Control and Spectroscopy of Excited States of Positronium

尽管粒子物理学的标准模型（SM）取得了成功，但在我们的知识中仍然存在一些巨大的差距。也许其中最引人注目的是暗物质和能量的未知特性，以及宇宙中反物质的缺乏：根据宇宙相对论，大爆炸应该产生等量的物质和反物质，但我们似乎生活在一个物质主导的宇宙中。这些谜团正在推动粒子天体物理学和宇宙学领域的大量研究，但仍未得到解释。当这些宇宙问题困扰着我们的时候，我们至少可以为自己掌握了普通物质的物理学而感到欣慰，比如电子和质子之类的，对吧？好吧，也许不是；也可能有一些谜团：使用外来的介子氢原子对质子半径的精确测量发现与氢光谱学测量的值严重不一致[R]。Pohl等人（2010）。质子的大小。自然科学学报，2016,35(3):393 - 398。我们的实验可能有助于揭示这些看似不相干的问题。我们的目标是产生由电子和正电子组成的原子（称为正电子，或Ps原子），并对它们进行高分辨率光谱分析。在我们能做到这一点之前，我们必须控制它们，把热气体变成冷的准直光束。我们还必须使用激光将Ps原子置于高度激发的里德伯态：这将防止正电子和电子（它们是彼此的反粒子）湮灭。创造里德伯态还为我们提供了一种控制Ps原子的方法：一对分离的电荷将具有较大的偶极矩，这使得使用电场对这些长寿命原子施加力成为可能。我们已经证明了，我们可以产生正确的，长寿命的里德伯态，而且我们可以用静电场来控制它们。下一步是完善我们所学到的，并在我们使用时变场使其减速后产生更高质量的Ps光束。一旦我们有了这些冷原子束，我们就可以进行两种实验：首先，我们将用微波照射原子，观察状态之间的转变。因为原子是缓慢的，不会湮灭，我们可以对它们进行很长时间的探测，以获得对它们能量水平的精确测量。这让我们可以测试基本的QED理论，并测量里德伯常数，这是质子半径测量所需要的。这个数字将原子的能级与原子结构联系起来，但也有必要知道质子半径来建立这种联系。正常氢实验和介子氢实验之间存在分歧的一个可能原因是，对里德伯常数的了解不够准确。一个更令人兴奋的原因可能与量子引力或额外维度有关，但无论哪种方式，我们都需要理解这个问题。正电子很像氢，除了它没有任何质子，这意味着在这个系统中测量里德伯常数不会因为不知道质子的大小而变得复杂。当然，还有其他问题需要克服，但原则上，这种测量方法可能有助于理解目前的差异。如果我们可以用大量的微波激发里德伯Ps原子，那么我们就可以创造出特殊的状态（称为循环状态），这种状态的寿命很长，大约是毫秒。对于寿命如此之长的原子，我们可以测量它们在地球引力场中的下落。这将有助于回答这个问题：反物质是否以不同的方式落向物质？如果答案不是“否”，那将对我们现有的物理理论产生深远的影响。目前还没有直接的测试，但我们希望能够对寿命很长（而且很冷）的Ps进行测量。

Despite the success of the Standard Model (SM) of particle physics there are still some large gaps in our knowledge. Perhaps the most striking of these are the unknown properties of Dark Matter and Energy, and the lack of antimatter in the Universe: the Big Bang should have produced equal amounts of matter and antimatter according to the SM, but we seem to live in a matter-dominated Universe. These mysteries are driving much current research in particle astrophysics and cosmology, but remain unexplained. As these cosmic problems vex us, we can at least take comfort in our mastery of the physics of ordinary matter, electrons and protons and the like, right? Well, perhaps not; there may be some mysteries there as well: using exotic muonic hydrogen atoms accurate measurements of the proton radius have been found to be in serious disagreement with values measured by hydrogen spectroscopy [R. Pohl, et al. (2010). The size of the proton. Nature. 466 (7303): 213-216]. Our experiments may help to shed some light on these seemingly disparate problems. Our goal is to produce atoms composed of electrons and positrons (known as positronium, or Ps, atoms) and perform high resolution spectroscopy on them. Before we can do this we have to control them, turning a hot gas into a cold collimated beam. We also have to use lasers to put the Ps atoms into highly-excited Rydberg states: this will prevent the positrons and electrons (which are antiparticles of each other) from annihilating. Creating Rydberg states also gives us a way to control the Ps atoms: a pair of separated charges will have a large dipole moment, and that makes it possible to use electric fields to exert a force on these long-lived atoms. We have already shown that we can produce the right long-lived Rydberg states and that we can control them using electrostatic fields. The next step is to refine what we have learned, and to produce higher quality Ps beams, after we have used time-varying fields to slow them down. Once we have these cold atoms beams we can perform two kinds of experiments: first we will irradiate the atoms with microwaves and observe transitions between states. Because the atoms will be slow and won't annihilate we can probe them for a long time in order to obtain accurate measurements of their energy levels. This lets us test basic QED theory and measure the Rydberg constant, which is needed in the proton radius measurements. This number relates atomic energy levels to the atomic structure, but it is also necessary to know the proton radius to make this connection. One possible reason for the disagreement between the normal hydrogen and muonic hydrogen experiments could be if the Rydberg constant is not known accurately enough. A more exciting reason could be to do with quantum gravity or extra dimensions, but either way we need to understand the problem. Positronium is a lot like hydrogen except it doesn't have any protons, which means that measurements of the Rydberg constant in this system are not complicated by not knowing the proton size. Of course there are other problems to be overcome, but in principle this measurement might help to understand the present discrepancy. If we can excite our Rydberg Ps atoms with lots of microwaves then we can create special states (called circular states) that have very long lifetimes, of the order of milliseconds. With atoms that live this long we can measure how they fall in the gravitational field of the earth. This will help answer the question: does antimatter fall differently to matter? If the answer is not "no" there will be profound implications for our existing physical theories. There has never been a direct test, but with very long-lived (and cold) Ps we hope to be able to do the measurement.