Study of e+e- ???J /? and Observation of X (3872) ??J /?

Abstract

We study the e+e-???J/? process using 11.6 fb-1 e+e- annihilation data taken at center-of-mass energies from s=4.008 GeV to 4.600 GeV with the BESIII detector at the BEPCII storage ring. The X(3872) resonance is observed for the first time in the ?J/? system with a significance of more than 5?. The relative decay ratio of X(3872)??J/? and ?+?-J/? is measured to be R=1.6-0.3+0.4±0.2, where the first uncertainty is statistical and the second systematic (the same hereafter). The s-dependent cross section of e+e-??X(3872) is also measured and investigated, and it can be described by a single Breit-Wigner resonance, referred to as the Y(4200), with a mass of 4200.6-13.3+7.9±3.0 MeV/c2 and a width of 115-26+38±12 MeV. In addition, to describe the ?J/? mass distribution above 3.9 GeV/c2, we need at least one additional Breit-Wigner resonance, labeled as X(3915), in the fit. The mass and width of the X(3915) are determined. The resonant parameters of the X(3915) agree with those of the Y(3940) in B?K?J/? and of the X(3915) in ????J/? observed by the Belle and BABAR experiments within errors.