Proton structure comes to light

The proton is not a point particle, but rather a composite of quarks and gluons. Particle physicists characterize the proton’s structure with so-called form factors. The two most significant of those relate to the charge distribution and to the distribution of internal magnetic moments. Those two form factors can be extracted from measurements of elastic scattering of electrons off protons, but experiments have yielded different results depending on whether the electron beam was polarized or unpolarized. Theorists proposed a reason for the observational mismatch: Interpretations of the experiments ignored processes in which the electron and proton exchange two virtual photons. Now, as the culmination of a seven-year-long effort, the OLYMPUS collaboration has precisely measured the effects of two-photon exchange in an experiment that probes proton structure by comparing how electrons and positrons elastically scatter off protons. Two years ago, the CLAS and VEPP-3 collaborations reported on comparable but less definitive experiments.

Because electrons and positrons have charges of opposite sign, the interference between one-photon- and two-photon-exchange contributions to their scattering off the positively charged proton also differs by a sign. The theoretical models that reconcile the earlier experimental discords predict that (after appropriate corrections are made) the ratio of positron to electron detections increases as the scattering angle and momentum transferred to the proton increase. Those general trends, observed by all three collaborations, provide evidence that two-photon exchange is germane to understanding the previous measurements of proton structure. The OLYMPUS experiment, however, garnered enough precise data to generate tension with some model predictions. To seal the case that two-photon exchange fully explains the earlier experimental discrepancies, future experiments will need to test theory further by probing regimes of higher momentum transfer. (B. S. Henderson et al., OLYMPUS collaboration, Phys. Rev. Lett. 118, 092501, 2017 .)