Search for B_s^0 → ℓ ∓ τ ± with the semi-leptonic tagging method at Belle

We present a search for the lepton-flavor-violating decays B_s^0 → ℓ ∓ τ ± , where ℓ = e, μ , using the full data sample of 121 fb − 1 collected at the Υ(5 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. We use B_s^0B_s^0 events in which one B_s^0 meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for B_s^0 → ℓ ∓ τ ± decays and set upper limits on their branching fractions at 90