First Measurement of the Relative Phase between Proton Psionic Form Factors

Abstract

The relative phase between the time-like form factors of the proton is a crucial observable for a complete understanding of its internal structure, yet it has remained unmeasured due to the formidable experimental challenge of determining the final-state polarization or having available polarized beams. With a novel technique that measures polarization via secondary scattering on spectrometer material, we use $10.09\times 10^9$ $J/\ddot{I}ˆ$ events collected at BESIII to analyze the reaction $e^{+}{e}^{-}\to J/\ddot{I}ˆ\to p\bar{p}$. This allows the first determination of the sine of the relative phase between the proton psionic form factors, $\sin \hat{I}”\hat{I}¦=-0.20\pm 0.34_{\text{stat}}\pm 0.11_{\text{syst}}$. This result provides the first direct insight into the complex dynamics of proton formation, and offers valuable new information to constrain theoretical models of nucleon structure.

4 pages, 6 figures