As known, the proton is from two up and a single down quark, while the neutron is from a single up and two down quarks. The down quark is a little bit more massive and the up, and so the neutron is also a little bit more massive as the proton. It was my naive layman's hypothese.

But the masses of the quarks (4 and 8 MeV) take only a tiny part in the whole mass of the baryons ($\approx 1\textrm{ GeV}$). Their masses are coming mainly from the gluon field around them.

And, considering the relative recent charming result of the calculation of the protons and neutrons from the principles, I think maybe this question could have a more detailed answer.

I think the mass difference is coming from the difference of the gluon field, is it okay? What is that difference?

Extension: comment says, that calculation couldn't yet differentiate between protons and neutrons. I hope, maybe the last 8 years made possible this improvement.

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    $\begingroup$ Related: physics.stackexchange.com/q/85/2451 and links therein. $\endgroup$ – Qmechanic Jun 24 '16 at 6:15
  • $\begingroup$ Only related, but not duplicate. This question asks for the difference of the gluon fields, the other doesn't (and the answers miss this info). $\endgroup$ – peterh Jun 24 '16 at 6:19

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