Observation of the1P1state of charmonium

We have performed a search for the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}_1$ state of charmonium resonantly formed in p¯p annihilations, close to the center of gravity of the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_{\mathit{J}}$ states. We report results from the study of the J/ψ+${\mathrm{\pi }}^0$ and J/ψ+2π final states. We have observed a statistically significant enhancement in the p¯+p→J/ψ+${\mathrm{\pi }}^0$ cross section at √s ≃3526.2 MeV. This enhancement has the characteristics of a narrow resonance of mass, total width, and production cross section consistent with what is expected for the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}_1$ state. In our search we have found no candidates for the reactions p¯+p→J/ψ+${\mathrm{\pi }}^0$+${\mathrm{\pi }}^0$ and p¯+p→J/ψ+${\mathrm{\pi }}^{+}$+${\mathrm{\pi }}^{\mathrm{-}}$.