Collaborative Research: Beta-decay Angular Correlation Measurements with Cold and Ultracold Neutrons

该奖项资助了一项合作研究计划，该计划涉及两个独立的与中子的角关联测量。这些测量将使我们能够提取关于引起中子衰变的弱相互作用的基本信息，包括核子的轴向耦合常数。这个耦合常数出现在非常广泛的核物理和粒子物理环境中，从作为太阳聚变率高精度计算的关键输入，到作为更高精度晶格QCD计算的目标，再到与中子寿命一起限制新的粒子物理相互作用的存在。第一个实验UCNA测量了β不对称性，即中子极化和衰变β粒子动量之间的相关性。UCNA是第一个使用超冷中子进行这种测量的实验，超冷中子是指能量低于300 Nev的中子，可以储存在材料瓶中。超冷中子在中子偏振测量和中子本底测量方面都具有优势，这是现有中子角关联实验中系统误差的主要来源。UCNA实验目前正在运行，按照洛斯阿拉莫斯超冷中子源目前可用的中子密度，计划再运行一段时间。目标精度低于0.4%，大约比目前贝塔不对称的世界平均水平高出2倍。第二个实验，NAB，使用一束未极化的冷中子束(温度接近30K)来测量电子-中微子角关联，计划中电子-中微子关联的灵敏度约为0.1%。这项实验还应该能够对标量和张量耦合的粒子物理标准模型的扩展施加重要限制。这些实验成功的关键是本科生和研究生以及博士后学者的参与。这些年轻的科学家将在实验技术、粒子探测器和数据分析方面接受出色的培训。

This award funds a collaborative research program involving two separate angular correlation measurements with neutrons. These measurements will enable us to extract fundamental information concerning the weak interaction which causes neutron decay, including the axial coupling constant of the nucleon. This coupling constant appears in a very wide variety of nuclear and particle physics contexts, from being a critical input to high precision calculations of the solar fusion rates, to being a target for higher precison lattice QCD calculations, to placing, together with the neutron lifetime, limits on the existence of new particle physics interactions.The first experiment, UCNA, measures the beta-asymmetry, or the correlation between the neutron polarization and the decay beta-particle's momentum. UCNA is the first experiment to use ultracold neutrons, or neutrons with energies below about 300 neV that can be stored in material bottles, for this kind of measurement. Ultracold neutrons have advantages both for neutron polarimetry and neutron-generated backgrounds, key sources of systematic error in existing neutron angular correlation experiments. The UCNA experiment is currently operating, with one more run period planned at the neutron densities currently available at the Los Alamos ultracold neutron source. The targeted precision is below 0.4%, roughly a factor of 2 more precise than the current world average for the beta-asymmetry. The second experiment, Nab, uses an unpolarized beam of cold neutrons (with temperatures near 30K) to measure the electron-neutrino angular correlation, with a planned sensitivity at about the 0.1% level for the electron-neutrino correlation. This experiment should also be able to place important limits on extensions to the standard model of particle physics for scalar andtensor couplings.Key to the success of these experiments is the involvement of undergraduate and graduate students, as well as postdoctoral scholars. These young scientists will receive excellent training in experimental techniques, particle detectors, and data analysis.