Precision Neutron Decay Measurements

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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 ‘weak’ nuclear force - one of the four fundamental forces of nature. In this project the group will pass beams of free neutrons through specialized detectors that precisely measure the neutron decay lifetime and the angles between the emitted particles. From these results it is possible to test and refine the theory of the weak nuclear force and better understand the physics of the sun, stars, the Big Bang, and other 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 program is 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 semi-leptonic weak interaction and the 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. However, there is a serious problem with recent neutron lifetime experiments: results obtained using the traditional beam method, and those using the newer ultracold neutron storage method, disagree by about 1% (4 standard deviations). An important goal of this research program is to resolve this discrepancy. The program consists of three projects: 1) completing the BL2 neutron lifetime experiment now running at the NIST Center for Neutron Research; 2) design and construction of the new BL3 beam neutron lifetime experiment that will investigate and test systematic effects that could help resolve the neutron lifetime discrepancy and improve the precision of the beam method to less than 0.04%; and 3) analysis and publication of the recently completed run of the aCORN experiment, which measures the electron-antineutrino correlation a-coefficient, and develop improvements to reduce the uncertainty to 1% in a future run.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.