Observation of the ^1P_1 State of Charmonium

TL;DR

This paper reports the first observation of the spin-singlet P-wave charmonium state hc(1P1), measuring its mass and branching ratios through decay analysis, providing insights into quarkonium spectroscopy.

Contribution

The paper presents the first experimental observation and precise mass measurement of the hc(1P1) state in charmonium, using inclusive and exclusive decay analyses.

Findings

Measured hc mass: 3524.4 MeV/c^2

Determined hyperfine splitting: 1.0 MeV/c^2

Observed hc production in psi(2S) decays

Abstract

The spin-singlet P-wave state of charmonium, hc(1P1), has been observed in the decay psi(2S) -> pi0 hc followed by hc -> gamma etac. 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 -> gamma etac or a range of values of M(etac). These analyses, consistent with one another within statistics, yield M(h_c) =[3524.9 +/- 0.7 (stat) +/- 0.4 (sys)]MeV/c^2 and a product of the branching ratios B_psi(psi(2S) -> pi0 hc) x B_h(hc -> gamma etac) = [3.5 +/- 1.0 (stat) +/- 0.7 (sys)] x 10^{-4}. When the etac 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/c^2, and B_psi x B_h = [5.3 +/- 1.5 (stat) +/- 1.0 (sys)] x 10^{-4}. Because the inclusive and exclusive data samples are largely independent they are combined to yield an overall mass M(hc) = [3524.4 +/- 0.6 (stat) +/- 0.4 (sys)]MeV/c^2 and product of branching ratios B_psi x B_h = [4.0 +/- 0.8 (stat) +/- 0.7 (sys)] x 10^{-4}. The hc mass implies a P-wave hyperfine splitting Delta M_{HF}(1P) \equiv M(1^3P)-M(1^1P_1) = [1.0 +/- 0.6 (stat) +/- 0.4 (sys)] MeV/c^2.