"Forbidden beta decays" are nuclear beta decays in which a nucleus transforms into another nucleus, an electron, and a neutrino while violating certain selection rules imposed by parity and rotational symmetry. These violations suppress the decay rate compared to the more common allowed beta decays.

In particular, the unique first-forbidden beta decay occurs when the initial and final nuclear spins differ by two units and the parity is reversed. Because of angular momentum conservation, this decay mode is completely blocked at lowest order if the final-state nucleus does not recoil. Its distinctive decay spectrum provides an excellent probe for testing the Standard Model of particle physics.

In this talk, I will show that this blockage can be lifted by quantum corrections involving the emission of a virtual or real photon by the nucleus. The photon provides an additional kick to the nucleus, compensating for the angular momentum mismatch and enabling the decay even when the final-state nucleus remains at rest. This mechanism opens a new window for exploring physics beyond the Standard Model.