Precision Neutron Decay Measurements

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The neutron is one of the building blocks of ordinary matter. However, when free from the confines of a stable atomic nucleus, the neutron is unstable and 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. We pass beams of cold neutrons (moving less than 2000 m/s) through a variety of apparatus designed to make precise measurements of the neutron decay lifetime and angular correlations. We use these results to test the Standard Model of particle interactions and look for evidence of new physics. The neutron lifetime is used to calculated the abundances of light elements produced in the early universe from the big bang model.A number of graduate and undergraduate students, as well as postdoctoral researchers, participate in this research program. In addition to learning the science of neutron decay, they are educated broadly in the physics and methodology of research at neutron sources, which prepares them for a wide range of careers related to neutron science in academia and industry.