Observation of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi>P</mml:mi><mml:mn>1</mml:mn><mml:none /><mml:mprescripts /><mml:none /><mml:mn>1</mml:mn></mml:mmultiscripts></mml:math>state of charmonium

The spin-singlet $P$-wave state of charmonium, ${h}_{c}{(}^{1}{P}_{1})$, has been observed in the decay $\ensuremath{\psi}(2S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{h}_{c}$ followed by ${h}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}$. Inclusive and exclusive analyses of the $M({h}_{c})$ spectrum have been performed. Two complementary inclusive analyses select either a range of energies for the photon emitted in ${h}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}$ or a range of values of $M({\ensuremath{\eta}}_{c})$. These analyses, consistent with one another within statistics, yield $M({h}_{c})=[3524.9\ifmmode\pm\else\textpm\fi{}0.7\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }(\mathrm{sys})]\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a product of the branching ratios ${\mathcal{B}}_{\ensuremath{\psi}}(\ensuremath{\psi}(2S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{h}_{c})\ifmmode\times\else\texttimes\fi{}{\mathcal{B}}_{h}({h}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c})=[3.5\ifmmode\pm\else\textpm\fi{}1.0\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.7\text{ }\text{ }(\mathrm{sys})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. When the ${\ensuremath{\eta}}_{c}$ is reconstructed in seven exclusive decay modes, $17.5\ifmmode\pm\else\textpm\fi{}4.5$ ${h}_{c}$ events are seen with an average mass $M({h}_{c})=[3523.6\ifmmode\pm\else\textpm\fi{}0.9\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{ }(\mathrm{sys})]\text{ }\text{ }\mathrm{MeV}/{c}^{2}$, and ${\mathcal{B}}_{\ensuremath{\psi}}{\mathcal{B}}_{h}=[5.3\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.0\text{ }\text{ }(\mathrm{sys})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. If combined, the inclusive and exclusive data samples yield an overall mass $M({h}_{c})=[3524.4\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }(\mathrm{sys})]\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and product of branching ratios ${\mathcal{B}}_{\ensuremath{\psi}}{\mathcal{B}}_{h}=[4.0\ifmmode\pm\else\textpm\fi{}0.8\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.7\text{ }\text{ }(\mathrm{sys})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. The ${h}_{c}$ mass implies a $P$-wave hyperfine splitting $\ensuremath{\Delta}{M}_{\mathrm{HF}}(1P)\ensuremath{\equiv}⟨M({1}^{3}P)⟩\ensuremath{-}M({1}^{1}{P}_{1})=[1.0\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }(\mathrm{sys})]\text{ }\text{ }\mathrm{MeV}/{c}^{2}$.