Observation of thehc(P11)State of Charmonium

The $h_c{(}^1{P}_1)$ state of charmonium has been observed in the reaction $\psi (2S)\to {\pi }^0{h}_c\to (\gamma \gamma )(\gamma {\eta }_c)$ using $3.08\times {10}^6$ $\psi (2S)$ decays recorded in the CLEO detector. Data have been analyzed both for the inclusive reaction, where the decay products of the ${\eta }_c$ are not identified, and for exclusive reactions, in which ${\eta }_c$ decays are reconstructed in seven hadronic decay channels. We find $M(h_c)=3524.4\pm 0.6\pm 0.4\mathrm{MeV}$ which corresponds to a hyperfine splitting $\Delta M_{\mathrm{hf}}(1P)\equiv ⟨M{(}^3{P}_J)⟩-M{(}^1{P}_1)=+1.0\pm 0.6\pm 0.4\mathrm{MeV}$, and $\mathcal{B}\mathbf{(}\psi (2S)\to {\pi }^0{h}_c\mathbf{)}\times \mathcal{B}(h_c\to \gamma {\eta }_c)=(4.0\pm 0.8\pm 0.7)\times {10}^{-4}$.