arxiv: 2606.22759 · v1 · ★pith:56ETWVK7new · submitted 2026-06-22 · ✦ hep-ex
Observation of psi(3770)to pbar p and Measurement of Electromagnetic Form Factors of Proton at sqrt{s} = 3.510-4.946 GeV
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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 $\psi(3770)\to p\bar{p}$ is observed with a significance of 6.6$\sigma$ including systematic uncertainties. Furthermore, a structure near 4.2 GeV is observed with significances of $4.6\sigma$ or $4.8\sigma$ for the $\psi(4160)$ 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.
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