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，还没有找到解决这一问题的不充分失衡，谜团依然存在。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物质对应的原子的性质是不可区分的这一常见假设来寻求这个问题的答案。为了实现这一点，我们已经着手应用最强大的精密测量工具来解决这个问题。我们的方法是捕获反氢，这是一种由反质子和正电子组成的原子，并使用原子物理学中发展起来的光谱技术研究其内部状态。基本的方法与给我们提供原子钟的方法相同；目前是人类工具箱中最精确的量具。具体地说，我们将研究磁囚禁中反氢的基态到第一激发态的跃迁，以检验这种跃迁的频率与氢原子(物质)的频率相同的假设。这个转变是用氢的14位小数精度确定的，令人震惊。在这个项目中，我们计划第一个研究反氢中相应的量子跃迁，并预计初始实验的精度在9-10位小数点左右。在这个项目的第二个线索中，我们利用我们在反氢捕获方面的专业知识来执行反物质引力加速的教科书测量。这是一项可能的壮举，因为我们可以使用电荷中性反氢原子，这消除了如果使用带电反粒子可能产生的系统误差。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)，但引力问题的性质不同。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价原理的假设，即惯性(运动)质量等于引力质量。如果我们对引力的理解是正确的，那么给定的反物质质量，尽管可能与物质具有不同的性质，也应该遵守这一原理。因此，通过实验测试这一点对于进一步加深我们对引力的知识非常感兴趣，到目前为止，这一理论与公认的量子场理论不相容。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，这些基础给了我们对物理世界的许多洞察。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宗反物质。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，而反氢性质是令人信服的主题，因为对它们的价值进行了非常具体的、迄今未经检验的预测。在这条道路上找不到线索的风险(尽管这样的结果当然意味着排除一些可能的解释，因此也不是没有兴趣)，超过了如果反氢和氢的行为之间存在任何可测量的差异将带来的壮观和无法量化的后果。

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.