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.
The neutron is an elementary particle that is a constituent of the nucleus of nearly every atomic species. While the neutron can exist stably within the nuclei of most atoms, free neutrons are unstable and decay with a lifetime of about 880 seconds. When a neutron decays, it usually emits three particles: a proton, an electron and an antineutrino; however an observable photon (x-ray) may be emitted in coincidence with the three particles. The study of the properties of neutron decay, that is the rate of decays (or lifetime) and the relative directions of the particles and photons emitted is being studied in order to precisely determine the nature of the forces that govern neutron decay and many other processes in nature. The neutron is also a particle with spin and it is possible to control the direction of the spin and thus extend the studies to include the emission directions with respect to the spin of the neutron before it decays. We have been analyzing the data from an experiment that detected the photons in coincidence with the proton and electron emitted in neutron decay. The analysis is begin led primarily by a University of Michigan graduate student and an undergraduate under his supervision and includes detailed study of the data as well as detailed computer simulations of the apparatus and decay processes. These data, from the RDK (Radiative-neutron decay) experiment, will provide the most precise determination of the fraction of decays that include an observable photon and help to validate the understanding of such effects. In addition, the apparatus used for the RDK experiment is being employed in a new measurement of the neutron lifetime, and the detailed simulations are crucial to designing this new measurement and anticipating systematic errors. The neutron lifetime is a crucial quantity in nature that impacts our understanding of elementary particle interactions as well as the details of element creation in the early universe - Big Bang Nucleosynthesis. In addition to providing computer simulations for the design of the new measurement, we are working on a way to calibrate the measurement. Only about one in a million of the neutrons passing through the detector decays, and the measurement relies on knowing how many neutrons pass through the apparatus as well as the number that decay. A major contribution of our group over the years has been the ability to control direction of the neutron spin and the spin of other systems and to use this to enhance the signals. One of the crucial elements is an isotope of helium, 3He, which has been a workhorse in many fields because of our ability to control its spin. We continue to improve ways to control the spin of 3He and other noble gases, and are working on a way to use this to make very accurate measurements of magnetic field over a very broad range. One crucial component of 3He-magnetometry is knowledge of the rate at which the spin moves in a magnetic field of specific strength, that is the magnetic moment. This can be determined completely independently from other spin measurements, which usually use NMR of protons in water, if we can isolate 3He nuclei in electromagnetic traps. We have begun to develop such a measurement with the goal of 3He magnetometry. One particularly interesting application would be in providing the calibration of the magnetic fields for the measurement of the magnetic moment of the muon, an unstable elementary particle. The muon magnetic moment is sensitive to a variety of elementary particle forces that may also reveal new physics. The muon magnetic-moment experiment is underway at Fermilab.
