Observation of $\ddot{I}ˆ(3770)\to p\bar{p}$ and Measurement of Electromagnetic Form Factors of Proton at $\sqrt{s}=3.510-4.946$ GeV

Abstract

We search for possible charmonium(-like) states decaying into the $p\bar{p}$ final state by studying the Born cross sections of the $e^{+}{e}^{-}\to p\bar{p}$ reaction, and we determine the proton electromagnetic form factors by analyzing the proton angular distribution. The measurement is performed using a sample of $e^{+}{e}^{-}$ collision data collected at 47 center-of-mass energies from 3.510 to 4.946 GeV, corresponding to an integrated luminosity of 26 fb${}^{-1}$, recorded by the BESIII detector collected at the BEPCII collider. The decay $\ddot{I}ˆ(3770)\to p\bar{p}$ is observed with a significance of 6.6$\ddot{I}ƒ$ including systematic uncertainties. Furthermore, a structure near 4.2 GeV is observed with significances of $4.6\ddot{I}ƒ$ or $4.8\ddot{I}ƒ$ for the $\ddot{I}ˆ(4160)$ or $Y(4230)$ hypotheses including systematic uncertainties, respectively; these interpretations cannot presently be distinguished. In addition, the moduli of the form factor ratios $|G_E/G_M|$ and of the magnetic form factors $|G_M|$ are extracted by analyzing the proton polar angle distribution with higher precision at large time-like squared momentum transfer. These results provide important experimental insights into both the decay mechanisms of charmonium(-like) states in the open charm region and the internal structure of proton.

11 pages, 5 figures, etc