Beta-Decay Measurements with Ultracold Neutrons

该奖项将支持北卡罗来纳州立大学对超冷中子不对称A(UCNA)实验的贡献。UCNA实验的目标是通过确定贝塔不对称性来提供对中子贝塔衰变的高精度测量。β不对称性是指在中子衰变后，中子自旋与电子发射方向之间的角关联，它对带电弱相互作用中轴耦合常数的取值非常敏感。这个常数是控制弱相互作用强度的基本参数，是晶格计算的重要目标，也是研究核子自旋结构的输入参数。它在天体物理学中也很重要，是太阳产生能量的关键输入，也是确定中子寿命大小的关键。如果所提出的测量成功，将在最精确的当前实验中提高β不对称性的精度，并提供与当前最精确的中子寿命测量相媲美的轴向形状因子值。UCNA实验是独一无二的，因为它是第一个使用极化超冷中子(UCN)测量中子衰变中角关联的实验。UCN在控制角关联测量固有的系统不确定性方面具有显著的优势，但使用这些优势的实验受到UCN可用密度的限制。UCNA的这项实验，使用了一个专门的固体氢源，也是第一个，与洛斯阿拉莫斯B区800 MeV质子束的散裂靶相耦合。这一来源现在产生的UCN密度约为1/cc，与法国格勒诺布尔劳埃朗之万研究所(世界上最强的UCN来源)的实验密度相当，正在进行进一步升级。为UCNA实验开发的实验设备可能会扩展到其他几个也在该项目中开发的中子β衰变观测值(例如寿命，以及对新张量和标量相互作用敏感的角关联)。拟议研究的更广泛影响部分源于我们计划的测量对核物理、粒子物理和天体物理领域的根本影响。源开发计划还有助于该项目的更广泛影响，有可能扩大我们的基本对称性实验的范围，并有可能将中子和核实验转移回大学校园。在该项目的第一年，将对在NCSU的PULSTAR反应堆建造的UCN源进行原理验证测试，从而能够评估这些下一代UCN源在核物理研究中可能产生的影响。

This award will support the North Carolina State University contribution to the Ultracold Neutron asymmetry A (UCNA) experiment. The goal of the UCNA experiment is to provide a high precision measurement of neutron beta-decay through a determination of the beta-asymmetry. The beta-asymmetry, known as "A", is the angular correlation between the neutron spin and the emission direction of the electron following beta-decay of the neutron, and is very sensitive to the value of the axial coupling constant in the charged weak interaction. This constant is a fundamental parameter governing the strength of the weak interaction, is an important target for lattice calculations, and is an input parameter in spin-structure studies of the nucleon. It is also of importance in astrophysics, being a critical input for energy generation in the sun and in fixing the magnitude of the neutron lifetime. The proposed measurements, if successful, will improve the precision of the beta-asymmetry over the most precise current experiments, and provide a value for the axial form factor competitive with the most precise current measurements of the neutron lifetime.The UCNA experiment is unique, in that it is the first experiment to measure angular correlations in neutron decay using polarized ultracold neutrons (UCN). UCN have significant advantages for the control of systematic uncertainties intrinsic to angular correlations measurements, but experiments which use them have been limited by the available densities of UCN. The UCNA experiment, uses a dedicated solid deuterium source, also the first of it's kind, coupled to a spallation target for the 800 MeV proton beam in Area B of at Los Alamos. This source now produces UCN densities of about 1/cc, comparable to densities in experiments at the Institut Laue Langevin in Grenoble, France (the strongest UCN source in the world), with further upgrades on the way. The experimental facility developed for the UCNA experiment can potentially be extended to several other neutron beta-decay observables (such as the lifetime, and angular correlations sensitive to new tensor and scalar interactions) also under development in this project. The broader impact of the proposed research stems in part from the fundamental impact of our planned measurements on the fields of nuclear and particle physics and astrophysics. The source development program also contributes to the broader impact of the project, potentially expanding the reach of our fundamental symmetries experiments, and by potentially shifting neutron and nuclear experiments back onto the university campus. Proof-of-principle tests of a UCN source constructed at the PULSTAR reactor at NCSU will be carried out in the first year of this project, permitting an assessment of the possible impact for these next-generation UCN sources in nuclear physics rearch.