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, 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 subatomic interactions. This award funds a program to incisively and carefully advance both the precision and the accuracy of the neutron lifetime, measurement of the radiative decay branch and measurement of correlation coeffcients in neutron decay. Three intertwined projects are underway: 1) advances toward a new cold-neutron beam measurement of the neutron lifetime by transfer of calibration from a calorimetric measurement of the neutron beam flux; 2) precision measurement of the radiative decay branch in neutron decay over an extended range of photon energies; 3) development of experimental principles and apparatus that will measure the proton asymmetry and search for Scalar currents and other new physics with a precision polarized cold neutron beam. 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. The techniques of He-3 polarization, the spin filter and spin flipping will be greatly improved. 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 He-3 and Xe-129 used in related experiments are now used in biomedical research, materials science, and quantum information research.This project 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. 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 as well as interdisciplinary research. 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 leadership in 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 experiments that probe the origin of matter all follow from the control of nuclear magnetism and neutron spins.