Results of a search for thehc(P11)state of charmonium in theηcγandJ/ψπ0decay modes

We report evidence for the $h_c$ state of charmonium in its ${\eta }_c\gamma$ decay mode and lack of evidence in the $J/\psi {\pi }^0$ mode. We studied these channels in $\overline{p}p$ annihilations near the center of gravity of the ${}^{3}P_J$ states, where the $h_c$ was reported in the $J/\psi {\pi }^0$ mode by E760, our previous experiment, at $3526.2\pm 0.15\pm 0.2\mathrm{MeV}$, with ${\Gamma }_R\le 1\mathrm{MeV}$. We observe an event excess in the ${\eta }_c\gamma$ mode near 3526 MeV. Testing the null hypothesis of a linearly varying background cross section against the alternate hypothesis that includes a resonance near 3526 MeV, we reject the null hypothesis with $\mathcal{P}\sim 0.001$. The resonance mass is $3525.8\pm 0.2\pm 0.2\mathrm{MeV}$ and the resonance width $\le 1\mathrm{MeV}$. We estimate $10.0\pm 3.5<{\Gamma }_{\overline{p}p}{\mathcal{B}}_{{\eta }_c\gamma }<12.0\pm 4.5\mathrm{eV}$, corresponding to fixed values $0.5<{\Gamma }_R<1.0\mathrm{MeV}$. We find no event excess within the search region in the $J/\psi {\pi }^0$ mode.