We report a measurement of $CP$ asymmetry parameters in the decay $B^0({\overline{B}}^0)\to J/\psi {\pi }^0$, which is governed by the $b\to c\overline{c}d$ transition. The analysis is based on a $140{\mathrm{f}\mathrm{b}}^{-1}$ data sample accumulated at the ${\rm Y}(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. One neutral $B$ meson in the $J/\psi {\pi }^0$ final state is fully reconstructed for events used in this analysis. 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 $CP$-violating parameters: $S_{J/\psi {\pi }^0}=-0.72\pm 0.42(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.09(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$ and $A_{J/\psi {\pi }^0}=-0.01\pm 0.29(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.03(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$.

• Received 20 August 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.261801