Precision Measurements with Polarized Cold Neutron Beams and 3He

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The neutron is a sub--atomic particle thatcomprises more than half of the matter in theworld. Within the nuclei of most atoms, theneutron remains stable, but when freed from thenucleus, it is unstable. Free neutrons are thusan important tool for study ofsub--atomic physics, because the decay andinteractions of free neutrons reveal theinteractions of its constituents and decayproducts. Neutrons also have spin, the quantummechanical property of the most fundamentalpieces of matter that distinguishes two states calledspin--up and spin--down. Spin is responsiblefor the nuclear magnetism exploited, for example,in NMR and MRI. The spin states also affect thedecay and interactions of neutrons, and so thecontrol of neutron spin becomes useful for moredetailed study of sub--atomic interactions. Inthe proposed research, free neutrons produced bya particle accelerator at Los Alamos NationalLaboratory and at the nuclear reactor at NISTwill be used in experiments that control theneutron spin to reveal weak interactions andpossible new physics beyond the Standard Model that summarizes known elementaryparticle interactions.The neutron spins in a beam of neutron will be manipulated byselecting predominantly one spin state with aspin filter based on laser polarized $^3$He. Theneutron spin can be reversed usingmagnetic fields that couple to the nuclearmagnetic moment. Very precise measurement of theneutron polarization ({\it i.e.} the excessfraction of the selected spin state) and precisespin reversal are required to mitigate falseeffects that may arise. The techniques of$^3$He polarization, the spin filter, and spinflipping will be greatly improved as we undertakethese experiments.The objective of the Los Alamos experiment ismeasurement of the weak interaction effects onthe absorption of neutrons by hydrogen. The weakinteractionis characterized by absorption and emission ofparticles that have a handedness that correlatesthe spin orientation with the direction of motion.This handedness allows observation of the weakinteraction amidst the much stronger interactionsof neutrons and protons. Definitive characterization of the weakinteraction between the neutron and proton iscrucial to fully understanding the forces thatbind the nucleus. The experiment at NIST measuresthe dependence of neutron decays on the spinstate. In a dedicated detector, neutron decaysare observed by simultaneously detecting the proton andelectron. Thethird particle emitted, the neutrino, is too elusive todetect with high efficiency. A dependence ofthe electron's and proton's relative motion on the neutron spinstate would reveal interactions that are notsymmetric under reversal of time, a specialsignature that could reveal new physics beyondthe Standard Model, physics that mayexplain the origin of matter in the Big Bang. The advancedtechniques of spin state selection, precisionpolarimetry, and spin flipping will be applied toa planned experiment with the objective of preciselymeasuring the handedness of neutron decays. Whencombined with other neutron decay measurementsthese measurements will also probe physics beyond theStandard Model.This work probes deep intellectual questionsabout the most fundamentalpieces of matter. The aim is to collect datathat will help complete the picture of elementaryparticles and their interactions. The techniqueshave much broader impact. Laser polarized$^3$He and $^{129}$Xe used in related experimentsare now used in biomedical research, materialsscience, and may be applied in advances ofquantum computation. This research is a remarkabletraining ground for undergraduate and graduatestudents. The technical challenges combined withthe deep intellectual issues provide motivationand develop technical skills. Several undergraduates willgain research experience working along with graduate students andsenior researchers. Graduate students emergebroadly capable and move on to prepare forfaculty positions as well as interdisciplinaryresearch. This work has also led to developmentof new courses for non--physics majors and to aset of public lectures on Nuclear Magnets.The theme for the public isthat, in the context of probing fundamentalproblems of physics -- exciting in their ownright, the hardest problems produce the mostinnovative solutions with spin--offs unimaginableat the outset. Atomic clocks, enhanced MRI, andprobing the origin of matter all follow from thecontrol of nuclear magnetism.