Two-Photon exchange in elastic lepton-proton scattering

The proton elastic form factor ratio can be directly accessed via measurements exploiting polarization degrees of freedom. Compared to the ratio formed with the Rosenbluth-separated form factors from unpolarized measurements Rosenbluth, they show a different trend growing with Q². The proposed explanation is two-photon exchange, which has recently been measured in three precision experiments. From these new data, a hard two-photon exchange effect at the couple-of-percent level can be extracted, in significant disagreement from theoretical calculation. Theory at larger momentum transfer remains untested, as well as the accuracy of the descriptions at lowest Q², relevant for the extraction of the proton's rms. charge and magnetic radii.