Search for B±→[K±π±]DK± and upper limit on the b→u amplitude in B±→DK±

We search for B±→[K±π±]DK± decays, where [K±π±]D indicates that the K±π± pair originates from the decay of a D0 or D-bar 0. Results are based on 120x106 Υ(4S)→BB-bar decays collected with the BABAR detector at SLAC. We set an upper limit on the ratio RKπ≡([Γ(B+→[K-π+]DK+)+Γ(B-→[K+π-]DK-)]/[Γ(B+→[K+π-]DK+)+Γ(B-→[K-π+]DK-)]) <0.026 (90% C.L.). This constrains the amplitude ratio rB≡vertical bar A(B-→D-bar 0K-)/A(B-→D0K-) vertical bar <0.22 (90% C.L.), consistent with expectations. The small value of rB favored by our analysis suggests that the determination of the Cabibbo-Kobayashi-Maskawa phase γ from B→DK will be difficult