Study of time-dependent CP-violation in $B^0 \to J/\psi ~\pi^0$ decay

We report a measurement of CP asymmetry parameters in the $b \to c\bar{c}d$-transition-induced decays $B^0(\bar{B^0}) \to J/\psi pi^0$. The analysis is based on a 140 fb$^{-1}$ data sample accumulated at the $\Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We fully reconstruct one neutral $B$ meson in the $J/\psi ~\pi^0$ final state. The accompanying $B$ meson flavor is identified by its decay products. From the distribution of proper time intervals between the two B decays, we obtain the following CP violation parameters: S(J/\psi \pi^0)= -0.72 \pm0.42 (stat) \pm 0.08(syst) A(J/\psi \pi^0)= -0.01 \pm0.29 (stat) \pm 0.07(syst).