Electromagnetic contribution to the mass of proton

Question

It is a well-established fact that the main contribution to the mass of proton is the gluonic field inside a proton, as some answers explain (see here and here). But being the proton and the neutron remarkably similar in structure, I wonder how much the electromagnetic field contributes to the mass difference. And if it is not due to the electromagnetic field, what would be the cause for the mass difference between the proton and the neutron.

Answer 1

The electromagnetic contribution to the proton-neutron mass difference is a tiny effect of $0.58 \pm 0.16$ MeV. The main contribution to $m_p-m_n$ is a QCD effect (due to the mass difference of up and down quark): $-1.87 \pm 0.16$ MeV. See J. Gasser, H. Leutwyler, A. Rusetsky, arXiv 2003.13612 for details.