Search for the lepton-flavor-violating τ− → e∓ℓ±ℓ− decays at Belle II

Abstract

We present the result of a search for the charged-lepton-flavor violating decays τ− → e∓ℓ±ℓ−, where ℓ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb−1 recorded by the Belle II experiment at the SuperKEKB e+e− collider. The selection of e+e− → τ+τ− events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10−8 are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes.