Search for the lepton-flavour-violating decays B_s⁰ → τ ^±µ^∓ and B⁰ → τ ^±µ^∓

A search for B_s⁰ → τ^±µ^∓ and B⁰ → τ^±µ^∓ decays is performed using data corresponding to an integrated luminosity of 3 fb⁻¹ of proton-proton collisions, recorded with the LHCb detector in 2011 and 2012. For this search, the τ lepton is reconstructed in the τ⁻ → π⁻π⁺π⁻ν_τ channel. No significant signal is observed. Assuming no contribution from B⁰ → τ^±µ^∓ decays, an upper limit is set on the B_s⁰ → τ^±µ^∓ branching fraction of B (B_s⁰ → τ^±µ^∓) < 4.2 × 10⁻⁵ at 95% confidence level. If instead no contribution from B_s⁰ → τ^±µ^∓ decays is assumed, a limit of B (B⁰ → τ^±µ^∓) < 1.4 × 10⁻⁵ is obtained at 95% confidence level. These are the first limit on B (B_s⁰ → τ^±µ^∓) and the world’s best limit on B (B⁰ → τ^±µ^∓).