First Observation of the Direct Production of the chi(c1) in e(+)e(-) Annihilation

Abstract

We study the direct production of the J(PC) = 1(++) charmonium state chi(c1)(1P) in electron-positron annihilation by carrying out an energy scan around the mass of the chi(c1)(1P). The data were collected with the BESIII detector at the BEPCII collider. An interference pattern between the signal process e(+)e(-) -> chi(c1)(1P) -> gamma J/psi -> gamma mu(+)mu(-) and the background processes e(+)e(-) -> gamma(ISR) J/psi -> gamma(ISR)mu(+)mu(-) and e(+)e(-) -> gamma(ISR)mu(+)mu(-) is observed by combining all the data samples. The chi(c1)(1P) signal is observed with a significance of 5.1 sigma. This is the first observation of a C-even state directly produced in e(+)e(-) annihilation. The electronic width of the chi(c1)(1P) resonance is determined to be Gamma(ee) = (0.12(-0.08)(+0.13)) eV, which is of the same order of magnitude as theoretical calculations.