Observation of e+e-›??c1,2e+e-›??c1,2 near s?s = 4.42 and 4.6 GeV
KünyeKolcu, O. B., ... (2016). Observation of e+e-›??c1,2e+e-›??c1,2 near s?s = 4.42 and 4.6 GeV. Physical Review Letters. 93, 011102.
BEPC-BES. Based on data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies s?>s> 4.4 GeV, the processes e+e-›??c1,2e+e-›??c1,2 are observed for the first time. With an integrated luminosity of 1074pb-11074pb-1 near s?=s= 4.42 GeV, a significant ??c2??c2 signal is found, and the cross section is measured to be (20.9±3.2±2.5)(20.9±3.2±2.5)pb-1-1. With 567567pb-1-1 near s?=s= 4.6 GeV, a clear ??c1??c1 signal is seen, and the cross section is measured to be (9.5±2.1±1.3)(9.5±2.1±1.3)pb-1-1, while evidence is found for an ??c2??c2 signal. The first errors are statistical and the second are systematic. Due to low luminosity or low cross section at other energies, no significant signals are observed. In the ??c2??c2 cross section, an enhancement is seen around s?=s= 4.42 GeV. Fitting the cross section with a coherent sum of the ?(4415)?(4415) Breit-Wigner function and a phase space term, the branching fraction B(?(4415)›??c2)B(?(4415)›??c2) is obtained to be of the order of 10-310-3.