Collaborative Research: Development of an Apparatus to Measure the Electron-Anitneutrino Correlation in Free Neutron Beta Decay

中子是普通物质的组成部分之一。然而，当脱离稳定原子核的限制时，中子衰变为质子、电子和反中微子，平均寿命约为15分钟。这种衰变是由弱核力控制的，弱核力是自然界四种基本力之一。通过仔细测量发射粒子的能量和方向以及原始中子的自旋轴，可以揭示弱力基本性质的微妙细节。如果有足够的精确度，我们就可以看到超越粒子物理学标准模型的更深层次理论的线索。一个关键参数，“a”系数，描述了中子衰变中电子和反中微子之间的平均发射角（相关性）。它的实验不确定度为5%，远大于其他中子衰变参数。反中微子实际上是无法探测到的，所以它的方向和能量必须通过应用动量和能量守恒定律从电子和质子中确定。在以前的实验中，“a”系数是从它对质子能量分布的微小影响推断出来的。在目前的工作中，一种新的方法是用来提取的“a”系数的分布之间的时间检测的电子和质子在一个特殊的设备专门为此目的而设计的。这种方法保证了实验的不确定性在“a”的系数小于1%，一个因素的五个改进。这将是对弱核力更深入理解的重要一步

The neutron is one of the building blocks of ordinary matter. However, when free from the confines of a stable atomic nucleus, a neutron decays into a proton, electron, and antineutrino with an average lifetime of about 15 minutes. This decay is governed by the weak nuclear force, one of the four basic forces of nature. By carefully measuring the energies and directions of the emitted particles and the spin axis of the original neutron, subtle details of the fundamental nature of the weak force can be revealed. With enough precision, hints of a deeper theory, beyond our Standard Model of particle physics, may be seen. A key parameter, the "a" coefficient, describes the average emission angle (correlation) between the electron and antineutrino from neutron decay. It has an experimental uncertainty of 5%, much larger than other neutron decay parameters. The antineutrino is practically undetectable, so its direction and energy must be determined from the electron and proton by applying the laws of conservation of momentum and energy. In previous experiments the "a" coefficient was inferred from its small effect on the proton energy distribution. In the present work a novel method is used to extract the "a" coefficient from the distribution of times between detection of the electron and proton in a special apparatus specifically designed for this purpose. This approach promises an experimental uncertainty in the "a" coefficient of less than 1%, a factor of five improvement. It will be an important step towards a deeper understanding of the weak nuclear force