Fundamental Physics with Cold Polarized Neutrons

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The neutron is a subatomic particle that comprises more than half of the matter in the world. Within the nuclei of most atoms, the neutron remains stable, but when freed from the nucleus, it is unstable. Free neutrons are an important tool for study of subatomic physics, because the decay and interactions of free neutrons reveal the interactions of its constituents and decay products. Neutrons also have spin, a quantum mechanical property of fundamental particles. Spin is responsible for the nuclear magnetism exploited, for example, in NMR and MRI. The spin states also affect the decay and interactions of neutrons, and so the control of neutron spin becomes useful for more detailed study of subatomic interactions. In the proposed research, free neutrons produced at Los Alamos National Laboratory, at the new Spallation Neutron Source at Oak Ridge National Laboratory and at the nuclear reactor at NIST will be used in experiments that control the neutron spin. The results will reveal weak interactions and possible new physics beyond the Standard Model that summarizes known elementary particle interactions. The neutron spins in a beam of neutrons will be manipulated by selecting predominantly one spin state with a spin filter based on laser polarized 3He. The neutron spin can be reversed using magnetic fields that couple to the nuclear magnetic moment. Very precise measurement of the neutron polarization (i.e. the excess fraction of the selected spin state) and precise spin reversal are required to mitigate false effects that may arise in these experiments. Several of the techniques used in these experiments will be greatly improved over those of previous experiments.Three major experiments will be undertaken. The objective of the first is measurement of the weak interaction effects on the absorption of neutrons by hydrogen. The weak interaction is characterized by absorption and emission of particles that have a handedness that correlates the spin orientation with the direction of motion. This handedness allows observation of the weak interaction amidst the much stronger interactions of neutrons and protons. Definitive characterization of the weak interaction between the neutron and proton is crucial to fully understanding the forces that bind the nucleus. This experiment will take data at Los Alamos before moving to Oak Ridge in 2007. An experiment at NIST aims to report the discovery of a rare mode of neutron decay, radiative decay, in which a photon is produced along with the neutrino, electron, and proton emitted in most neutron decays. The third experiment will be designed and apparatus built to detect the dependence of the direction of proton emission with respect to the decaying neutron spin. This proton direction dependence depends on the relationship of the multiple quantum paths from the initial neutron to the final decay products. It is affected by the intrinsic interactions among the fundamental particles that make up the neutron and proton and the emitted electron and neutrino. Advanced techniques of spin state selection, precision polarimetry, and spin flipping will be applied with the objective of precisely measuring the handedness of neutron decays. When combined with other neutron decay measurements these measurements will also probe physics beyond the Standard Model.This work probes deep intellectual questions about fundamental issues in science. The aim is to collect data that will aid in understanding elementary particles and their interactions, but the techniques have much broader impact. Laser polarized 3He and 129Xe experiments can be applied to biomedical research, materials science, and quantum information research. This project is an unusual training ground for undergraduate and graduate students. The technical challenges combined with the deep intellectual issues provide motivation and develop technical skills. Undergraduates will gain research experience working along with graduate and post doctoral fellows. Graduate students emerge broadly capable and move on to prepare for faculty or national lab positions. This work has also led to development of new courses for nonphysics majors, to a set of public lectures on Nuclear Magnets and Neutrinos and to communicating science to the interested general population. In the context of probing fundamental problems of physics, exciting in its own right, the hardest problems produce the most innovative solutions with spin-offs unimaginable at the outset. Atomic clocks, enhanced MRI, and probing the origin of matter all follow from the control of nuclear magnetism and neutron spins.