Neutron-proton mass ratio

A 0.14 % excess sounds trivial and is anything but. It exceeds the electron's rest mass, so a free neutron can decay to a proton, an electron and an antineutrino with 0.782 MeV to spare, and does so with a mean life near 879 s. Had the sign been reversed, protons would decay into neutrons, hydrogen could not exist, and there would be no water and no organic chemistry.

The difference comes from a tug-of-war: the down quark is heavier than the up quark, which pushes the neutron up in mass, while electromagnetic self-energy pushes the charged proton up — and the quark-mass term wins by about 1.3 MeV. Lattice QCD reproduced this split from first principles only in 2015. In the first seconds of the universe the same number set the neutron-to-proton freeze-out ratio near 1:7, which fixed the primordial helium fraction at about 25 % — a prediction confirmed by observation and one of the pillars of Big Bang cosmology.