Time Reversal, Neutron Structure, and Neutron Beta Decay

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The tools of nuclear and neutron polarization allow investigation of the symmetries of time reversal and parity that can be used to study weak interactions in the presence of much stronger interactions. Time reversal symmetry and its violation are key to understanding physics at the elementary particle level and to address the question: how has the predominance of matter over anti matter been generated. The reversal of time or motion of particles and spins in the decay of polarized neutrons provides a probe of parity even - time reversal odd physics and a unique view of certain kinds of interactions not yet observed. An experiment with the cold neutron beam at NIST, Gaithersburg addresses is designed to measure the triple vector correlation of electron and proton momenta with decaying neutron spin. The charge distribution of the neutron can best be studied by scattering of polarized neutrons by electrons or of electrons from 3He. Though neutral, the subtle distribution of charge characterized by the neutron charge radius and electric form factor has a fundamental origin in the strong interaction. Experiments with a cold polarized neutron beam at Los Alamos and with polarized electrons scattering at MIT/Bates are designed to address these questions. The weak interaction between neutrons and protons has never been directly observed, though features of the current distribution of the heavy nuclei as well as parity mixing of nuclear states has been observed. At Los Alamos, an experiment is under development to use a polarized cold neutron beam captured on liquid parahydrogen to measure this coupling for the first time. Models of the nucleon-nucleon force including the weak interaction corrections predict a range of values. Measurement to 10% should restrict this range and corroborate other observations including the anapole moment of 113CS measured in atomic parity violation.