Precision Measurements with Polarized Cold Neutron Beams and 3He

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

The neutron is a sub--atomic 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 thus an important tool for study of sub--atomic physics, because the decay and interactions of free neutrons reveal the interactions of its constituents and decay products. Neutrons also have spin, the quantum mechanical property of the most fundamental pieces of matter that distinguishes two states called spin--up and spin--down. 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 sub--atomic interactions. In the proposed research, free neutrons produced by a particle accelerator at Los Alamos National Laboratory and at the nuclear reactor at NIST will be used in experiments that control the neutron spin to reveal weak interactions and possible new physics beyond the Standard Model that summarizes known elementary particle interactions. The neutron spins in a beam of neutron will be manipulated by selecting predominantly one spin state with a spin filter based on laser polarized $^3$He. The neutron spin can be reversed using magnetic fields that couple to the nuclear magnetic moment. Very precise measurement of the neutron polarization ({\it i.e.} the excess fraction of the selected spin state) and precise spin reversal are required to mitigate false effects that may arise. The techniques of $^3$He polarization, the spin filter, and spin flipping will be greatly improved as we undertake these experiments. The objective of the Los Alamos experiment 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. The experiment at NIST measures the dependence of neutron decays on the spin state. In a dedicated detector, neutron decays are observed by simultaneously detecting the proton and electron. The third particle emitted, the neutrino, is too elusive to detect with high efficiency. A dependence of the electron's and proton's relative motion on the neutron spin state would reveal interactions that are not symmetric under reversal of time, a special signature that could reveal new physics beyond the Standard Model, physics that may explain the origin of matter in the Big Bang. The advanced techniques of spin state selection, precision polarimetry, and spin flipping will be applied to a planned experiment 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 the most fundamental pieces of matter. The aim is to collect data that will help complete the picture of elementary particles and their interactions. The techniques have much broader impact. Laser polarized $^3$He and $^{129}$Xe used in related experiments are now used in biomedical research, materials science, and may be applied in advances of quantum computation. This research is a remarkable training ground for undergraduate and graduate students. The technical challenges combined with the deep intellectual issues provide motivation and develop technical skills. Several undergraduates will gain research experience working along with graduate students and senior researchers. Graduate students emerge broadly capable and move on to prepare for faculty positions as well as interdisciplinary research. This work has also led to development of new courses for non--physics majors and to a set of public lectures on Nuclear Magnets. The theme for the public is that, in the context of probing fundamental problems of physics -- exciting in their 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.