Study ofCP-Violating Asymmetries inB0→π+π−Decays

We present a measurement of $CP$-violating asymmetries in $B^0\to {\pi }^{+}{\pi }^{-}$ decays based on a $41.8{\mathrm{f}\mathrm{b}}^{-1}$ data sample collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We fully reconstruct one neutral $B$ meson as a $B^0\to {\pi }^{+}{\pi }^{-}$ $CP$ eigenstate and identify the flavor of the accompanying $B$ meson from its decay products. From the asymmetry in the distribution of the time intervals between the two $B$ meson decay points, we obtain the $CP$-violating asymmetry parameters ${\mathcal{S}}_{\pi \pi }=-{1.21}_{-0.27}^{+0.38}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}{)}_{-0.13}^{+0.16}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$ and ${\mathcal{A}}_{\pi \pi }=+{0.94}_{-0.31}^{+0.25}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.09(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$.