Evidence of B^{+}→τ^{+}ν decays with hadronic B tags

We present a search for the decay B+→τ+ν using 467.8×106 BB ̄ pairs collected at the Υ(4S) resonance with the BABAR detector at the SLAC PEP-II B-Factory. We select a sample of events with one completely reconstructed B- in the hadronic decay mode (B-→D(*)0X- and B-→J/ψX-). We examine the rest of the event to search for a B+→τ+ν decay. We identify the τ+ lepton in the following modes: τ+→e+νν ̄, τ+→μ+νν ̄, τ+→π+ν ̄ and τ+→ρ+ν ̄. We find an excess of events with respect to the expected background, which excludes the null signal hypothesis at the level of 3.8σ (including systematic uncertainties) and corresponds to a branching fraction value of B(B+→τ+ν)=(1.83-0.49+0.53(stat)±0.24(syst))×10-4.