Observation of $B^{+}\to a_1^{+}(1260)K^0$ and $B^0\to a_1^{-}(1260)K^{+}$

We present branching fraction measurements of the decays $B^{+}\to a_1^{+}(1260)K^0$ and $B^0\to a_1^{-}(1260)K^{+}$ with $a_1^{\pm }(1260)\to {\pi }^{\mp }{\pi }^{\pm }{\pi }^{\pm }$. The data sample corresponds to $383\times {10}^6$ $B\overline{B}$ pairs produced in $e^{+}{e}^{-}$ annihilation through the ${\rm Y}(4S)$ resonance. We measure the products of the branching fractions $\mathcal{B}\mathbf{(}{B}^{+}\to a_1^{+}(1260)K^0\mathbf{)}\mathcal{B}\mathbf{(}{a}_1^{+}(1260)\to {\pi }^{-}{\pi }^{+}{\pi }^{+}\mathbf{)}=(17.4\pm 2.5\pm 2.2)\times {10}^{-6}$ and $\mathcal{B}\mathbf{(}{B}^0\to a_1^{-}(1260)K^{+}\mathbf{)}\mathcal{B}\mathbf{(}{a}_1^{-}(1260)\to {\pi }^{+}{\pi }^{-}{\pi }^{-}\mathbf{)}=(8.2\pm 1.5\pm 1.2)\times {10}^{-6}$. We also measure the charge asymmetries ${\mathcal{A}}_{\mathrm{ch}}\mathbf{(}{B}^{+}\to a_1^{+}(1260)K^0\mathbf{)}=0.12\pm 0.11\pm 0.02$ and ${\mathcal{A}}_{\mathrm{ch}}\mathbf{(}{B}^0\to a_1^{-}(1260)K^{+}\mathbf{)}=-0.16\pm 0.12\pm 0.01$. The first uncertainty quoted is statistical and the second is systematic.

Figure 1

Projections of (a) $\Delta E$, (b) $m_{\mathrm{ES}}$, (c) $m_{a_1}$, and (d) $\mathcal{F}$ for the $B^{+}\to a_1^{+}(1260)K^0$ decay mode. Points represent on-resonance data, dashed lines the continuum and $B\overline{B}$ backgrounds, and solid lines the full fit function. These plots are made with a cut on the signal likelihood which includes 35%–45% of the signal.

Figure 2

Projections of (a) $\Delta E$, (b) $m_{\mathrm{ES}}$, (c) $m_{a_1}$, and (d) $\mathcal{F}$ for the $B^0\to a_1^{-}(1260)K^{+}$ decay mode. Points represent data, dashed lines the continuum and $B\overline{B}$ backgrounds, and solid lines the full fit function. These plots are made with a cut on the signal likelihood which includes 25%–35% of the signal.