Observation of the 1P1 state of charmonium

The spin-singlet P-wave state of charmonium, h_c(¹P₁), has been observed in the decay ψ(2S)→π⁰h_c followed by h_c→γη_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→γη_c or a range of values of M(η_c). These analyses, consistent with one another within statistics, yield M(h_c)=[3524.9±0.7(stat)±0.4(sys)]MeV/c² and a product of the branching ratios B_ψ(ψ(2S)→π⁰h_c)×B_h(h_c→γη_c)=[3.5±1.0(stat)±0.7(sys)]×10^-4. When the η_c is reconstructed in seven exclusive decay modes, 17.5±4.5 h_c events are seen with an average mass M(h_c)=[3523.6±0.9(stat)±0.5(sys)]MeV/c², and B_ψB_h=[5.3±1.5(stat)±1.0(sys)]×10^-4. If combined, the inclusive and exclusive data samples yield an overall mass M(h_c)=[3524.4±0.6(stat)±0.4(sys)]MeV/c² and product of branching ratios B_ψB_h=[4.0±0.8(stat)±0.7(sys)]×10^-4. The h_c mass implies a P-wave hyperfine splitting ΔM_HF(1P)≡⟨M(1³P)⟩-M(1¹P₁)=[1.0±0.6(stat)±0.4(sys)]MeV/c².