  The proton radius conundrum [R. Pohl et al., Nature vol.466, p.213 (2010) and
A. Antognini et al., Science vol.339, p.417 (2013)] highlights the need to
revisit any conceivable sources of electron-muon nonuniversality in
lepton-proton interactions within the Standard Model. Superficially, a number
of perturbative processes could appear to lead to such a nonunversality. One of
these is a coupling of the scattered electron into an electronic as opposed to
a muonic vacuum polarization loop in the photon exchange of two valence quarks,
which is present only for electron projectiles as opposed to muon projectiles.
However, we can show that this effect actually is part of the radiative
correction to the proton's polarizability contribution to the Lamb shift,
equivalent to a radiative correction to double scattering. We conclude that any
conceivable genuine nonuniversality must be connected with a nonperturbative
feature of the proton's structure, e.g., with the possible presence of light
sea fermions as constituent components of the proton. If we assume an average
of roughly 0.7*10^(-7) light sea positrons per valence quark, then we can show
that virtual electron-positron annihiliation processes lead to an extra term in
the electron-proton versus muon-proton interaction, which has the right sign
and magnitude to explain the proton radius discrepancy.
