Measurements of $CP$-Violating Asymmetries in $B^0\to a_1^{\pm }(1260){\pi }^{\mp }$ Decays

We present measurements of $CP$-violating asymmetries in the decay $B^0\to a_1^{\pm }(1260){\pi }^{\mp }$ with $a_1^{\pm }(1260)\to {\pi }^{\mp }{\pi }^{\pm }{\pi }^{\pm }$. The data sample corresponds to $384\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric $B$ factory at SLAC. We measure the $CP$-violating asymmetry ${\mathcal{A}}_{CP}^{a_1\pi }=-0.07\pm 0.07\pm 0.02$, the mixing-induced $CP$ violation parameter $S_{a_1\pi }=0.37\pm 0.21\pm 0.07$, the direct $CP$ violation parameter $C_{a_1\pi }=-0.10\pm 0.15\pm 0.09$, and the parameters $\Delta C_{a_1\pi }=0.26\pm 0.15\pm 0.07$ and $\Delta S_{a_1\pi }=-0.14\pm 0.21\pm 0.06$. From these measured quantities we determine the angle ${\alpha }_{\mathrm{eff}}=78.6°\pm 7.3°$.

Figure 1

Projections of (a) $\Delta E$, (b) $m_{\mathrm{ES}}$. Points represent on-resonance data, dotted lines the sum of all backgrounds, and solid lines the full fit function. These plots are made with a cut on the signal likelihood.

Figure 2

Projections onto $\Delta t$ of the data (points) for (a) $B^0$ and (b) ${\overline{B}}^0$ tags, showing the fit function (solid line), and the background function (dotted line), and (c) the asymmetry between $B^0$ and ${\overline{B}}^0$ tags.