When not bound up in an atomic nucleus, the neutron decays into a proton, electron and anti-neutrino in on average about 15 minutes. Detailed measurements of this process – the overall rate, correlations between neutron spin and final-state particle momenta, etc. – can be used to determine parameters of the underlying standard model of particle physics. The problem is overconstrained: there are many possible experimental observables dependent on just a few standard model parameters. The parameters thus determined can be compared with results of other experiments to test the fundamental theory, possibly revealing new physics as experimental precision improves. An experiment at Los Alamos National Laboratory, using ultracold neutrons produced at the LANSCE accelerator, recently published the world’s most precise single measurement of the neutron lifetime, with overall uncertainty half that of the previous best measurements. This precision begins to probe an apparent discrepancy in the fundamental theory that has arisen in a class of nuclear beta decay measurements, in particular a violation of unitarity in the first row of the CKM quark mixing matrix when using Vud from nuclear decay. In this talk, I will explain the motivation to measure neutron beta decay precisely, discuss the experimental landscape of neutron lifetime experiments in particular, and comment a bit on the “beam vs. bottle” neutron lifetime puzzle, in which two broadly different experimental techniques appear to give different physics results.
M. Pepe-Altarelli, P. Silva