Precision measurement of the mass of the hc(P11) state of charmonium

A precision measurement of the mass of the h c ( 1 P 1 ) state of charmonium has been made using a sample of 24.5 X 10 6 ψ(2S) events produced in e + e - annihilation at the Cornell Electron Storage Ring (CESR). The reaction used was ψ(2S) → π 0 h c , π 0 → γγ, h c → γη c , and the reaction products were detected in the CLEO-c detector. Data have been analyzed both for the inclusive reaction and for the exclusive reactions in which η c decays are reconstructed in 15 hadronic decay channels. Consistent results are obtained in the two analyses. The averaged results of the present measurements are M(h c ) = 3525.28 ± 0.19(stat.) ± 0.12(syst) MeV, and B(ψ(2S) →π 0 h c ) X B(h c → γη c ) = (4.19 ± 0.32 ± 0.45) X 10 -4 . Using the 3 P J centroid mass, ΔM hf (1P) ≡ - M(h c ) = +0.02 ± 0.19 ± 0.13 MeV.