flavor violation is carried by the scalar unparticle mediation. We observe that their branching ratios are strongly sensitive to the unparticle scaling dimension and they can reach the values of the order of 10^-4, for the heavy lepton flavor case and for the small values of the scaling dimension.

We review various TeV scale models for neutrino masses utilizing different types of seesaw mechanisms, higher dimensional operators, expanded gauge symmetries, or low scale extra dimensions. In addition, we discuss the implications of these models for the collider experiments and low energy lepton flavor violation searches.

Recently, there is a growing interest of a light leptophilic gauge boson $Z^{\prime }$, which might explain the ${(g-2)}_{\mu }$ puzzle. Considering the constraints on the $Z^{\prime }$ coupling $Z\ell {\ell }^{\prime }$, we calculate its contributions to the lepton flavor violation (LFV) decay $Z\to \tau \mu$. We find that this kind of new light gauge boson $Z^{\prime }$ might make the LFV decay $Z\to \tau \mu$ to be probed in future $e^{+}{e}^{-}$ colliders.

Lepton flavor violating (LFV) processes, $e^{+}{e}^{-}\to e^{+}{\ell }^{-}$ and $e^{-}{e}^{-}\to e^{-}{\ell }^{-}$ ($\ell =\mu$ or $\tau$), via four-Fermi contact interactions at future International Linear Collider (ILC) are studied. Taking account of previous experimental results of LFV processes $\mu \to 3e$ and $\tau \to 3e$, we find that the upper limits on the LFV parameters for $\ell =\tau$ could be improved at the ILC experiment using the polarized electron beam. The improvement of the upper limits could be nearly an order of magnitude as compared to previous ones.

Taking account of the constraint from radiative two-body decays $l_i\to l_j\gamma$, we investigate the lepton flavor violation decays $B_q^0\to {\bar{l}}_l{l}_k$ in the framework of the minimal extension of the Standard Model with one neutral singlet scalar. The couplings $C_{e\mu }$, $C_{e\tau }$ and $C_{\mu \tau }$ between the different generation leptons and scalar $S^0$ are constrained by the current bounds of $l_i\to l_j\gamma$. The numerical results show that the theoretical prediction of $B_q^0\to {\bar{l}}_l{l}_k$ strongly depends on the couplings $C_{qb}$ ($q=d$ or $s$) between down-type quarks and new scalar. The contributions from couplings $C_{uc}$, $C_{ut}$ and $C_{ct}$ between up-type quark and new scalar are less dominant.

Lepton flavor violation decays are channels which may lead to fundamental discoveries in the forthcoming years and this makes it an exciting research field for beyond the Standard Model (SM) searches. In this work, we present an analysis of the lepton flavor violation decays $l_{\alpha }\to 3l_{\beta }$ in Minimal R-symmetric Supersymmetric Standard Model. The prediction for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$ depends on the off-diagonal entries of the slepton mass matrix. The contributions to Wilson coefficients can be classified into Higgs penguins, photon penguins, Z penguins and box diagrams. It shows the contribution from Z penguins dominates the predictions for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$, and the contributions from Higgs penguins and box diagrams play different roles in different decay channels. The theoretical predictions for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$ can reach the future experimental limits, and the channels are very promising to be observed in near future experiment.

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 $B^{+}\to K^{+}{\mu }^{-}{\tau }^{+}$ 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 $L_{\mu }-L_{\tau }$ model, both of which are introduced to address the $B$ 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 $0.0696\pm 0.0010\mathrm{\text{fb}}$ (CMS, LHC Run-3), $0.0070\pm 0.0001\mathrm{\text{fb}}$ (CMS, HL-LHC) and $0.1389\pm 0.0006\mathrm{\text{ab}}$ (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.

Heavy neutrinos are usually introduced to accommodate tiny neutrino masses via seesaw mechanism, or to alleviate the cosmology problem, and there may exist charged Higgs which couple to the leptons with different flavors. These two features can appear in the Twin Higgs models. What interests us is that such new particles and interactions may lead to new contributions to the lepton flavor violating processes ${\ell }_i\to {\ell }_j\gamma$ and ${\ell }_i\to {\ell }_j{\ell }_k{\ell }_l$. We find that current experimental data can constrain the parameter spaces and certain lepton flavor violating processes can possibly be tested by the next generation experiments.

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 $\tau \to 3\mu$  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.

We present a recent BABAR search for lepton flavor violation in tau decays. The search for τ → ℓℓℓ(ℓ = e, μ) decays is based on a data sample of 91.6 fb^-1. No signal is found in any of six channels and 90% CL upper limits are set in the range of (1 - 3) × 10^-7. The search for τ → μγ decay is done with a 63 fb^-1 data sample. The number of observed events is consistent with expected background level and a preliminary upper limit is set at 2 × 10^-6 at 90% CL.

We report mainly on a search for lepton-flavor-violating τ decays involving π⁰/η/η′ final states using a sample of 153.8 fb^-1, which was recorded with the Belle detector at the KEKB e⁺e^- asymmetric-energy collider. We also present searches for other possible LFV tau decays, and discuss the relationship of our results to SUSY parameters.

Based on results of a search for the lepton-family-number-violating decay K⁺ → π⁺μ⁺e^- with data collected by experiment E865 in 1998 at the Alternating Gradient Synchrotron of Brookhaven National Laboratory we report an upper limit on the branching ratio at 2.2 × 10^-11 (90% C.L.). In combination with results based on the data collected in 1995, 1996, and a previous experiment, E777, an upper limit on the branching ratio of 1.2 × 10^-11 (90% C.L.) is obtained.

We conduct a detailed analysis of the phenomenology of two predictive see-saw scenarios which lead to the Quark-Lepton Complementarity relation θ₁₂+θ_C = π/4. The neutrino mixing observables and their correlations, neutrinoless double beta decay, lepton flavor violating decays such as μ → eγ and leptogenesis are discussed. The features which allow to distinguish the scenarios are identified.

We study the radiative lepton flavor violating l_i → l_jγ decays in the two-Higgs doublet model, respecting the Randall–Sundrum scenario and estimate the contributions of the KK modes of left- (right-)handed charged lepton doublets (singlets) on the branching ratios. We observe that the branching ratios are sensitive to the contributions of the charged lepton KK modes.

We study the lepton-flavor-violating processes of τ^- → μ^-PP decays with PP = K⁺K^-, , π⁺π^-, π⁰π⁰ in the framework of two-Higgs-doublet model III by virtue of the chiral perturbation theory. In this model, only three neutral Higgs bosons contribute to these decays. With the current experimental constraints, we show that (a) the contributions of the |λ_uu(dd)| term are very small for these four decays; (b) we get the correlation between |λ_ss| and |λ_τμ|; for |λ_τμ| ~ 10–400, one has |λ_ss| ~ 40–1; (c) in the existing model parameter space, Br(τ^- → μ^- K⁺K^-) could reach the order of , but Br(τ → μ^-π⁺π^-/π⁰π⁰) are too small to be observed.