Researchers in Institute of Modern Physics, Chinese Academy of Sciences (IMP) investigated the two-photon exchange effects of lepton-proton elastic scattering process, the lepton helicity broken caused by the mass of the lepton was considered for the first time.

The muon was about 200 times heavier than electron, thus the lepton mass effect would be more significant in muon-proton scattering. The estimation provided some theoretical information for the coming muon-proton scattering measurements at Paul Scherrer Institute.

The proton charge radius could be determined by the spectroscopy of hydrogen, the electron-proton elastic scattering and muonic hydrogen. The radius measured from the latter method was smaller than those obtained from the former two methods, this phenomenon was called as “proton radius puzzle”. The muon-proton scattering measurements were considered to be the key of solving this puzzle.　 

The related results have been published inPhys. Rev. C 87, 045209 (2013).

Fig.1 Two-photon exchange corrections to lepton-proton elastic scattering. (Image by IMP)