RUI: The aCORN Experiment to Measure the Beta-Neutrino Asymmetry in Neutron Decay

aCORN合作计划以1%的相对不确定度测量中子衰变中的β中微子不对称性。自由中子的衰变提供了一个接近理想的系统，在其中探测电弱相互作用标准模型的极限。 目前标准模型自洽性的一些测试受到4%的实验不确定性的限制。一种改进的α测量方法也提供了一种新的方法来测量一个相关的量，称为Vud，不需要中子偏振法。aCORN实验依赖于将NIST冷中子束中的中子衰变分为两类;一类是β和中子几乎平行发射，另一类是反平行发射。 一组电场和磁场将粒子引导到位于3米长的真空容器两端的检测器，从而将a的测定减少到计算两类质子的数量。 相当大的努力已经进入研究系统误差的潜在来源，该设备已被设计为减少系统误差低于0.5%。该装置目前正在印第安纳州大学组装，准备转移到NIST。 系统误差的主要来源是磁场和限定实验轴的准直器之间的未对准。 我们已经设计了一个映射和对齐程序，将在印第安纳州进行测试，我们现在建议在NIST提出的更困难的磁场环境中重复该程序。 此外，我们将通过测量质子探测器位置处的热灯丝的电子分布来直接测量系统的对准。在汉密尔顿进行原型测试后，我们将在NIST的aCORN装置中进行最终对准检查。这项工作将继续涉及大量的本科生，无论是在汉密尔顿和NIST。 这将使一个小学院的本科生能够拥有汉密尔顿实验的一部分，然后在国家实验室的新环境中看到他们的作品融入最终的系统。

The aCORN collaboration intends to measure the beta-neutrino asymmetry, a, in neutron decay with a relative uncertainty of 1%. The decay of the free neutron provides a nearly ideal system in which to probe the limits of the Standard Model of the electroweak interaction. Some current tests of the self-consistency of the Standard Model are limited by the 4% experimental uncertainty in a. An improved measurement of a also provides a new method to measure a related quantity known as Vud that does not require neutron polarimetry. The aCORN experiment relies on separating neutron decays from a cold neutron beam at NIST into two classes; one in which the beta and neutron are emitted nearly parallel, and one in which they are anti-parallel. A set of electric and magnetic fields guide the particles to detectors at opposite ends of a 3m long vacuum vessel reducing the determination of a to counting the numbers of protons in the two classes. Considerable effort has gone into studying potential sources of systematic error, and the apparatus has been designed to reduce systematic errors below 0.5%. The apparatus is currently being assembled at Indiana University in preparation for a move to NIST. A principle source of systematic error is mis-alignment between the magnetic field and the collimators that define the experimental axis. We have devised a mapping and alignment procedure that will be tested at Indiana, and we now propose to repeat the procedure in the more difficult magnetic environment posed by NIST. In addition we will directly measure the alignment of the system by measuring the distribution of electrons from a hot filament at the proton detector position. After prototype testing at Hamilton, we will implement the final alignment check in the aCORN apparatus at NIST. The work will continue to involve significant numbers of undergraduates, both at Hamilton and at NIST. This will allow undergraduates from a small college to take ownership of a portion of the experiment Hamilton, and then to see their piece integrated into the final system in the new environment of a national laboratory.