Observation of h_c(^1P_1) State of Charmonium

Abstract

The h_c(1P1) state of charmonium has been observed in the reaction ψ(2S) -> \pi0 h_c -> (γγ)(γη_c) using 3.08 million ψ(2S) decays recorded in the CLEO detector. Data have been analyzed both for the inclusive reaction, where the decay products of the η_c are not identified, and for exclusive reactions, in which η_c decays are reconstructed in seven hadronic decay channels. We find M(h_c)=3524.4 +/- 0.6 +/- 0.4 MeV which corresponds to a hyperfine splitting ΔM_{hf}(1P) \equiv <M(3PJ)>-M(1P1)=+1.0+/-0.6+/-0.4 MeV, and B(ψ(2S)->\pi0 h_c)xB(h_c->γη_c)=(4. +/-0.8+/-0.7)x10^{-4}.

9 pages postscript,also available through http://www.lns.cornell.edu/public/CLNS/2005/, submitted to PRL