Department of Physics & Astronomy
University of New Mexico

Physics and Astronomy Colloquium

Precision measurements of neutron beta decay and the neutron lifetime puzzle

Presented by Steven Clayton, Los Alamos National Lab

Precision measurements of neutron beta decay and the neutron lifetime puzzle

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 over constrained: 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. There is a complication though, a discrepancy in different measurements of ostensibly the same observable. While neutron lifetime measurements based on storage of neutrons in a trap ("bottle"), including LANL's new result, are in reasonably good agreement with each other, previous experiments using the quite different technique of counting the protons produced in a known flux ("beam") of neutrons infer a significantly longer neutron lifetime. Either one or both classes of experiments has an undiscovered systematic error, or somehow less than 100% of neutron decays leave protons in the final state, inconsistent with the standard theory. 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.

3:30 pm, Friday, November 5, 2021
Via Zoom. Please take the Satisfaction Survey,


Disability NoticeIndividuals with disabilities who need an auxiliary aid or service to attend or participate in P&A events should contact the Physics Department (phone: 505-277-2616, email: well in advance to ensure your needs are accomodated. Event handouts can be provided in alternative accessible formats upon request. Please contact the Physics front office if you need written information in an alternative format.

A schedule of talks within the Department of Physics and Astronomy is available on the P&A web site at