The UCNA Experiment: Measuring Angular Correlations in Neutron Decay using Ultracold Neutrons

UCNA实验首次利用超冷中子（UCN）来测量中子β衰变中中子自旋和电子动量（β不对称性）之间的角相关。 β-不对称性测量，连同中子衰变寿命和μ子衰变寿命的测量，提供了足够的信息，为模型无关提取的参数表征半轻子带电电流弱相互作用的第一代夸克。 建议的测量可用于确定Vud，CKM矩阵元素的重要核β衰变，并提供各种测试的电弱标准model.UCN产生的散裂驱动的固体氘超冷中子源在洛斯阿拉莫斯，通过7特斯拉的极化磁铁和自旋翻转器，然后引导到衰变谱仪。 光谱仪由一个安装在均匀的7特斯拉磁场中的衰变“单元”组成。 在衰变过程中发射的带电粒子围绕场线盘旋到探测器，探测器位于衰变体积之外，以拦截衰变过程中发射的所有粒子。 利用超冷中子进行这些测量，而不是更传统的冷中子束，具有一定的优势：中子可以被极化到基本上100%，只需通过高场区域，与通常在基于反应堆的实验中发现的水平相比，背景可以降低到极低的水平。 在提案期间，我们试图在1%的水平上建立β不对称性的测量，然后开始推动这项技术达到最终的精度。我们测量的更广泛的影响部分在于它对整个物理学界的兴趣：UCNA实验确定了电弱标准模型中的基本参数，并提供了一种识别弱电弱标准模型之下的新物理的方法相互作用尺度，物理学中普遍感兴趣的主题。 我们还在开发UCN技术，该技术可应用于其他各种领域，包括与固体物理学，材料科学和生物物理学相关的表面阿萨探针。 最后，我们的计划将导致在核物理，中子学和一般与中子研究相关的问题的本科生，研究生和博士后助理的培训。

The UCNA experiment utilizes, for the first time, ultracold neutrons (UCN) to measure the angular correlation between the neutron spin and the electron momentum (the beta-asymmetry) in neutron beta-decay. A beta-asymmetry measurement, together with a measurement of the lifetime for neutron decay and the lifetime for muon decay, provides enough information for a model-independent extraction of the parameters characterizing semi-leptonic charged current weak interactions in the first generation of quarks. The proposed measurement can be used to determine Vud, the CKM matrix element important for nuclear beta-decay, and to provide various tests of the electroweak standard model.UCN are produced in a spallation-driven solid deuterium ultracold neutron source at Los Alamos, directed through a 7 Tesla polarizing magnet and spin-flipper, and then guided into a decay spectrometer. The spectrometer consists of a decay "cell" mounted within a uniform,7 Tesla magnetic field. Charged particles emitted during the decay spiral around the field lines to detectors, positioned outside the decay volume to intercept all particles emitted during the decay. The utilization of ultracold neutrons for these measurements, instead of a more traditional cold neutron beam, has certain advantages: the neutrons can be polarized to essentially 100 percent, simply by passing them through a high field region, and backgroundscan be reduced to extremely low levels compared to those typically found at reactor-based experiments. In the proposal period we seek to establish a measurement of the beta-asymmetry at the one percent level, and then begin pushing for the ultimate precision achievable with this technique.The broader impact of our measurement lies in part because of its interest to the physics community at large: the UCNA experiment determines fundamental parameters in the electroweak standard model and provides a means to identify new physics below the weak interactions scale, a subject of general interest in physics. We also are in the process of developing UCN technology, which may have applications in diverse other fields, including asa probe for surface properties relevant to solid state physics, materials science and biophysics. Finally, our program will result in the training of undergraduate students, graduate students and post-doctoral associates in nuclear physics, neutronics and issues associated with neutron research in general.