So a free neutron will decay into a proton and an electron. This already confuses me because the only difference between the nucleons is an up quark (uud vs ddd) and an electron an a lepton is a fundamental particle.

But an electron in an atom won’t merge with a proton to form a neutron. There seems to be some hand waving here around the strong nuclear force and zero energy state that I don’t really understand.

But in a neutron star the forces are so great that I believe this can happen (or how else does neutronium form?).

And now we see an antiproton can annihilate a proton. Doesn’t this have the same zero energy problem?

Can someone ELI5?