Study of exclusive two-body $B^0$ meson decays to charmonium

We present a study of three $B^0$ decay modes useful for time-dependent $\mathrm{CP}$ asymmetry measurements. From a sample of $9.7\times {10}^6\hspace{0.5em}B\overline{B}$ meson pairs collected with the CLEO detector, we have reconstructed $B^0\to J/\psi K_S^0,$ $B^0\to {\chi }_{c1\mathrm{}}{K}_S^0,$ and $B^0\to J/\psi {\pi }^0$ decays. The latter two decay modes have been observed for the first time. We describe a $K_S^0\to {\pi }^0{\pi }^0$ detection technique and its application to the reconstruction of the decay $B^0\to J/\psi K_S^0.$ Combining the results obtained using $K_S^0\to {\pi }^{+}{\pi }^{-}$ and $K_S^0\to {\pi }^0{\pi }^0$ decays, we determine $\mathcal{B}{(B}^0\to J/\psi K^0)=(9.5\mathrm{}\pm 0.8\pm 0.6)\times {10}^{-4},$ where the first uncertainty is statistical and the second one is systematic. We also obtain $\mathcal{B}{(B}^0\to {\chi }_{c1\mathrm{}}{K}^0{)=(3.9\mathrm{}}_{-1.3}^{+1.9\mathrm{}}\pm 0.4)\times {10}^{-4}$ and $\mathcal{B}{(B}^0\to J/\psi {\pi }^0{)=(2.5\mathrm{}}_{-0.9}^{+1.1\mathrm{}}\pm 0.2)\times {10}^{-5}.$