Magnetic Trapping of Ultracold Neutrons and The Neutron Lifetime Puzzle

Chen-Yu Liu, professor of nuclear physics, Indiana University

Description

Neutrons, when liberated from a nucleus, are unstable. A free neutron spontaneously decays into a proton (of a lighter mass), electron, and an anti- neutrino, with a lifetime of approximately 10
minutes. This process is the basis of all nuclear beta-decays, which, together with neutron capture and alpha decay in stellar environments, produced the diverse nuclear isotopes in the universe. The decay rate of the neutron is key input for predicting the abundances of light nuclei in the early universe, it is used to estimate the rate of nuclear fusion and energy production in the sun, and it can be used to search for physics beyond the Standard
Model. There exists, however, a 10-second discrepancy in the neutron lifetime as determined by two modern techniques. By counting the decays of free neutrons in a beam, the lifetime is 887.7 $\pm$ 2.2 s; by counting surviving ultracold neutrons stored in a material bottle, it is 878.5 $\pm$ 0.8 s. I will describe the challenges of performing these measurements and the innovations used to meet these challenges.