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、未解答的问题。 到目前为止，尚未发现的更大质量的粒子可以解释这些谜团。 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的高度不平衡的情况。 通过精确的测量，可以探测到大爆炸时常见的这种大质量粒子的回波，其数量类似于中子的电偶极矩，它反映了中子的复合正负电荷的分离。 除了推进我们在粒子物理学和宇宙学方面的知识，并产生技术进步外，这项实验将涉及博士后学者，研究生和本科生作为一个重要组成部分，为年轻的研究人员提供了在核物理前沿领域推进教育和培训的绝佳机会。技术说明中子电偶极矩（nEDM）是一种明确的时间反演，违反了在基本粒子物理学的描述中起着重要作用的可观察性;测量的上限继续限制流行模型的扩展。例如，目前，超对称性理论（SUSY）预测，当超对称性质量标度为1太电子伏特且混合最大时，其值约为10^-25 e-cm。 很明显，在这里提出的10^-28 e-cm尺度下的测量将在非常高的质量尺度和小的混合角的组合下提供重要的输入。 中子EDM对于理解T-（CP-）破坏的一般模式以及宇宙中物质和反物质的不对称性的原因也很重要。这个实验将在ORNL的Spanish中子源的基本中子物理束线上进行，基于一种与以前测量中采用的策略有质的不同的技术。 本实验的基本技术是在温度T ~ 0.5K的超流氦-4浴中形成极化中子和氦-3原子的三组分流体。 这个体积中的超冷中子被困在一个塑料测量池中，是由8.9毫微微克的中子与超流体的声子碰撞产生的。 中子和氦-3磁偶极子在垂直于所施加的外部磁场的平面中进动，在传统的核磁共振布置中为B 0。 通过测量强电场E0存在下的中子进动频率来确定nEDM，dn。 平行（反平行）于B 0的电场的应用改变了拉莫尔进动频率，nu，与dn成比例。 当B 0 = 30毫高斯，E0 = 50千伏/厘米时，nu = 88赫兹，对于10^-28 e-cm的EDM，频移为4.8毫微赫兹。 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相关的核俘获反应和通过在液态氦-4中产生的闪烁检测反冲质子和triton来测量的。 极化的氦-3原子（与中子体积相同）还包括一个共磁力计（因为氦-3原子的任何EDM都受到其原子电子的抑制）;该项目为本科生，研究生和博士后提供了重要的培训机会，因为他们参与了前沿物理研究，包括低温，原子，核物理和粒子物理。 实验结果可能会影响粒子物理学以及天体物理学和宇宙学。 一些技术的发展，预计作为几个SBIR和STTR赠款有关的实验装置已被授予日期。

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.