Lower bound on the proton charge radius from electron scattering data

The proton charge-radius determinations from the electromagnetic form-factor measurements in electron-proton (ep) scattering require an extrapolation to zero momentum transfer Q² = 0 which is prone to model-dependent assumptions. We show that the data at finite momentum transfer can be used to establish a rigorous lower bound on the proton charge radius, while bypassing the model-dependent assumptions that go into the fitting and extrapolation of the ep data. The near-future precise ep experiments at very low Q², such as PRad, are expected to set a stringent lower bound on the proton radius.