However, β decay cannot occur in an isolated proton because it requires energy, due to the mass of the neutron being greater than the mass of the proton. β decay can only happen inside nuclei when the daughter nucleus has a greater binding energy (and therefore a lower total energy) than the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron, and a neutrino and into the kinetic energy of these particles. This process is opposite to negative beta decay, in that the weak interaction converts a proton into a neutron by converting an up quark into a down quark resulting in the emission of a W or the absorption of a W. When a W boson is emitted, it decays into a positron and an electron neutrino :