Observation of ψ ( 3770 ) → p p ‾ and measurement of electromagnetic form factors of proton at s = 3.510 − 4.946 GeV

We search for possible charmonium(-like) states decaying into the pp̄ final state by studying the Born cross sections of the e⁺e⁻→ pp̄ 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 ψ(3770)→ pp̄ is observed with a significance of 6.6$σ$ including systematic uncertainties. Furthermore, a structure near 4.2 GeV is observed with significances of $4.6σ$ or $4.8σ$ for the $ψ(4160)$ or $Y(4230)$ hypotheses including systematic uncertainties, respectively; these interpretations cannot presently be distinguished. In addition, the moduli of the form factor ratios |GE/GM| and of the magnetic form factors |GM| 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.