Search for b→u transitions in B−→D0K− and B−→D*0K−

We search for B−→˜D0K− and B−→˜D*0K− and charge conjugates. Here the symbol ˜D0 indicates decay of a D0 or ¯¯¯D0 into K+π−, while the symbol ˜D*0 indicates decay of a D*0 or ¯¯¯D*0 to ˜D0π0 or ˜D0γ. These final states can be reached through the b→c transition B−→D(*)0K− followed by the doubly Cabibbo-Kobayashi-Maskawa (CKM)-suppressed D0→K+π−, or the b→u transition B−→¯¯¯D(*)0K− followed by the CKM-favored ¯¯¯D0→K+π−. The interference of these two amplitudes is sensitive to the angle γ of the unitarity triangle. Our results are based on 232×106 Υ(4S)→B¯¯¯B decays collected with the BABAR detector at SLAC. We find no significant evidence for these decays. We set a limit rB≡|A(B−→¯¯¯D0K−)/A(B−→D0K−)|<0.23 at 90% C.L. using the most conservative assumptions on the values of the CKM angle γ and the strong phases in the B and D decay amplitudes. In the case of the D∗ we set a 90% C.L. limit r*2B≡|A(B−→¯¯¯D*0K−)/A(B−→D*0K−)|2<(0.16)2 which is independent of assumptions on γ and strong phases.