The neutron is one of the building blocks of ordinary matter. However, when free from the confines of a stable atomic nucleus, a neutron decays into a proton, electron, and antineutrino with an average lifetime of about 15 minutes. This decay is governed by the weak nuclear force, one of the four basic forces of nature. By carefully measuring the energies and directions of the emitted particles and the spin axis of the original neutron, subtle details of the fundamental nature of the weak force can be revealed. With enough precision, hints of a deeper theory, beyond our Standard Model of particle physics, may be seen.
A key parameter, the "a" coefficient, describes the average emission angle (correlation) between the electron and antineutrino from neutron decay. It has an experimental uncertainty of 5%, much larger than other neutron decay parameters. The antineutrino is practically undetectable, so its direction and energy must be determined from the electron and proton by applying the laws of conservation of momentum and energy. In previous experiments the "a" coefficient was inferred from its small effect on the proton energy distribution. In the present work a novel method is used to extract the "a" coefficient from the distribution of times between detection of the electron and proton in a special apparatus specifically designed for this purpose. This approach promises an experimental uncertainty in the "a" coefficient of less than 1%, a factor of five improvement. It will be an important step towards a deeper understanding of the weak nuclear force.
