We report the first observation of the radiative hyperonic B decay B+→pΛ̄γ, using a 140fb-1 data sample recorded on the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider. The measured branching fraction is B(B+→pΛ̄γ)= (2.16-0.53+0.58±0.20)×10-6. We examine its MpΛ̄ distribution and observe a peak near threshold. This feature is expected by the short-distance b→sγ transition. A search for B+→pΣ̄ 0γ yields no significant signal, and we set a 90% confidence-level upper limit on the branching fraction of B(B+→pΣ̄0γ)<4. 6×10-6.
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