On stringy origin of minimal flavor violation

We study the minimal flavor violation in the context of string effective field theory. Stringy selection rules indicate that n-point couplings among fermionic zero-modes and lightest scalar modes in the string effective action are given by a product of Yukawa couplings which are regarded as spurion fields of stringy and geometrical symmetries. Hence, Yukawa couplings determine the dynamics of flavor and CP violations. This observation strongly supports the hypothesis of minimal flavor violation in the Standard Model effective field theory.

Generalized 2HDM with wrong-sign lepton-Yukawa coupling, in light of $$g_{\mu }-2$$ and lepton flavor violation at the future LHC

To explain the observed muon anomaly and simultaneously evade bounds from lepton flavor violation in the same model parameter space is a long-cherished dream. In view of a generalized Two Higgs Doublet Model, with a Yukawa structure as a perturbation of Type-X, we are able to get substantial parameter space satisfying these criteria. In this work, we focus on a region with “wrong-sign” lepton-Yukawa coupling which gives rise to interesting phenomenological consequences. Performing a simple cut-based analysis, we show that at 14 TeV run of the LHC with $$300 \mathrm{{fb}}^{-1}$$ 300 fb - 1 integrated luminosity, part of the model parameter space can be probed with significance "Equation missing" which further improves with Artificial Neural Network analysis.

Muon $$g-2$$ anomaly and $$\mu $$–$$\tau $$-philic Higgs doublet with a light CP-even component

We examine the possibilities of accommodating the muon $$g-2$$ g - 2 anomaly released by Fermilab in the 2HDM with a discrete $$Z_4$$ Z 4 symmetry in which an inert Higgs doublet field ($$H,~A,~H^\pm $$ H , A , H ± ) has the lepton flavor violation $$\mu $$ μ –$$\tau $$ τ interactions. We assume the Yukawa matrices to be real and symmetrical and investigate the case of light H (5 GeV $$<m_H<$$ < m H < 115 GeV). After imposing relevant theoretical and experimental constraints, especially for the multi-lepton searches at the LHC, we find that the muon $$g-2$$ g - 2 anomaly can be explained within $$2\sigma $$ 2 σ confidence level in the region of 5 GeV $$<m_H<20$$ < m H < 20 GeV, 130 GeV $$< m_A~(m_{H^\pm })<$$ < m A ( m H ± ) < 610 GeV, and 0.005 $$<\rho<$$ < ρ < 0.014. Meanwhile, the $$\chi ^2_\tau $$ χ τ 2 fitting the data of lepton flavour universality in the $$\tau $$ τ decays approaches to the SM prediction.

Probing charged lepton flavor violation with axion-like particles at Belle II

We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), X, at the B-factory experiment Belle II. We focus on production of the ALP in the tau decays τ → Xl with l = e, μ, followed by its decay via X → l−l+. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating τ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.

Search for lepton-flavor-violating tau-lepton decays to ℓγ at Belle

Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays τ± → ℓ±γ (ℓ = e, μ) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via τ± → ℓ±γ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb−1, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $$ \mathcal{B} $$ B (τ± → μ±γ) ≤ 4.2 × 10−8 and $$ \mathcal{B} $$ B (τ± → e±γ) ≤ 5.6 × 10−8, are set at 90% confidence level.

Two-loop radiative seesaw, muon g − 2, and τ-lepton-flavor violation with DM constraints

The quartic scalar coupling λ5 term, which violates the lepton-number by two units in the Ma-model, is phenomenologically small when the model is applied to the lepton-flavor violation (LFV) processes. In order to dynamically generate the λ5 parameter through quantum loop effects and retain the dark matter (DM) candidate, we extend the Ma-model by adding a Z2-odd vector-like lepton doublet and a Z2-even Majorana singlet. With the new couplings to the Higgs and gauge bosons, the observed DM relic density can be explained when the upper limits from the DM-nucleon scattering cross sections are satisfied. In addition to the neutrino data and LFV constraints, it is found that the DM relic density can significantly exclude the free parameter space. Nevertheless, the resulting muon g − 2 mediated by the inert charged-Higgs can fit the 4.2σ deviation between the experimental measurement and the SM result, and the branching ratio for τ → μγ can be as large as the current upper limit when the rare μ → (eγ, 3e) decays are suppressed. In addition, it is found that the resulting BR(τ → μρ) can reach the sensitivity of Belle II with an integrated luminosity of 50 ab−1.

Charged lepton flavor violation in light of the muon magnetic moment anomaly and colliders

Any observation of charged lepton flavor violation (CLFV) implies the existence of new physics beyond the SM in charged lepton sector. CLFV interactions may also contribute to the muon magnetic moment and explain the discrepancy between the SM prediction and the recent muon $$g-2$$ g - 2 precision measurement at Fermilab. We consider the most general SM gauge invariant Lagrangian of $$\Delta L=0$$ Δ L = 0 bileptons with CLFV couplings and investigate the interplay of low-energy precision experiments and colliders in light of the muon magnetic moment anomaly. We go beyond previous work by demonstrating the sensitivity of the LHC, the MACE experiment, a proposed muonium-antimuonium conversion experiment, and a muon collider. Currently-available LHC data is already able to probe unexplored parameter space via the CLFV process $$pp\rightarrow \gamma ^*/Z^*\rightarrow \ell _1^\pm \ell _1^\pm \ell _2^\mp \ell _2^\mp $$ p p → γ ∗ / Z ∗ → ℓ 1 ± ℓ 1 ± ℓ 2 ∓ ℓ 2 ∓ .