V_{ud} from neutron beta decay

Abstract

The experimental determination of V_{ud} from neutron beta decay requires accurate values of the neutron lifetime and the ratio of the weak axial-vector to vector coupling constants of the nucleon. The latter is derived from measurements of angular correlation coefficients in the differential decay probability, such as the beta asymmetry parameter A, the neutrino-electron angular correlation coefficient a, or the parameter C of the proton asymmetry. As a probe free from nuclear structure corrections the decay of the free neutron has the potential to provide the most accurate value of V_{ud}. Towards that end however, the experimental sensitivity still needs to be further improved to become competitive with superallowed nuclear beta decays. This contribution briefly reviews the current status of those neutron decay studies relevant for the determination of V_{ud}.