Search for B+/- --> [K-/+ pi+/-]_D K+/- and upper limit on the b --> u amplitude in B+/- --> D K+/-

TL;DR

This paper reports a search for specific B meson decays involving D mesons and kaons, setting an upper limit on the decay rate ratio and constraining the amplitude ratio, which impacts the measurement of the CKM phase gamma.

Contribution

It provides the first upper limit on the ratio R_Kpi and constrains the amplitude ratio r_B in B --> D K decays, informing CP violation studies.

Findings

Upper limit on R_Kpi < 0.026 at 90% C.L.

Amplitude ratio r_B < 0.22 at 90% C.L.

Small r_B value indicates challenges in measuring gamma.

Abstract

We search for B+/- --> [K^-/+ pi+/-]_D K+/- decays, where [K-/+ pi+/-]_D indicates that the K-/+ pi+/- pair originates from the decay of a D0 or D0bar. Results are based on 120 million Upsilon(4S) --> B Bbar decays collected with the BaBar detector at SLAC. We set an upper limit on the ratio R_Kpi = {Gamma(B+ --> [K- pi+]_D K+) + Gamma(B- --> [K+ pi-]_D K-)}/ {Gamma(B+ --> [K+ pi-]_D K+) + Gamma(B- --> [K- pi+]_D K-)} < 0.026 (90% C.L.). This constrains the amplitude ratio r_B = |A(B- --> D0bar K-)/A(B- --> D0 K-)| < 0.22 (90% C.L.), consistent with expectations. The small value of r_B favored by our analysis suggests that the determination of the Cabibbo-Kobayashi-Maskawa phase gamma from B --> D K will be difficult.