Search for b → u transitions in B− → [K+π−π0]DK−

We search for decays of a B meson into a neutral D meson and a charged kaon, with the D meson decaying into a charged kaon, a charged pion, and a neutral pion. This final state can be reached through the b → c transition B − → D 0 K − followed by the doubly Cabibbo-suppressed D 0 → K + π − π 0 , or the b → u transition B − → ¯¯¯ D 0 K − followed by the Cabibbo-favored ¯¯¯ D 0 → K + π − π 0 . The interference of these two amplitudes is sensitive to the angle γ of the unitarity triangle. We present results based on 226 × 10 6 e + e − → Υ ( 4 S ) → B ¯¯¯ B events collected with the BABAR detector at SLAC. We find no significant evidence for these decays and we set a limit R ADS ≡ Γ ( [ K + π − π 0 ] D K − ) + Γ ( [ K − π + π 0 ] D K + ) Γ ( [ K + π − π 0 ] D K + ) + Γ ( [ K − π + π 0 ] D K − ) < 0.039 at 95% confidence level, which we translate with a Bayesian approach into r B ≡ | A ( B − → ¯¯¯ D 0 K − ) / A ( B − → D 0 K − ) | < 0.19 at 95% confidence level