Precision Experiments with Antihydrogen

粒子宇宙目前主要由物质组成的观察似乎毫不含糊，就像宇宙诞生时包含等量的物质和反物质的断言一样。这种不平衡或不对称为什么会演变目前还不清楚，事实上，这是标准模型以外的物理学的核心问题之一。在基础物理中实验探索对称性的传统方法是研究动能越来越高的粒子碰撞，以努力重现宇宙开始(大爆炸)时的条件。然而，越来越清楚的是，这种研究可以通过小规模实验来补充和丰富，例如，在设置粒子电偶极矩的限制方面，通过新的暗物质搜索，以及在物质和反物质性质的精确比较方面。我们选择利用通过原子捕获和冷却物理学以及原子光谱学开发的强大工具箱来解决这个问题。简而言之，我们在研究反氢原子的性质和行为之前，先创建、捕获并冷却反氢原子。在一组实验中(Alpha-III，将专门用于光谱学研究)，我们打算使用一种称为双光子无多普勒光谱的技术，系统地探索反原子的基态和它的第一激发态之间的跃迁。我们希望以类似于目前氢原子达到的精度来确定它的频率，氢原子的小数点后14位令人震惊。这将提供一个非常直接的对称性测试。我们已经测量了反氢到小数点后12位的相同转变，现在我们的目标是氢的精确度。此外，我们打算通过将基态到第一激发态的工作与额外跃迁的光谱测量相结合，来确定反原子的基本常数，如反里德堡常数和反质子电荷半径。在我们的第二个主要实验途径，即所谓的Alpha-g中，我们将分析反氢原子离开专门建造的原子陷阱时的轨迹，其磁场经过精心设计，以提高实验对反原子引力行为的敏感性。我们希望第一次确定反物质因重力而产生的加速度。最终，我们希望将反氢的g值提取到1%或更高的精确度。对(反)原子系统中引力行为的兴趣部分源于现代物理学的另一个谜题，即我们的引力理论(爱因斯坦的广义相对论)与目前公认的量子场论不相容。虽然等价原理规定所有物体，无论其内容如何(例如，在这种情况下，无论它们是由物质还是反物质组成)，都应该以相同的加速度向地球坠落，但对量子物体测试(经典)引力理论是基本感兴趣的。电中性反物质系统更可取，因为它们不受电场的影响，电场可以淹没带电粒子的重力效应，而反氢特别合适，因为它现在可以被捕获和冷却。因此，我们通过探索反氢物理对对称性和引力的双管齐下的攻击，有望发展对自然的新见解。我们确定反物体的属性和行为的能力是前所未有的，我们的目标是通过本提案中规定的工作进一步发展这一能力。物质和反物质之间的任何差异，无论多么微小，都将对我们对物理学和自然规律的理解产生深远的影响。

The observation that the particulate Universe is currently comprised mostly of matter seems unequivocal, as does the assertion that at its birth, the Universe contained equal amounts of matter and antimatter. Just why this imbalance, or asymmetry, has evolved is currently not understood, and indeed it is one of the central questions of physics beyond what is known as the Standard Model. The conventional approach to experimentally explore symmetry in fundamental physics is to study particle collisions at ever-higher kinetic energies, in an effort to reproduce conditions further back towards the beginning of the Universe (the Big Bang). It is becoming increasingly clear, though, that such investigations can be complemented and enriched via small scale experiments, for instance in setting limits on particle electric dipole moments, with novel dark matter searches and, as here, in precision comparisons of the properties of matter and antimatter.We have chosen to bring the powerful toolbox developed via the physics of atom trapping and cooling and atomic spectroscopy to bear on this problem. In short, we create, capture and then cool antihydrogen atoms before studying their properties and behaviour. In one set of experiments (ALPHA-III, which will be devoted to spectroscopic investigations) we intend to systematically probe the transition between the ground state of the anti-atom and its first excited state using a technique known as two-photon Doppler-free spectroscopy. We hope to determine its frequency with a precision similar to that currently achieved for the hydrogen atom, for which it is known to a staggering 14 decimal places. This will deliver a very direct test of symmetry. We have already measured the same transition in antihydrogen to 12 decimal places and we are now aiming for the hydrogen precision. Additionally, we intend to determine fundamental constants in anti-atoms, such as the anti-Rydberg constant and the antiproton charge radius, by combining the ground-to-first excited state work with spectroscopic measurements of additional transitions.In our second major experimental avenue, so-called ALPHA-g, we will analyse the trajectories of antihydrogen atoms as they leave a purpose-built atom trap whose magnetic fields have been carefully tailored to enhance experimental sensitivity to the gravitational behaviour of the anti-atom. We expect to make the first determination of the acceleration of antimatter due to gravity. Eventually we hope to extract the value of g for antihydrogen to an accuracy of 1% or better. Interest in the behaviour of gravity on (anti-)atomic systems stems in part from another puzzle of modern physics, namely that our theory of gravity (Einstein's General Relativity) is incompatible with currently accepted quantum field theories. And whilst the equivalence principle dictates that all objects, irrespective of their content (e.g., in this context independently of whether they are comprised of matter or antimatter), should fall with the same acceleration towards the Earth, testing the (classical) theory of gravity on quantum objects is of fundamental interest. Electrically neutral antimatter-systems are preferable, since they are immune to the influence of electric fields, which can swamp the effects of gravity for charged particles, and antihydrogen is particularly suitable, since it can now be trapped and cooled.Thus, our two-pronged attack on symmetry and gravity by exploring the physics of antihydrogen promises the development of new insights into nature. Our ability to pin down the properties and behaviour of anti-objects is unprecedented, and we aim to further develop this with the work set out in this proposal. Any difference between matter and antimatter, however small, will have profound consequences for our understanding of physics and the laws of nature.