It's a bit different than that. The decay of a neutron into a proton and an electron conserved charge, mass-energy (to an expected degree), and momentum. Howe

It's a bit different than that. The decay of a neutron into a proton and an electron conserved charge, mass-energy (to an expected degree), and momentum. However, spin was not conserved. The neutrino was dreamed up as kind of a placeholder for the spin. However, it turned out that it was a real thing! The mass-energy arithmetic should not be the thing you look at for a couple of reasons. First, it's quite difficult to measure with exactitude. Second, the binding energy for particles and their constituents plays a part that is easily within error bounds.