Precision Experiments with Cold and Ultracold Neutrons

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

Research in Fundamental Nuclear and Particle Physics is being categorized in three areas: At the "Energy Frontier", physicists hope to discover new physics, e.g. new symmetries or particles, by colliding particles with increasingly high energy. At the "Cosmic Frontier", physicists use the cosmos as a laboratory that contains detectable information about new physics. And finally, at the "Precision Frontier", physicists attempt to measure quantities that are believed to be known very precisely, and to interpret deviations from the known physics predictions as a signal for new physics. This award supports a measurement of the third type, a precision measurement of several observables in free neutron beta decay which serve to better understand the fundamental interactions between elementary particles. The process of beta decay has been incorporated into the standard model of elementary particle physics, which gives us a successful description of elementary particle processes. A discovery of new physics would have a critical impact on Nuclear and High Energy Physics, and on Cosmology. Being on the forefront of scientific research and on the edge of what is technologically feasible, the project will serve as a training ground for future generations of scientists. Most of the research will be done by students at the undergraduate and graduate level. This grant supports the effort to design, construct, commission, and operate the Nab spectrometer. The spectrometer will use the state of the art neutron beamline (FNPB) at the new Spallation Neutron Source (SNS) in Oak Ridge, TN to determine the two-dimensional energy distribution of decay electron and protons in neutron beta decay. The redundancy inherent in the standard model description of the neutron beta decay process allows uniquely sensitive checks of the model's validity and limits, with strong implications in astrophysics. One of the expected observables, the determination of the neutrino-electron-correlation coefficient with a relative uncertainty of 0.1%, will allow an extraction of Vud, the upper left element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix, in order to test its unitarity. The current NSAC long range plan finds that "The only realistic way to improve Vud is via neutron decay." New results from high energy physics cast a shadow on the unitarity of the CKM matrix, and re-open the possibility of finding hints for new exchange bosons, or a fourth quark generation. The other observable, the determination of the Fierz term, which parametrizes a deviation from the known shape of the spectrum of the decay electrons, to about 0.001, allows finding new particles that mediate scalar or tensor interactions at low energy, in a complementary way and with sensitivity competitive to what is expected from LHC.