Elastic positron–proton scattering at low Q$$^2$$

Systematic differences in the proton's charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest elastic lepton scattering measurements. The defined slope form factor at Q$^2$=0, does not depend on probe. Any difference apparent size proton, when from versus hydrogen, could point to new physics or need for higher order corrections. While recent measurements seem now be agreement, there is date no high precision data with both electrons positrons. A proton radius measurement performed Hall B Jefferson Lab positron beam calorimeter based setup PRad experiment. This also extended deuterons where similar discrepancy has been observed between electronic determination deuteron radius. new, positrons, viewed alongside electron forthcoming MUSE muon measurement, help provide insights into origins puzzle, experimental constraints radiative correction calculations.