PHYSICAL REVIEW LETTERS 130, 181803 (2023)

ISSN: 0031-9007, SCIE

Search for Lepton-Flavor-Violating τ Decays to a Lepton and an Invisible Boson at Belle II

The Belle II collaboration (I. Adachi, ..., N. Anh Ky, ...)

We search for lepton-flavor-violating τ− → e−α and τ− → μ−α decays, where α is an invisible spin-0 boson. The search uses electron-positron collisions at 10.58 GeV center-of-mass energy with an integrated luminosity of 62.8 fb−1, produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known τ− → e− ¯νeν τ and τ− → μ− ¯ν μ ντ decays. We report 95% confidence-level upper limits on the branching-fraction ratio Bðτ− → e−αÞ=Bðτ− → e− ¯νeν τÞ in the range ð1.1–9.7Þ × 10−3 and on Bðτ− → μ−αÞ=Bðτ− → μ− ¯ν μ ν τÞ in the range ð0.7–12.2Þ × 10−3 for α masses between 0 and 1.6 GeV=c2. These results provide the most stringent bounds on invisible boson production from τ decays.

DOI: DOI: 10.1103/PhysRevLett.130.181803