Towards Precision Experiments with Antihydrogen

在今天的宇宙中，反物质的虚拟缺失以及相应的物质主导地位仍然是现代物理学面临的最大难题之一。早在1967年，著名的萨哈罗夫条件就描述了这样一个不对称的宇宙是如何通过要求物质和反物质之间的对称性破坏而产生的。然而，到目前为止，还没有找到足够的不平衡来解决这个问题，难题仍然存在。我们的项目将通过直接测试由反物质组成的原子的性质与其对应的物质的性质无法区分的常见假设来寻求这个问题的答案。为了实现这一点，我们已经着手将最强大的精确测量工具应用于这个问题。我们的方法是捕获反氢，一个由反质子和正电子组成的原子，并使用原子物理学中发展的光谱技术研究其内部状态。基本的方法论与原子钟相同，原子钟是目前人类工具箱中最精确的仪表。具体来说，我们将调查地面第一激发态的反氢磁阱中的过渡，以测试的假设，这种过渡的频率是相同的氢原子（物质）。这种转变已经在氢中以惊人的14位小数的精度确定。在这个项目中，我们计划首先研究反氢原子的量子跃迁，并期望在初始实验中达到小数点后9-10位的精确度。在这个项目的第二个线程中，我们将利用我们在反氢原子捕获方面的专业知识，对反物质的引力加速度进行教科书式的测量。这是一个壮举，这是唯一可能的，因为我们可以使用电荷中性的反氢原子，这消除了系统的错误，可能会出现，如果使用带电的反粒子。这些困难源于静电相互作用的大小，它完全淹没了预期的引力效应。虽然上面讨论的基本对称性要求反氢与氢相同，并且宇宙中存在等量的物质和反物质（即，早期难题的核心），引力问题是一个不同的性质。我们目前对引力的理解依赖于爱因斯坦的广义相对论，该理论基于被称为弱等效原理的假设，即惯性（运动）质量等于引力质量。一个给定质量的反物质，虽然可能具有与物质不同的性质，但如果我们对引力的理解是正确的，也应该遵守这一原则。因此，通过实验验证这一点对我们进一步了解引力有着极大的意义。迄今为止，引力与公认的量子场论是不相容的。这个项目中的反物质研究测试了物理学的基础：经过几十年的成功，这些基础给了我们许多关于物理世界的见解。尽管取得了这些成就，我们仍然不明白为什么宇宙中似乎没有大量的反物质。在这个项目中，我们将寻找与我们目前理解的微小偏差。过去的经验表明，仔细观察自然是取得突破的途径，反氢性质是引人注目的主题，因为它们的价值非常具体，迄今为止尚未经过测试。在这条道路上找不到任何线索的风险（尽管这样的结果当然意味着排除了一些可能的解释，因此并不是没有兴趣）被如果反氢和氢的行为之间存在任何测量差异所带来的壮观而无法量化的后果所超过。

The virtual absence of antimatter, and the corresponding dominance of matter, in the Universe today remains one of the biggest conundrums facing modern physics. Already in 1967, the famous Sakharov conditions described how such an asymmetric Universe could arise by requiring symmetry violations between matter and antimatter. However, up to the present, insufficient imbalance has been found to resolve this matter, and the puzzle remains. Our project will seek answers to this question by directly testing the common supposition that the properties of atoms made of antimatter are indistinguishable from their matter counterparts.To achieve this we have set out to apply the most powerful tools of precision measurements to the problem. Our approach is to trap antihydrogen, an atom made of an antiproton and a positron, and study its internal states using spectroscopic techniques developed in atomic physics. The underlying methodologies are the same as those that have given us atomic clocks; currently the most precise gauges in the human toolbox. Specifically, we will investigate the ground to first excited state transition in antihydrogen held in a magnetic trap to test the hypothesis that the frequency of this transition is identical to that of the hydrogen atom (matter). This transition has been determined with a staggering 14 decimal places of precision in hydrogen. In this project we plan to be the first to investigate the corresponding quantum jump in antihydrogen, and expect accuracies of around 9-10 decimal places for the initial experiment.In the second thread of this project we exploit our expertise in antihydrogen trapping to perform a text-book measurement of the gravitational acceleration of antimatter. This is a feat that is only possible because we can use the charge-neutral antihydrogen atom, which eliminates systematic errors that may arise if charged antiparticles are used. These difficulties originate from the size of the electrostatic interaction, which completely swamps the expected gravitational effects. Whilst the fundamental symmetries discussed above require both that antihydrogen is identical to hydrogen and that there are equal amounts of matter and antimatter in the Universe (i.e., the heart of earlier conundrum), the gravitational question is of a different nature. Our current understanding of gravity relies on Einstein's general theory of relativity, which is based on the postulate, known as the weak equivalence principle, that inertial (movement) mass is equal to gravitational mass. A given mass of antimatter, though potentially of a different nature to matter, should also obey this principle if our understanding of gravity is correct. Testing this experimentally is therefore of great interest to further our knowledge of gravity, which to date is incompatible with accepted quantum field theories.The antimatter research in this project tests the very foundations of physics: foundations that have, through decades of success, given us many insights into the physical world. In spite of these achievements, we still do not understand why there appears to be no bulk antimatter in the Universe. In this project we will search for tiny deviations from our current understanding. Past experience demonstrates that careful observation of Nature is the way to make breakthroughs and antihydrogen properties are compelling subjects due to the very specific, and thus far untested, predictions of their values. The risk of finding no clues on this path (though such an outcome would of course mean the exclusion of some possible explanations, and so is not devoid of interest) is outweighed by the spectacular and unquantifiable consequences that would follow if there were any measured difference between the behaviour of antihydrogen and hydrogen.