2026/07/22 by BESIII Collaboration, M. Ablikim, M. N. Achasov +754
#hep-ex
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 10.09×109 J/ψ events collected at BESIII to analyze the reaction e+e-→ J/ψ→ pp. This allows the first determination of the sine of the relative phase between the proton psionic form factors, sinΔΦ=-0.20±0.34_\textrmstat±0.11_\textrmsyst. 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.