cLFV Processes Research Articles

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

Study of cLFV with ℓiℓjγγ effective vertex

In this work we analyze cLFV processes using a low-energy EFT that induces the effective interaction between two charged leptons of different flavor and two photons. We compute ℓi → ℓjγ, ℓi → ℓjγγ decays and ℓi → ℓj conversion in nuclei. We derived indirect upper limits on the double photon decays, which turned out to be below current experimental bounds. Our prediction for ℓ → τ conversion in nuclei is below the expected sensitivity of the NA64 experiment

Charged lepton flavor violating processes in the Grimus-Neufeld model

Charged Lepton Flavour Violating (cLFV) decays constrain the relationship between the neutrino and the scalar sectors of the Grimus-Neufeld model (GNM), an appealing minimal model of neutrino masses. It turns out, that in the scenario, where the seesaw scale is lower than the electroweak one, cLFV is completely defined by the new Yukawa interactions between the additional single heavy Majorana neutrino, the second Higgs doublet and the lepton doublets. Therefore, we derive a useful parameterization for the Yukawa couplings which reproduces by construction the correct PMNS matrix and the correct neutrino masses for both Normal and Inverted ordering at one-loop level. We embed this scenario in the FlexibleSUSY spectrum-generator generator to perform parameter scans. Focusing on the tiny seesaw scale, we show that current μ → eγ limits provide significant constraints on the scalar sector, and we evaluate the impact of future cLFV τ-decay searches for the cases of discovery or non-discovery. The tiny seesaw scale makes the neutrino sector and the cLFV processes in the GNM similar to the scotogenic and the scoto-seesaw models, so we provide constraints for these models as well.

Effect of Sterile Neutrino on Low-Energy Processes in Minimal Extended Seesaw With Δ(96) Symmetry and TM1 Mixing

We study the effect of sterile neutrino on some low-scale processes in the framework of the minimal extended seesaw (MES). MES is the extension of the seesaw mechanism with the addition of sterile neutrino of intermediate mass. The MES model in this work is based on Δ(96) ×C2×C3flavor symmetry. The structures of mass matrices in the framework lead toTM1mixing withμ–τsymmetry. The model predicts the maximal value of the Dirac CP phase. We carry out our analysis to study the new physics contributions from the sterile neutrino to different charged lepton flavor violation (cLFV) processes involving muon and tau leptons as well as neutrinoless double beta decay (0νββ). The model predicts normal ordering (NO) of neutrino masses, and we perform the numerical analysis considering normal ordering (NO) only. We find that a heavy sterile neutrino can lead to cLFV processes that are within the reach of current and planned experiments. The sterile neutrino present in our model is consistent with the current limits on the effective neutrino mass set by 0νββexperiments.

Impact of sterile neutrinos on nuclear-assisted cLFV processes

We discuss charged lepton flavour violating processes occurring in the presence of muonic atoms, such as muon-electron conversion in nuclei $\text{CR}(\mu -e, \text{ N})$, the (Coulomb enhanced) decay of muonic atoms into a pair of electrons BR($\mu^- e^- \to e^- e^-$, N), as well as Muonium conversion and decay, $\text{Mu}-\bar{\text{Mu}}$ and $\text{Mu}\to e^+ e^-$. Any experimental signal of these observables calls for scenarios of physics beyond the Standard Model. In this work, we consider minimal extensions of the Standard Model via the addition of sterile fermions, providing the corresponding complete analytical expressions for all the considered observables. We first consider an "ad hoc" extension with a single sterile fermion state, and investigate its impact on the above observables. Two well motivated mechanisms of neutrino mass generation are then considered: the Inverse Seesaw embedded into the Standard Model, and the $\nu$MSM. Our study reveals that, depending on their mass range and on the active-sterile mixing angles, sterile neutrinos can give significant contributions to the above mentioned observables, some of them even lying within present and future sensitivity of dedicated cLFV experiments. We complete the analysis by confronting our results to other (direct and indirect) searches for sterile fermions.

On the interrelationship among leptonicg− 2, EDMs and lepton flavor violation

We present a concise review of the status of charged lepton flavor violation (cLFV) in scenarios beyond the SM. We emphasize that the current experimental resolutions on cLFV processes are already testing territories of new physics (NP) models well beyond the LHC reach. On the other hand, with the expected sensitivities of next-generation experiments, cLFV will become the most powerful probe of NP signals at our disposal. Finally, the interrelationship among leptonic g − 2, EDMs and cLFV will turn out to be of outmost importance to disentangle among different NP scenarios.

Interrelationship among [formula omitted], EDMs and cLFV

We summarize the status of charged lepton flavor violation (cLFV) in models beyond the SM. We stress that the current experimental bounds on cLFV processes are already constraining new physics (NP) scenarios more than the direct bounds from the LHC. On the other hand, the interrelationship among leptonic g−2, EDMs and cLFV will turn out to be of outmost importance to disentangle among different NP scenarios.

Lepton Flavour Violation Experiments

Lepton Flavour Violation in the charged lepton sector (CLFV) is forbidden in the Minimal Standard model and strongly suppressed in extensions of the model to include finite neutrino mixing. On the other hand, a wide class of Supersymmetric theories, even coupled with Grand Unification models (SUSY-GUT models), predict CLFV processes at a rate within the reach of new experimental searches operated with high resolution detectors at high intensity accelerators. As the Standard model background is negligible, the observation of one or more CLFV events would provide incontrovertible evidence for physics beyond Standard model, while a null effect would severely constrain the set of theory parameters. Therefore, a big experimental effort is currently (and will be for incoming years) accomplished to achieve unprecedented sensitivity on several CLFV processes. In this paper we review past and recent results in this research field, with focus on CLFV channels involving muons and tau's. We present currently operating experiments as well as future projects, with emphasis laid on how sensitivity enhancements are accompanied by improvements on detection techniques. Limitations due to systematic effects are also discussed in detail together with the solutions being adopted to overcome them.

Muon to Electron Conversion Experiment

The search for CLFV with muons has large potential to obtain hints for new physics beyond the Standard Model. The physics of CLFV has received glowing attention from both theorists and experimentalists. In this article, the phenomenology, the experimental status and future experimental prospects for various CLFV processes with muons are described.

Signature of the minimal supersymmetric standard model with right-handed neutrinos in long baseline experiments

The effective interactions which violate lepton flavor accompanied with neutrinos (nLFV) are considered. Such new physics effects are expected to be measured in future neutrino oscillation experiments with a long baseline. They are induced by radiative corrections in the framework of the minimal supersymmetric standard model with right-handed neutrinos. We numerically evaluate the size of nLFV interactions in this framework. The slepton mixing is not only the origin of lepton flavor violation in the charged lepton sector (cLFV) but also that of the nLFV. We find that the nLFV couplings are strongly correlated with the corresponding cLFV processes, and they are constrained to be $\mathcal{O}({10}^{\ensuremath{-}5})$ times smaller than the standard four-Fermi couplings.