Precision Neutron Decay Measurements

Tulane University, New Orleans LA

Investigators

Abstract

The neutron is a key building block of ordinary matter, but when freed from the confines of an atom the neutron is unstable and decays into other elementary particles: a proton, an electron, and an antineutrino (a lightweight neutral particle), with an average lifetime of about 15 minutes. Neutron decay is a useful laboratory for studying details of the force responsible - the weak nuclear force - one of the four fundamental forces of nature. This award will support an experiment in which beams of free neutrons pass through specialized detectors that precisely measure the neutron decay lifetime and the angles between the emitted particles. From these results the scientists can test and refine the theory of the weak nuclear force and better understand the physics of the sun, stars, the Big Bang, and important nuclear reactions. This work, at the "precision frontier" of particle and nuclear physics, complements research at the "high energy frontier", for example at the Large Hadron Collider in Europe. This project is also an excellent opportunity to train undergraduate and graduate students in the general methods and theory of neutron science that are applicable to diverse fields in physics, chemistry, materials science, and biology research at major neutron sources around the world. The beta decay of the free neutron is the prototype semileptonic weak interaction and simplest nuclear beta decay. There are no complications from nuclear structure, and the decay energy is small compared to the nucleon mass so recoil-order weak form factors enter below the 0.1% level. Therefore neutron decay is an attractive system for precise low energy weak interaction measurements. The neutron lifetime establishes the time scale and temperature of nucleon "freeze out" shortly after the Big Bang, which sets the neutron to proton ratio during the era of primordial nucleosynthesis and hence the helium abundance, and indirectly constrains the effective number of light neutrinos. The electron-antineutrino correlation "a" gives lambda, the ratio of axial vector (GA) and vector (GV) weak nucleon couplings. With the neutron lifetime it can be used to determine Vud, the first element of the Cabbibo-Kobayashi-Maskawa matrix, and constrain new physics beyond the Standard Model such as weak scalar and tensor forces. The present experimental situations with both the neutron lifetime and lambda are unsatisfactory, there is relatively poor agreement among previous measurements. The proposed program consists of three projects: 1) A continuation of the aCORN experiment at the NIST Center for Neutron Research toward an ultimate goal of measuring "a" to < 0.5% uncertainty (a factor of 10 improvement); 2) a phased program to improve the neutron lifetime measurement using the beam method; and 3) development of a 3He gas scintillation absolute neutron flux counter to support the neutron lifetime program.