ACME: Advanced Cold Molecule Electron Electric Dipole Moment Search

这个项目的目标是寻找电子的一种新的基本性质，电子是物质的主要成分之一，也是所有原子的带电成分。这种被称为电偶极矩的新性质，可以被描述为一个原本完美的电荷球上的轻微凸起。这一看似深奥的性质可能揭开了自然界最根本的谜团之一：为什么宇宙中的一切都是由物质而不是反物质组成的？在加速器实验室中，只要能量被转化为粒子(根据E=mc2)，就会产生相同数量的物质粒子和反物质粒子。例如，电子有一个对应的反物质粒子，即反电子，它具有相同的质量，但电荷相反。能量可以转化为电子/反电子对，反之，电子和反电子可以相互湮灭并转化为能量。就在大爆炸之后，能量被转化为粒子和反粒子。天文观测表明，从那时起，基本上所有的反物质都被物质湮灭了--但留下了少量物质。这一微小的过剩构成了今天在宇宙中看到的所有物体。目前的框架描述了基本粒子之间所有已知的基本力，被称为“标准模型”，无法解释这种过剩的物质是如何存活的。然而，许多数学理论已经被设计出来，通过假设还没有在任何实验中发现的新的力和粒子来解释这种“物质-反物质不对称”。根据同样的理论，这些新的力和粒子也经常导致电偶极矩，大到足以在这里支持的实验中观察到。因此，这个项目本质上是在寻求一个问题的答案：为什么在过去的某个时候，物质比反物质略受青睐，导致了今天的物理宇宙？在一般意义上，该项目还推进了精密测量科学的技术范围，这在过去导致了GPS(全球定位系统)、新型传感器等技术的意想不到的突破。如果存在电偶极矩(EDM)，它必须位于电子的自旋轴上。在非零电火花加工存在的情况下，电场会对电子产生扭矩，导致自旋绕场进动。这个自旋进动角是实验信号。极性分子的巨大内部电场THO被用来放大这种可观察到的效应。THO的内部结构也抑制了可能的系统误差。使用了能够提供前所未有的高通量分子的低温分子束源。激光和光学技术将THO分子置于可用的相干叠加中，然后探测发出电子自旋进动信号的量子干涉。在之前的资助期间，这些方法的初始版本被用来进行迄今为止最灵敏的电子电火花测量。这一结果与EDM的零值是一致的，但许多关于粒子物理标准模型之外的理论预测，随着灵敏度的提高，EDM有可能被检测到。在这个项目中，将介绍大大提高实验灵敏度的方法；这些方法包括聚焦分子束和使用基于绝热通道的更有效的状态准备过程。诸如使用低吸收光学元件之类的改进将减少在先前实验中观察到的系统误差。

The goal of this project is to search for a new fundamental property of the electron, one of the main constituents of matter and a charged component of all atoms. This new property, called an electric dipole moment, can be described as a slight bulge on an otherwise perfect sphere of charge. This seemingly abstruse property may hold the key to one of the most fundamental mysteries of nature: why is everything in the universe made of matter rather than of antimatter? In accelerator laboratories, whenever energy is converted into particles (according to E=mc2), equal numbers of matter particles and antimatter particles are created. For example, the electron has a counterpart antimatter particle, the anti-electron, which has identical mass but opposite electric charge. Energy can be converted into an electron/anti-electron pair, and conversely an electron and anti-electron can annihilate each other and turn into energy. Just after the Big Bang, energy was converted into particles and anti-particles. Astronomical observations show that since then, essentially all the antimatter annihilated with matter--but a tiny bit of matter was left over. That small excess makes up all of the objects seen in the Universe today. The current framework that describes all known fundamental forces between elementary particles, known as the "Standard Model", cannot explain how this excess of matter survived. However, many mathematical theories have been devised that can explain this "matter-antimatter asymmetry", by positing new forces and particles not yet discovered in any experiment. These same new forces and particles also often lead, according to the same theories, to an electric dipole moment that is large enough to observe in the experiment supported here. Hence, this project is essentially seeking an answer to the question: how is it that matter was slightly preferred over anti-matter at some time in the past, resulting in the physical Universe seen today? In a general sense, this project also advances the range of techniques for precision measurement science, which in the past has led to unexpected breakthroughs in technology such as GPS (the Global Positioning System), new types of sensors, etc. The electric dipole moment (EDM), if it exists, must lie along the spin axis of the electron. In the presence of a nonzero EDM, an electric field will induce a torque on the electron, resulting in precession of the spin about the field. This spin precession angle is the experimental signal. The huge internal electric field of a polar molecule, ThO, is used to amplify this observable effect. The internal structure of ThO also suppresses possible systematic errors. A cryogenic molecular beam source that delivers an unprecedented high flux of molecules is used. Lasers and optical techniques put the ThO molecules in usable coherent superpositions and then probe the quantum interference that signals the electron's spin precession. Over the previous grant period, an initial version of these methods was used to make by far the most sensitive measurement of the electron's EDM. This result was consistent with a zero value for the EDM, but many theories of what lies beyond the Standard Model of particle physics predict that, with improved sensitivity, detection of the EDM is likely. In this project, methods to greatly improve the sensitivity of the experiment will be introduced; these include focusing of the molecular beam and a use of a more efficient process for state preparation, based on adiabatic passage. Improvements such as the use of low-absorption optical elements will reduce systematic errors observed in the previous experiment.