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