Precision Measurement of the Mass of the h_c(1P1) State of Charmonium

TL;DR

This paper reports a highly precise measurement of the h_c(1P1) charmonium state's mass using a large sample of psi(2S) decays, employing both inclusive and exclusive analysis methods with consistent results.

Contribution

It provides the most precise measurement to date of the h_c(1P1) mass and its hyperfine splitting, using data from CLEO-c with multiple decay channels.

Findings

Measured M(h_c)=3525.28+-0.19(stat)+-0.12(syst) MeV

Determined the branching fraction B(psi(2S) -> pi0 h_c) x B(h_c -> gamma eta_c)

Found hyperfine splitting Delta M_hf(1P)=+0.02+-0.19+-0.13 MeV

Abstract

A precision measurement of the mass of the h_c(1P1) state of charmonium has been made using a sample of 24.5 million psi(2S) events produced in e+e- annihilation at CESR. The reaction used was psi(2S) -> pi0 h_c, pi0 -> gamma gamma, h_c -> gamma eta_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 eta_c decays are reconstructed in fifteen 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(psi(2S) -> pi0 h_c)xB(h_c -> gamma eta_c)= (4.19+-0.32+-0.45)x10^-4. Using the 3PJ centroid mass, Delta M_hf(1P)=<M(chi_cJ)> - M(h_c) = +0.02+-0.19+-0.13 MeV.