Production and manipulation of Rydberg positronium for a matter-antimatter gravitational free fall measurement

宇宙起源于一次被称为大爆炸的原始宇宙爆炸的观点现在已经得到了很好的证实。根据我们目前的理解，从这个初始奇点释放能量之后的膨胀和冷却应该导致等量的物质和反物质的产生。然而，今天观察到的宇宙中物质的优势与这一假设相矛盾，并且是当前许多反物质研究的主要动机。这种不平衡可能是由于物质和反物质的性质之间的基本不对称性造成的，这种不对称性尚未被理解，或者它可能是由于不同的引力相互作用造成的。尽管自20世纪30年代首次发现反物质以来，粒子物理学和宇宙学取得了巨大的进步，但这个问题仍然无法解释。研究反物质的性质有许多不同的方法。例如，欧洲核子研究中心（CERN）正在进行几项实验计划，以创造反氢原子（即反质子和正电子之间的束缚态）。这些反原子的精确激光光谱学将允许CPT守恒的测试，该理论使我们期望物质和反物质之间存在精确的对称性。其他实验试图观察反物质和地球引力场之间的相互作用。由于引力比电磁力弱得多，因此必须使用中性粒子进行这种测量，否则实验往往会被极小的杂散电场所控制（只需约10-10 V/m的电场就可以抵消电子或正电子上的引力）。我们建议进行的实验是针对寻找物质和反物质的引力相互作用之间可能存在的差异。我们将通过制造一束电子偶素原子来实现。在基态，这些原子将在不到一微秒的时间内自我湮灭，因为它们是由一个粒子和它的反粒子组成的。然而，为了观察引力对Ps的微小影响，我们将用激光和微波辐射将它们激发到里德伯态。这可以将寿命增加到几毫秒，这将是足够长的寿命，以允许观察的引力偏转的正电子素束。欧洲核子研究中心计划进行一项补充实验，研究重力对反氢原子的影响。然而，这项关于反物质的工作对我们理解宇宙的潜在意义意味着以不同的方式对各种系统进行测量是必不可少的。由于Ps只由轻子组成，因此研究Ps和反氢是很有意义的。任何涉及反物质和引力的测量都将具有重大意义，因为以前从未进行过。如果像许多人所期望的那样，物质和反物质在引力作用下是相同的，这仍然会限制一些理论上的可能性，并且是一个重要的结果。然而，即使观察到很小的差异，这种测量的重要性也将非常深远。此外，随着我们将发展到产生高质量Ps原子束的能力，我们将能够进行许多其他类型的实验。一个例子是用激光研究Ps本身的性质。由于Ps只由轻子构成，它（几乎）完全由量子电动力学（QED）描述。因此，精确的测量是对这一理论的一个很好的检验。目前有一个小的分歧QED预测和测量值的Ps超精细间隔。这种差异仅相当于百万分之十，但通常QED测量与理论非常准确地吻合。重要的是要解决这样的问题，以防他们隐藏任何新的物理。

The idea that the universe originated in a primordial cosmic explosion known as the Big Bang is now well established. According to our present understanding, the expansion and cooling that followed the release of energy from this initial singularity should have resulted in the production of equal quantities of matter and antimatter. However, the observed predominance of matter in the universe today contradicts this hypothesis, and is the primary motivation for much of the current research on antimatter. This imbalance may result from a fundamental asymmetry between the properties of matter and antimatter that has not yet been understood, or it may arise from differing gravitational interactions. Despite the great advances made in particle physics and cosmology since antimatter was first discovered in the 1930's, this problem is still unexplained. There are many different ways in which the properties of antimatter are being studied. For example, several experimental programmes are underway at CERN to create antihydrogen atoms (that is, the bound state between an antiproton and a positron). Precision laser spectroscopy of these antiatoms will permit tests of CPT conservation, the theory that leads us to expect that there is an exact symmetry between matter and antimatter. Other experiments seek to observe the interaction between antimatter and the gravitational field of the Earth. Because gravity is so much weaker than the electromagnetic force, such measurements must be carried out using neutral particles, otherwise experiments tend to become dominated by extremely small stray electric fields (it only takes an electric field of ~ 10-10 V/m to cancel out the force of gravity on an electron or positron). The experiments that we propose to carry out are directed toward the search for a possible difference between the gravitational interaction of matter and antimatter. We will do this by creating a beam of positronium atoms. In their ground states, these atoms will self-annihilate in less than a micro-second, since they are composed of a particle and its antiparticle. However, to observe the small effect of gravity on Ps we will excite them with lasers and microwave radiation to Rydberg states. This can increase the lifetime to many milliseconds, which will be sufficiently long-lived to permit the observation of the gravitational deflection of a positronium beam. A complimentary experiment is planned at CERN, in which the effects of gravity on antihydrogen atoms will be studied. However, the potential significance of this work on antimatter to our understanding of the universe means that it is essential to perform measurements of a variety of systems in different ways. It is of interest to study Ps as well as antihydrogen since Ps is composed only of leptons. Any measurement involving antimatter and gravity will be of great significance as none has ever been performed before. If, as many expect, matter and antimatter turn out to be identical gravitationally, this will still limit some theoretical possibilities and be a significant result. However, if even a small difference is observed the importance of such a measurement will be very profound indeed. Moreover, as we will develop to the capability to produce high quality beams of Ps atoms, we will be in a position to conduct many other kinds of experiment. One example is to study the properties of Ps itself with lasers. Since Ps is made from only leptons it is (almost) entirely described by the theory of quantum electrodynamics (QED). Precision measurements are therefore a good test of this theory. Currently there is a small disagreement between QED predictions and the measured value of the Ps hyperfine interval. This discrepancy only amounts to ~ 10 parts per million but usually QED measurements agree extremely accurately with theory. It is important to resolve problems like this in case they are hiding any new physics.