The Spectroscopy of Antihydrogen

几个世纪以来，理解和解释宇宙的起源和演化一直是科学研究的核心。近几十年来，随着粒子物理学和宇宙学的结合提供了一幅连贯图景的开端，我们看到了惊人的进步。我们的宇宙似乎诞生于一次被称为大爆炸的灾难性事件中，并在此后的130 - 140亿年里不断进化。尽管可见宇宙的大部分都可以解释，但仍有许多非常深奥的谜团，其中最令人费解的就是反物质的存在。简单地说，反物质对物理学来说仍然是一个谜。虽然自然规律的对称性，特别是量子力学的对称性要求它的存在，但宇宙似乎完全由物质组成。解决这个难题是基础科学面临的巨大挑战之一。在大爆炸后不久，随着热宇宙的冷却，所有的反物质似乎都消失了，只留下了少量多余的物质。十亿分之一听起来并不多，但整个物质宇宙都是由它创造的。问题是我们不明白这是怎么来的。物质和反物质的行为是不对称的，但它们太小了许多数量级，不足以解释宇宙的存在。解决这个问题的一种方法，也是我们选择的方法，是研究反氢原子——反物质的基本组成部分，也是宇宙从未有机会制造的一种原子。近年来，我们对低能反粒子（反质子和正电子）的研究取得了很大进展。我们可以常规地在真空中收集它们中的许多，并将它们储存起来，直到我们准备好在非常受控的条件下将它们轻轻混合形成反氢。虽然这种能力开辟了巨大的机会，但在反氢的性质可以与氢的性质相比较之前，还有很多工作要做。在这个项目中，我们将沿着这条路开始，对我们制造并困在特殊装置中的反氢原子进行一系列实验。该装置有几个部分，但最重要的是一个陷阱，它可以容纳中性物质，如反氢。陷阱是由一个复杂的线圈排列形成的磁场形成的，在反氢产生区域的中心形成一个最小的磁场。反氢，像氢一样，有一个微小的磁矩——把轨道上的正电子想象成一个微小的电流环——这意味着在外加磁场中能级会发生变化。那些在磁场中势能增加的原子会倾向于坐在磁场最小的位置，从而被困住。陷阱的深度非常浅，刚好低于相当于一度开尔文的温度，所以我们必须在非常可控的条件下制造反原子。一旦它们被捕获，我们将向它们发射光子，以询问它们的内部结构。第一个实验可能是用微波进行的，这将有助于我们与著名的21厘米氢线进行比较。最终我们将能够用激光照射反氢。如果发现氢和反氢的性质之间有任何差异，我们将发现新的物理学，也许在探索宇宙早期反物质发生了什么的道路上走了一段路。

Understanding and explaining the origin and evolution of our Universe has been at the heart of scientific endeavour for centuries. Recent decades have seen spectacular advances, as particle physics and cosmology have combined to provide the beginnings of a coherent picture. Our Universe seems to have been born in a cataclysmic event called the Big Bang, and has continuously evolved over the 13-14 billion years since then. Though much of the visible Universe can be explained, there are still many very profound mysteries, and none more so than that posed by the existence of antimatter.Simply put, antimatter remains a mystery to Physics. Whilst the symmetry of the laws of nature, and in particular quantum mechanics, demands its existence, the Universe appears to be composed entirely of matter. Addressing this conundrum is one of the great challenges of basic science. As the hot Universe cooled shortly after the Big Bang it appears that all of the antimatter vanished, leaving a tiny excess of matter. At one part in a billion, this doesn't sound much, but the entire material Universe is created from it. The problem is we don't understand how this came to be. There are asymmetries in the behaviour of matter and antimatter, but they are too small by many orders of magnitude to account for the existence of the Universe. One way to address this problem, and the way we have chosen, is to study the antihydrogen atom - the building block of antimatter, and an atom that the Universe never got the chance to make. Recent years have seen great progress in our capabilities with low energy antiparticles (antiprotons and positrons). We can routinely collect many of them in vacuum and store them until we are ready to gently mix them to form antihydrogen under very controlled conditions.Although this capability has opened up great opportunities, there is still much work to be done before the properties of antihydrogen can be compared to those of hydrogen. In this project we will begin along this road by performing a series of experiments on antihydrogen atoms which we have manufactured and trapped in a special device. The apparatus has several parts, but the most important is a trap which can hold neutral species, such as antihydrogen. The trap is formed by magnetic fields from a complicated coil arrangement that forms a magnetic field minimum in the centre of the antihydrogen production region. Antihydrogen, like hydrogen, has a tiny magnetic moment - think of the orbiting positron as a minute current loop - which means that the energy levels shift in an applied magnetic field. Those atoms whose potential energy increases in the field will prefer to sit at the magnetic field minimum, and will be trapped.The depth of the trap is very shallow, just below the equivalent of one degree Kelvin, so we have to make our anti-atoms under very controlled conditions. Once they are trapped we will shine photons on them to interrogate their internal structure. First experiments are likely to be with microwaves, which will help us to compare with the famous 21 cm line of hydrogen. Eventually we will be able to shine laser light onto the antihydrogen.If any differences between the properties of hydrogen and antihydrogen are found, we will have discovered new physics, and perhaps come some way along the road to discovering what happened to antimatter in the early Universe.