Contribution of \sigma-meson exchange to elastic lepton-proton   scattering
  Lepton mass effects play a decisive role in description of elastic lepton-proton scattering when the beam's energy is comparable to the mass of the lepton. The future MUSE experiment, which is devised to solve the "Proton Radius Puzzle", is going to cover the corresponding kinematic region for a scattering of muons by a proton target. We anticipate that helicity-flip meson exchanges will make a difference in comparison of elastic electron-proton versus muon-proton scattering in MUSE. In this article, we estimate the $\sigma$ meson exchange contribution in the $t$-channel. This contribution, mediated by two-photon coupling of $\sigma$, is calculated to be at most $\sim 0.1 \%$ for muons in the kinematics of MUSE and it is about 3 orders in magnitude larger than for electrons because of the lepton-mass difference. 
O. Koshchii, A. Afanasev
hep-ph