Search for B⁰→π^-τ⁺ν_τ with hadronic tagging at Belle

A search for the decay B⁰→π^-τ⁺ν_τ is presented. The search is performed on the full Belle data set containing 772 x 10⁶ B anti B pairs, collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider. The τ⁺ lepton is reconstructed in the decays τ⁺→e⁺ν_e anti ν_τ, τ⁺→μ⁺ν_μ anti ν_τ, τ⁺→π⁺ anti ν_τ and τ⁺→ρ⁺ anti ν_τ. A full reconstruction algorithm based on NeuroBayes is used to reconstruct one B⁰ meson, the B_tag, in a hadronic decay. The remainder of a signal event contains exactly two charged particles. The separation between signal and background events is performed using boosted decision trees. A fit is performed in the distribution of the extra energy in the electromagnetic calorimeter ECL, which is defined as all energy deposited by neither the B_tag nor the reconstructed signal final state particles. No significant signal is observed and an upper limit of B(B⁰→π^-τ⁺ν_τ)<2.5 x 10^-4 is obtained at the 90% confidence level. The result is in good agreement with the Standard Model predictions.