First Observation of the Direct Production of the χc1 in e+e- Annihilation

Abstract

We study the direct production of the JPC=1++ charmonium state χc1(1P) in electron-positron annihilation by carrying out an energy scan around the mass of the χc1(1P). The data were collected with the BESIII detector at the BEPCII collider. An interference pattern between the signal process e+e-→χc1(1P)→γJ/ψ→γμ+μ- and the background processes e+e-→γISRJ/ψ→γISRμ+μ- and e+e-→γISRμ+μ- is observed by combining all the data samples. The χc1(1P) signal is observed with a significance of 5.1σ. This is the first observation of a C-even state directly produced in e+e- annihilation. The electronic width of the χc1(1P) resonance is determined to be Γee=(0.12-0.08+0.13) eV, which is of the same order of magnitude as theoretical calculations.