Study of B Decays to Charmonium States $B\to \eta_{c}K$ and $B\to \chi_{C0}K$

In a sample of $9.66\times 10^6 B\bar{B}$ pairs collected with the CLEO detector we make the first observation of B decays to an $\eta_c$ and a kaon. We measure branching fractions $BR(B^+\to\eta_c K^+) = (0.69 + 0.26 - 0.21 +- 0.08 +- 0.20)\times 10^{-3}$ and $BR(B^0\to\eta_c K^0) = (1.09 + 0.55 - 0.42 +- 0.12 +- 0.31)\times 10^{-3}$, where the first error is statistical, the second is systematic and the third is from the $\eta_c$ branching fraction uncertainty. From these we extract the $\eta_c$ decay constant in the factorization approximation, $f_{\eta_c} = 335 +- 75$ MeV. We also search for B decays to a $\chi_{c0}$ and a kaon. No evidence for a signal is found and we set 90% CL upper limits: $BR(B^+\to\chi_{c0} K^+) < 4.8\times 10^{-4}$ and $BR(B^0\to\chi_{c0} K^0) < 5.0\times 10^{-4}$.