We report improved measurements of time-dependent $CP$ violation parameters for $B^0({\overline{B}}^0)\to J/\psi {\pi }^0$ decay. This analysis is based on $535\times {10}^6$ $B\overline{B}$ pairs accumulated at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. From the distribution of proper time intervals between the two $B$ decays, we obtain the following $CP$ violation parameters ${\mathcal{S}}_{J/\psi {\pi }^0}=-0.65\pm 0.21(\mathrm{stat})\pm 0.05(\mathrm{syst})\mathrm{\text{and}}{A}_{J/\psi {\pi }^0}=+0.08\pm 0.16(\mathrm{stat})\pm 0.05(\mathrm{syst}),$which are consistent with standard model expectations.

• Received 2 August 2007

DOI:https://doi.org/10.1103/PhysRevD.77.071101