Lepton Universality Test in the Photoproduction of e^{-}e^{+} Versus μ^{-}μ^{+} Pairs on a Proton Target.

recoiling proton in the p ! l l + p reaction, we show that a measurement of a ratio of e e + + + over e e + cross sections with an absolute precision of 7 10 4 , would allow for a test to distinguish, at the 3 level, between the two different proton charge radii currently extracted from muonic and electronic observables.