On the Neutron Lifetime and the Weak Coupling Constants

The beta-decay of the neutron, n + p + e+ Ce, either in its free or its bound state, is the most elementary of the semi-leptonic weak hadron decays, and, along with muon decay, is the prime source of experimental information underpinning the V-A description of weak interactions. We review its role in the context of the standard model of particle physics, and discuss the significance of both the neutron lifetime, and the various angular-polarization correlation coeficients which characterize the decay, in providing values for the polar and axial vector coupling constants, and relating these to the symmetry properties of the corresponding weak currents. We then describe a neutron decay detector, based on the use of a quasi-Penning trap to store and record recoil protons, which has been used to measure the neutron lifetime to better than 1% accuracy. A modified version of the same system is now being prepared to measure the proton spectrum, with a view to making an independent determination of the absolute coupling constant ratio, whose precise value is currently the subject of some debate. 1. Neutron betadecay in the standard model