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

UCNA实验首次利用超冷中子（UCN）来测量中子衰变中中子自旋与电子动量（β -不对称）之间的角相关性。β -不对称测量，以及中子衰变寿命和介子衰变寿命的测量，为第一代夸克中半轻子带电电流弱相互作用特征参数的模型无关提取提供了足够的信息。所提出的测量方法可用于确定对核β衰变重要的CKM矩阵元素Vud，并为电弱标准模型提供各种测试。UCN是在洛斯阿拉莫斯的一个散裂驱动的固体氘超冷中子源中产生的，通过一个7特斯拉的极化磁铁和自旋鳍片引导，然后引导到一个衰变光谱仪中。该光谱仪由一个安装在7特斯拉均匀磁场中的衰变“细胞”组成。在衰变过程中发射的带电粒子绕着场线旋转，到达位于衰变体积外的探测器，以拦截在衰变过程中发射的所有粒子。在这些测量中使用超冷中子，而不是更传统的冷中子束，有一定的优势：中子可以被极化到基本上100%，只需通过一个高场区域，与那些典型的基于反应堆的实验相比，背景扫描可以降低到极低的水平。在提案期间，我们寻求在1%的水平上建立β -不对称的测量，然后开始推动使用该技术可以实现的最终精度。我们测量的更广泛影响部分在于它对整个物理界的兴趣：UCNA实验确定了电弱标准模型中的基本参数，并提供了一种方法来识别弱相互作用尺度以下的新物理，这是物理学中普遍感兴趣的主题。我们还在开发UCN技术，该技术可能在其他各种领域得到应用，包括与固态物理、材料科学和生物物理学相关的表面特性探针。最后，我们的项目将培养本科生、研究生和博士后，研究核物理学、中子学和与中子研究相关的问题。

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.