Contribution of π0 exchange in elastic muon-proton scattering

The effect of the lepton's mass is significantly enhanced when the beam's energy is on the order of the lepton's mass. In the case of electrons, this corresponds to beam momenta on the order of a few $\text{MeV}/\text{c}$ and is negligible in higher energy experiments. In this paper, we calculate the differential cross section $d\sigma /d\mathrm{\Omega }$ for the helicity-flip meson exchange interference in elastic muon-proton $\left(\mu p\right)$ scattering ${\mu }^{-}p\to {\mu }^{-}p$. In particular, we examine the ${\pi }^0$ meson exchange in the $t$ channel for a longitudinally polarized beam and a transversely polarized target. We demonstrate the contribution to be larger for muons due to the lepton mass difference. Then we construct the corresponding beam-target double-spin asymmetries for the target polarized normal and parallel to the momentum transfer in the Breit frame, and then consider the model dependence of the calculation from the estimation of the ${\pi }^0\mu \mu$ vertex. The contribution was found to be on the order of $\approx 0.15\%$ for muons in the kinematic region of the Muon-Proton Scattering Experiment at the Paul-Scherrer Institute in Switzerland.