Beta decay:

In beta-decay, the nucleus is transformed to a different state where the number of nucleons remains the same but the number of protons either increases or decreases by one with the emission of an electron or a positron with its anti-neutrino or neutrino, respectively. The simplest case is where the free neutron decays into a proton and emits an electron and an electron anti-neutrino. The weak force (one of the four fundamental forces of nature) is responsible for nuclear beta-decay processes. Unlike the neutron where vector and axial-vector components of the weak force are involved, beta-decay in nuclei enable each of these compounds to be studied in isolation. Pure Fermi beta-decay transitions involve the vector component and pure Gamov-Teller transitions involve the axial-vector component. Only the axial-vector component is parity violating.