Search for Bs0 → ℓ^∓ τ^± with the semi-leptonic tagging method at Belle

We present a search for the lepton-flavor-violating decays Bs0 → ℓ^∓ τ^±, where ℓ = e, μ, using the full data sample of 121 fb⁻¹ collected at the Υ(5S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺ e⁻ collider. We use Bs0B¯s0 events in which one Bs0 meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for Bs0 → ℓ^∓ τ^± decays and set upper limits on their branching fractions at 90% confidence level as B (Bs0 → e^∓ τ^±) < 14 × 10⁻⁴ and B (Bs0 → μ^∓ τ^±) < 7.3 × 10⁻⁴. Our result represents the first upper limit on the Bs0 → e^∓ τ^± decay rate. [Figure not available: see fulltext.]