Simulations of antihydrogen formation

每一个粒子都有一个对应的反粒子。这个反物质伴侣具有相反的电荷，但被认为是粒子的完美镜像。人们相信，在大爆炸中，粒子和反粒子的数量是相同的，但我们只能看到我们周围的普通物质。反物质都到哪里去了？虽然不能完全排除遥远的反物质星系的存在，但到目前为止，所有的搜索都给出了一个无效的结果。另一种可能是镜像实际上并不完美。一个微小的不对称可能导致现在的宇宙是物质-反物质湮灭后遗留下来的。物质和反物质之间的细微差别可以通过对反原子的高精度研究来寻找。如果能探测到反氢和普通氢光谱之间的微小差异，那就意味着违反了理论物理学中最基本的定理之一——CPT定理。虽然这个定理可以在非常一般的假设下推导出来，但它仍然需要尽可能高精度的实验检验。如今，氢光谱学已经达到了惊人的精度，大约是一亿万分之一，因此该系统确实非常适合高精度测试。由于其深远的重要性，人们为这类研究投入了大量的精力来创造反氢原子。结果，2002年欧洲核子研究中心的雅典娜实验创造了第一个冷反氢原子。在对反氢进行高精度光谱分析之前，还有很多工作要做。最重要的是，反原子必须被捕获并储存在由磁场制成的原子阱中，这反过来又要求它们足够冷。反氢原子的结合也一定更紧密。目前的项目旨在通过计算机模拟反氢的形成过程来帮助这些努力。我们的模拟包括对整个过程的真实再现。我们从被电场和磁场捕获的正电子等离子体开始。反质子被注入到陷阱中，并在正电子等离子体中来回传递，由于与等离子体的相互作用，每次传递都会失去一些动能。而在等离子体内部，反质子也有可能通过碰撞或辐射过程形成反氢。然而，大多数形成的反氢会再次被破坏，或者被新的碰撞破坏，或者被阱中的电场破坏。只有在极少数情况下，反氢才能成功地穿过等离子体和陷阱，并被探测到。我们将努力详细了解这一复杂的多步骤过程。例如，我们将研究正电子等离子体的温度和密度如何决定其形成。在什么条件下我们最有可能形成可以被捕获的反氢原子？哪些过程是重要的，只有碰撞还是辐射形成？那些从未形成反氢，但不知何故丢失了的反质子会发生什么？回答这些问题将有助于设计冷却和捕获反氢的方法。

For every particle there is a corresponding antiparticle. This antimatter partner has opposite charge, but is believed to otherwise be a perfect mirror image of the particle. It is believed that at the big bang particles and antiparticles were created in the same amount, yet we only see ordinary matter around us. Where did all the antimatter go? Although the existence of faraway antimatter galaxies cannot be ruled out completely, so far all searches have given a null result. Another possibility is that the mirror image in fact is not perfect. A small asymmetry could have resulted in the current universe being left over from matter-antimatter annihilation.Tiny differences between matter and antimatter could be searched for in high-precision studies of anti-atoms. If a small difference between the spectra of antihydrogen and ordinary hydrogen would be detected that would mean a violation of one of the most fundamental theorems of theoretical physics, the CPT theorem. Although this theorem can be derived under very general assumptions, it still needs to be tested experimentally to as high precision as possible. Hydrogen spectroscopy has today reached a stunning accuracy of about 1 part in 100 000 000 000 000, so the system is indeed well suited for high-precision tests. Because of its profound importance, a lot of effort has gone into creating antihydrogen atoms for such studies. As a result the first cold antihydrogen atoms were created by the ATHENA experiment at CERN in 2002.Still a lot remains to be done before high-precision spectroscopy of antihydrogen can be performed. Most importantly the antiatoms must be caught and stored in atom traps made from magnetic fields, which in turn requires them to be cold enough. The antihydrogen atoms must also be more tightly bound. The present project aims to help these efforts through computer simulations of the process of formation of antihydrogen. Our simulations include a realistic representation of the entire process. We start from a plasma of positrons, trapped by electric and magnetic fields. Antiprotons are injected into the trap and pass back and forth through the positron plasma, with each pass losing some kinetic energy due to interactions with the plasma. While inside the plasma there is also a probability that the antiproton will form antihydrogen, either through collisions, or through radiative processes. Most of the antihydrogen formed will however be destroyed again, either by new collisions, or by the electric fields in the trap. Only on rare occasions will the antihydrogen successfully make in through the plasma and the trap, and be detected. We will strive to get a detailed understanding of this complicated multi-step process. For instance we will look at how formation depends on the temperature and the density of the positron plasma. Under which conditions are we most likely to form antihydrogen atoms which can be trapped? Which processes are important, only collisions or also radiative formation? What happens to the antiprotons which never form antihydrogen, but are somehow lost? Answering these questions will help in designing ways of cooling and trapping antihydrogen.