The suppressed decay chain B-→DK-, D→K +π-, where D indicates a D̄0 or D0 state, provides important information on the CP-violating angle ?3. We measure the ratio RDK of the decay rates to the favored mode B-→DK-, D→K-π+ to be RDK=[1.63-0.41+0.44(stat)-0.13+0.07(syst)]×10-2, which indicates the first evidence of the signal with a significance of 4.1σ. We also measure the asymmetry ADK between the charge-conjugate decays to be ADK=-0.39-0.28+0.26(stat)-0.03+0. 04(syst). The results are based on the full 772×106 BB̄ pair data sample collected at the Υ(4S) resonance with the Belle detector.
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