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、尚未解答的问题。到目前为止，还没有发现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，有比反物质多得多的普通物质。然而，任何已知的粒子相互作用机制都不能解释宇宙是如何从大爆炸时的物质-反物质平衡演化到现在的高度不平衡的情况的。通过进行精确的测量，有可能探测到这种大质量粒子的回波，这种粒子在大爆炸时期很常见，其量类似于中子的电偶极矩，它只反映了中子合成正负电荷的分离。除了增进我们在粒子物理和宇宙学方面的知识，并推动技术进步，这项实验还将包括博士后学者、研究生和本科生作为关键部分，为年轻的研究人员提供在核物理前沿领域推进他们的教育和培训的难得机会。技术说明中子电偶极矩(NEDM)是一种明显违反时间反转的可观测物质，在描述基本粒子物理中发挥了重要作用；测量的上限继续限制了现有模型的扩展。例如，目前，超对称性(SUSY)预测，对于1太电子伏特的超对称性质量标度和最大混合，其值约为10^-25e-cm。很明显，这里建议的10^-28 e-cm尺度的测量将在非常高的质量尺度和小的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-)破坏的一般模式和宇宙中观测到的物质和反物质不对称的原因也是重要的。这次实验将在ORNL散裂中子源的基本中子物理光束线上进行，基于一种与以前测量中采用的策略本质上不同的技术。本实验的基本技术是在温度T~0.5K的超流氦-4中形成一个由极化中子和氦-3原子组成的三组分流体。这个体积中的超冷中子是由8.9ngstrom中子与超流体中的声子碰撞产生的。中子和氦-3磁偶极子在垂直于外加磁场B0的平面内运动，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。将电场平行(反平行)施加到B0，改变Larmor进动频率nu，与dN成比例。当B0=30mliGauss，E0=50kV/cm时，nu=88赫兹，对于10^-28e-cm的电火花加工，频移为4.8纳赫兹。在操作上，通过利用强烈自旋相关的核俘获反应并通过在液态氦-4中产生的闪烁来探测反冲质子和Triton，测量了相对于氦-3的中子进动频率。极化的氦-3原子(与中子的体积相同)还包括一个共磁强计(因为氦-3原子的任何EDM都受到其原子电子的抑制)；它们的进动是直接使用SQUID观察的。这个项目为本科生、研究生和博士后提供了重要的培训机会，因为他们参与了跨越低温、原子、核物理和粒子物理等领域的前沿物理研究。该实验的结果可能会影响粒子物理学以及天体物理学和宇宙学。预计会有一些技术发展，因为到目前为止，已经授予了几项与实验仪器有关的SBIR和STR赠款。

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.