The Neutron Lifetime

Fred Wietfeldt
Department of Physics and Engineering Physics
Tulane University

The neutron is a basic component of ordinary matter, but when freed from the confines of an atomic nucleus the neutron is unstable; it decays to a proton, electron, and antineutrino with a lifetime of about 15 minutes. Neutron decay is the prototype for nuclear beta decay and other semileptonic weak interactions. The neutron lifetime provides direct access to key parameters in the Standard Model of particle physics. It was important in the early universe and is used in theoretical calculations of primordial element abundances. It has been the subject of more than 20 major experiments starting in 1951. Two main methods, the neutron beam method and the ultracold neutron storage method, have reached the 10-3 precision level in recent years but unfortunately now disagree by more than 8 seconds (4 standard deviations). I will discuss the motivation and physics of the neutron lifetime, briefly review past and current experiments along with ideas for resolving the discrepancy, and present details of a project BL3 to measure the neutron lifetime to < 0.3 s precision using the beam method.