In  decay, or positron emission, the weak interaction converts an atomic nucleus into a nucleus with atomic number decreased by one, while emitting a positron () and an electron neutrino ().  decay generally occurs in proton-rich nuclei. The generic equation is:
This may be considered as the decay of a proton inside the nucleus to a neutron:
:p → n +  + 
However,  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.  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  or the absorption of a . When a  boson is emitted, it decays into a positron and an electron neutrino: