Time Reversal, Neutron Structure, and Neutron Beta Decay

原子核和中子极化的工具可以研究时间反转和宇称的对称性，在存在更强的相互作用的情况下，可以用来研究弱相互作用。时间反转对称性及其破坏是在基本粒子水平上理解物理学和解决这个问题的关键：物质相对于反物质的优势是如何产生的。在极化中子的衰变中，粒子和自旋的时间或运动的反转提供了宇称偶数时间反转的奇特物理的探测，以及对某些尚未观察到的相互作用的独特看法。为了测量电子和质子动量与衰变中子自旋的三向量关联，在盖瑟斯堡NIST用冷中子束设计了一个实验。用极化的中子与电子或电子与3He的散射来研究中子的电荷分布是最好的。虽然是中性的，但由中子电荷半径和电形状因子表征的电荷的微妙分布在强相互作用中具有根本的起源。洛斯阿拉莫斯的冷极化中子束和麻省理工学院/贝茨的极化电子散射实验就是为了解决这些问题而设计的。尽管观测到了重核的电流分布特征和核态的宇称混合，但从未直接观测到中子与质子之间的弱相互作用。在洛斯阿拉莫斯，一项利用液态仲氢捕获的偏振冷中子束首次测量这种耦合的实验正在进行中。包括弱相互作用修正在内的核子-核子力模型预测了一系列数值。测量到10%应该限制这一范围，并证实其他观察结果，包括在原子宇称破坏中测量的113CS的极点矩。

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.