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

在粒子物理学的高能前沿，有几个关键的、未解决的问题。目前尚未发现的更大质量的粒子可以解释其中的许多谜团。例如，在今天的宇宙中，普通物质比反物质多得多。然而，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，我们有可能探测到大爆炸时期常见的大质量粒子的回声，比如中子的电偶极矩，它只是反映了中子复合正负电荷的分离。除了提高我们在粒子物理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。技术描述中子电偶极矩（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。例如，目前，超对称（SUSY）预测，对于1太电子伏特的SUSY质量尺度和最大混合，其值大约为10^-25 e-cm。很明显，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。本卷中的超冷中子被困在一个塑料测量单元中，由8.9 Ångstrom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。本项目为本科生、研究生和博士后从事低温、原子、核、粒子等领域的前沿物理研究提供了重要的培训机会。实验结果可能会影响粒子物理学、天体物理学和宇宙学。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。

At the high energy frontier in particle physics there are several key, unanswered questions. As yet undiscovered, more massive particles could explain a number of these mysteries. For example, in the universe today, there is much more ordinary matter than antimatter. However, 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 such massive particles, common at the time of the Big Bang, in quantities like the electric dipole moment of the neutron, which just reflects the separation of the neutron's composite positive and negative charges. In addition to advancing our knowledge in particle 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.Technical descriptionThe 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. For example, at present, supersymmetry (SUSY) predicts a value of roughly 10^-25 e-cm for a SUSY mass scale of 1 Teraelectronvolt and maximal mixing. It is clear that 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, trapped in a plastic measurement cell, are produced by the collision of 8.9 Ångstrom 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 project provides important training opportunities for undergraduates, graduate students and postdocs as they participate in forefront physics research that stretches across areas including low temperature, atomic, nuclear and particle physics. The results of the experiment potentially impact particle physics as well as astrophysics and cosmology. Some technological developments are expected as several SBIR and STTR grants related to the experimental apparatus have been awarded to date.