Evidence for $B^+ \rightarrow h_c K^+$ and observation of $\eta_c(2S) \to p \bar{p} \pi^+ \pi^-$

A search for the decays $B^+ \rightarrow h_c K^+$ and $B^0 \rightarrow h_c K_S^0$ is performed. Evidence for the decay $B^+ \rightarrow h_c K^+$ is found; its significance is $4.8\sigma$. No evidence is found for $B^0 \rightarrow h_c K_S^0$. The branching fraction for $B^+ \rightarrow h_c K^+$ is measured to be $(3.7^{+1.0}_{-0.9}{}^{+0.8}_{-0.8}) \times 10^{-5}$; the upper limit for the $B^0 \rightarrow h_c K_S^0$ branching fraction is $1.4 \times 10^{-5}$ at $90\%$ C.L. In addition, a study of the $p \bar{p} \pi^+ \pi^-$ invariant mass distribution in the channel $B^+ \to (p \bar{p} \pi^+ \pi^-) K^+$ results in the first observation of the decay $\eta_c(2S) \to p \bar{p} \pi^+ \pi^-$ with $12.1\sigma$ significance. The analysis is based on the 711 $\mathrm{fb}^{-1}$ data sample collected by the Belle detector at the asymmetric-energy $e^+ e^-$ collider KEKB at the $\Upsilon(4S)$ resonance.