Collaborative Research: A Search for the Electric Dipole Moment of the Neutron

为了寻找超越我们目前理解的新的自然规律，研究人员可以使用非常高能的加速器，例如欧洲核子研究中心的大型强子对撞机，或者他们可以对基本粒子特性进行精确的实验测量，并寻找与已知物理规律的偏差。例如，现在宇宙中的普通物质比反物质多得多，但没有任何已知的粒子相互作用机制可以解释宇宙是如何从大爆炸时的物质-反物质平衡演变到现在这种高度不平衡的情况的。通过精确的测量，可以探测到早期粒子和相互作用的回声，这在大爆炸时期很常见，比如中子的电偶极矩。中子的这种特性反映了中子内部正电荷和负电荷的分离。该奖项支持设计和建造高度专业化的仪器，以精确测量中子的电偶极矩。除了提高我们在亚原子物理学和宇宙学方面的知识，并促进技术进步，这项实验将涉及博士后学者，研究生和本科生作为重要组成部分，为年轻的研究人员提供特殊的机会，以促进他们在核物理前沿领域的教育和培训。由于迄今已颁发了与实验装置有关的若干SBIR和STTR赠款，预计会有一些技术发展。中子电偶极矩（nEDM）是一个明显违反时间反转的观测值，在基本粒子物理的描述中起着重要作用。测量的上限继续限制现行模式的扩展。这里提出的10^-28 e-cm尺度的测量将在非常高的质量尺度和小混合角的组合中提供重要的输入。中子电火花加工对于理解T-(CP-)违逆的一般模式和宇宙中观察到的物质和反物质不对称的原因也很重要。该实验将在ORNL的散裂中子源的基本中子物理光束线上进行，它基于一种与以前测量中采用的策略在质量上不同的技术。本实验的基本技术是在温度为T ~ 0.5 K的超流体氦-4中形成极化中子和氦-3原子组成的三组分流体。本卷中的超冷中子将由8.9埃的中子与超流体的声子碰撞产生。中子和氦-3磁偶极子在垂直于外加磁场的平面上进动，B0在传统的核磁共振排列中。nEDM dn是通过测量在强电场E0存在下的中子进动频率来确定的。平行（反平行）电场作用于B0时，拉莫尔进动频率nu与dn成正比。当B0 = 30毫高斯，E0 = 50千伏特/厘米时，电火花加工为10^-28 e-cm时，nu = 88赫兹，频移为4.8纳赫兹。在操作上，中子进动频率相对于氦-3的进动频率是通过利用强自旋依赖的核捕获反应和通过在液态氦-4中产生的闪烁检测反冲质子和氚来测量的。极化的氦-3原子（与中子的体积相同）还包括一个共磁计（因为氦-3原子的任何EDM都被其原子电子抑制）；它们的进动是用squid直接观察到的。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

To search for new laws of nature beyond our current understanding, researchers can use very high energy accelerators, such as the Large Hadron Collider at CERN, or alternatively they can make precise experimental measurements of basic particle properties and search for deviations from the known physical laws. For example, the universe now has much more ordinary matter than anti-matter but no mechanism of known particle interactions can explain how the universe evolved from the matter-antimatter balance at the time of the Big Bang to the present, highly unbalanced, situation. By making precision measurements, it is possible to detect the echoes of early particles and interactions, common at the time of the Big Bang, in quantities like the electric dipole moment of the neutron. That characteristic of neutrons reflects the separation of the neutron's internal positive and negative charges. This award supports design and construction of highly specialized instrumentation that is needed to precisely measure the neutron's electric dipole moment. In addition to advancing our knowledge in sub-atomic physics and cosmology, and generating technological progress, this experiment will involve post-doctoral scholars, graduate and undergraduate students as an essential part, affording the young researchers exceptional opportunities to advance their education and training in a forefront area of nuclear physics. Some technological developments are expected as several SBIR and STTR grants related to the experimental apparatus have been awarded to date.The neutron electric dipole moment (nEDM) is an explicitly time-reversal-violating observable that has played an important role in descriptions of elementary particle physics; measured upper limits continue to limit extensions of prevailing models. Measurements at the scale of 10^-28 e-cm as proposed here will provide important input at combinations of very high mass scales and small mixing angles. The neutron EDM is also important for understanding the general pattern of T-(CP-) violation and the cause of the observed asymmetry of matter and antimatter in the universe. This experiment, to be performed at the Fundamental Neutron Physics Beamline of the Spallation Neutron Source at ORNL, is based on a technique that is qualitatively different from the strategies adopted in previous measurements. The basic technique in the present experiment involves formation of a three-component fluid of polarized neutrons and Helium-3 atoms dissolved in a bath of superfluid Helium-4 at a temperature T ~ 0.5 K. The ultracold neutrons in this volume will be produced by the collision of 8.9 angstrom neutrons with the phonons of the superfluid. The neutron and Helium-3 magnetic dipoles precess in the plane perpendicular to an applied external magnetic field, B0 in a traditional Nuclear Magnetic Resonance arrangement. The nEDM, dn, is determined by measuring the neutron precession frequency in the presence of a strong electric field, E0. Application of the electric field parallel (antiparallel) to B0 changes the Larmor precession frequency, nu, in proportion to dn. With B0 = 30 milliGauss and E0 = 50 kiloVolt/cm, nu = 88 Hertz and the frequency shift is 4.8 nanoHertz for an EDM of 10^-28 e-cm. Operationally, the neutron precession frequency is measured relative to that of the Helium-3 by taking advantage of the strongly spin dependent nuclear capture reaction and detecting the recoiling proton and triton via scintillation produced in the liquid Helium-4. The polarized Helium-3 atoms (in the same volume as the neutrons) also comprise a co-magnetometer (since any EDM of the Helium-3 atoms is suppressed by its atomic electrons); their precession is observed directly using SQUIDS.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.