Proton mass

Rutherford named the proton in 1920 after showing that alpha particles knocked hydrogen nuclei out of nitrogen. Its mass is measured today by comparing cyclotron frequencies in a Penning trap, good to 3 parts in 10¹⁰. Almost none of that mass is the quarks: the up and down quarks inside contribute perhaps 1 %, and the rest is the energy of the gluon field binding them — E = mc² running backwards, mass made out of confinement energy.

The proton is very slightly lighter than the neutron, by 1.293 MeV/c², and that small difference is why the proton appears absolutely stable while a free neutron beta-decays in about 15 minutes. Experiment puts the proton lifetime beyond 10³⁴ years, longer than most grand-unified theories predicted. A useful check on any nuclear arithmetic: 1 u ≈ 931.494 MeV, so 1.007 276 u × 931.494 = 938.272 MeV.