Search for b-->u transitions in B- --> DK- and B- --> D*K- Decays

TL;DR

This study investigates suppressed B- decays involving D(*) mesons to measure parameters related to the CKM angle gamma, providing evidence with statistical significance and setting upper limits on decay amplitude ratios.

Contribution

It reports the first indications of suppressed B- to D(*)K- decay modes and measures their branching fraction ratios and CP asymmetries, advancing understanding of CKM matrix parameters.

Findings

Indications of suppressed decay signals with ~2 sigma significance.

Measured ratios R_{DK} and R*_{(D pi0)K} around 1-2%.

Set upper limits on the amplitude ratios r_B and r*_B.

Abstract

We report results from an updated study of the suppressed decays B- to D K- and B- to D* K- followed by D to K+pi-, where D(*) indicates a D(*)0 or an anti-D(*)0 meson, and D* decays to D pi0 or D gamma. These decays are sensitive to the CKM unitarity triangle angle gamma, due to interference between the b to c transition B- to D(*)0K- followed by the doubly Cabibbo-suppressed decay D0 to K+pi-, and the b to u transition B- to anti-D(*)0K- followed by the Cabibbo-favored decay anti-D0 to K+pi-. We also report an analysis of the decay B- to D(*)pi- with the D decaying into the doubly Cabibbo-suppressed mode D to K+pi-. Our results are based on 467 million Upsilon (4S) to B anti-B decays collected with the BaBar detector at SLAC. We measure the ratios R(*) of the suppressed ([K+pi-]_D K-/pi-) to favored ([K-pi+]_D K-/pi-) branching fractions as well as the CP asymmetries A(*) of those modes. We see indications of signals for the B- to D K- and B- to D*K-, D*->D pi0 suppressed modes, with statistical significances of 2.1 and 2.2 sigma, respectively, and we measure: R_{DK} = (1.1\pm 0.6 \pm 0.2)\times 10^{-2}, A_{DK} = -0.86 \pm 0.47 \ ^{+0.12}_{-0.16}, R*_{(D pi0)K} = (1.8\pm 0.9 \pm 0.4)\times 10^{-2}, A*_{(D pi0)K} = +0.77 \pm 0.35\pm 0.12, R*_{(D gamma)K} = (1.3\pm 1.4\pm 0.8 )\times 10^{-2}, A*_{(D gamma)K} = +0.36 \pm 0.94\ ^{+0.25}_{-0.41}, where the first uncertainty is statistical and the second is systematic. We use a frequentist approach to obtain the magnitude of the ratio r_B = |A(B- to anti-D0 K-) / A(B- to D0K-)|= (9.5^{+5.1}_{-4.1})%, with r_B<16.7% at 90% confidence level. In the case of B- to D* K- we find r*_B = |A(B^- to anti-D*0 K-)/A(B- to D*0 K-)|=(9.6^{+3.5}_{-5.1})%, with r*_B<15.0% at 90% confidence level.