Search for the lepton-flavor-violating decay ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$ at Belle II

Abstract

We present a search for the lepton-flavor-violating decay ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$ using a data sample that corresponds to an integrated luminosity of 428 fb${}^{-1}$ recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We employ a multivariate classifier to suppress the backgrounds from the Standard Model processes, and the signal extraction is performed using an extended maximum-likelihood fit. Since no significant excess over the expected background is observed, we set an upper limit on the branching fraction $\mathcal{B}({\tau }^{\pm }\to {\mu }^{\pm }\gamma )<9.5$ $(12.2)\times 10^{-8}$ at the 90\% (95\%) confidence level, using the CL${}_s$ technique.