Observation of the 1P1 state of charmonium

The spin-singlet P-wave state of charmonium, hc(1P1), has been observed in the decay ψ(2S)→π0hc followed by hc→γηc. Inclusive and exclusive analyses of the M(hc) spectrum have been performed. Two complementary inclusive analyses select either a range of energies for the photon emitted in hc→γηc or a range of values of M(ηc). These analyses, consistent with one another within statistics, yield M(hc)=[3524.9±0.7 (stat)±0.4 (sys)] MeV/c2 and a product of the branching ratios Bψ(ψ(2S)→π0hc)xBh(hc→γηc)=[3.5±1.0 (stat)±0.7 (sys)]x10-4. When the ηc is reconstructed in seven exclusive decay modes, 17.5±4.5 hc events are seen with an average mass M(hc)=[3523.6±0.9 (stat)±0.5 (sys)] MeV/c2, and BψBh=[5.3±1.5 (stat)±1.0 (sys)]x10-4. If combined, the inclusive and exclusive data samples yield an overall mass M(hc)=[3524.4±0.6 (stat)±0.4 (sys)] MeV/c2 and product of branching ratios BψBh=[4.0±0.8 (stat)±0.7 (sys)]x10-4. The hc mass implies a P-wave hyperfine splitting ΔMHF(1P)≡-M(11P1)=[1.0±0.6 (stat)±0.4 (sys)] MeV/c2