In  decay, the weak interaction converts an atomic nucleus into a nucleus with atomic number increased by one, while emitting an electron () and an electron antineutrino ().  decay generally occurs in neutron-rich nuclei. The generic equation is:
where  and  are the mass number and atomic number of the decaying nucleus, and X and X′ are the initial and final elements, respectively.
Another example is when the free neutron () decays by  decay into a proton ():
At the fundamental level (as depicted in the Feynman diagram on the right), this is caused by the conversion of the negatively charged () down quark to the positively charged () up quark by emission of a  boson; the  boson subsequently decays into an electron and an electron antineutrino: