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

Abstract

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.

Charged lepton EDM with extra Yukawa couplings

“…The decay τ → µγ arises from figure 2 with initial µ replaced by τ , which constrains the product ρ τ µ ρ tt . To comply with the Belle bound of B(τ → µγ) < 4.2 × 10 −8 [56], large ρ τ µ means small ρ tt . Even more surprising [38] is that the collider search for gg → H, A → τ µ provides a much better constraint.…”

Introduction to Charged Lepton Flavour Violation

“…No significant excess over background predictions from the Standard Model is observed, and the 90% C.L. upper limits on the branching fractions are set at B(τ ± → µ ± γ) ≤ 4.2 × 10 −8 and B(τ ± → e ± γ) ≤ 5.6 × 10 −8 [175].…”

“…Two-dimensional distributions of ∆E/√ s vs M BC for τ ± → µ ± γ (left) and τ ± → e ± γ) (right) events. Black points are data, blue squares are τ ± → l ± γ signal MC events, and magenta ellipses show the ±2σ signal regions used in this analysis (Figure and caption from[175]).…”

Search for charged lepton flavor violating decays of ϒ (1S)

“…Due to correlation, uncertainties in charged track finding are added linearly. The efficiency of photon reconstruction is estimated with radiative Bhabha events, and the associated uncertainty is 2.0% [24].…”