Simulations of antihydrogen formation

对于每个粒子，都有一个相应的反粒子。这个反物质伙伴具有相反的电荷，但被认为在其他方面是粒子的完美镜像。人们相信，在大爆炸中，粒子和反粒子的数量是相同的，但我们只看到了我们周围的普通物质。反物质都跑到哪里去了？尽管不能完全排除遥远的反物质星系的存在，但到目前为止，所有的搜索都没有给出任何结果。另一种可能性是，镜像实际上并不完美。微小的不对称性可能会导致物质-反物质湮灭留下当前的宇宙。物质和反物质之间的细微差别可以在反原子的高精度研究中寻找到。如果检测到反氢和普通氢光谱之间的微小差异，就意味着违反了理论物理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得到，但它仍然需要进行实验测试，以达到尽可能高的精度。今天氢谱学已经达到了惊人的精度，大约是百万分之一，所以该系统确实非常适合高精度的测试。由于其深远的重要性，人们在为这类研究创造反氢原子方面付出了大量努力。因此，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。在能够进行高精度的反氢光谱之前，仍然有很多工作要做。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陷阱中，这反过来又要求它们足够冷。反氢原子也必须更紧密地结合在一起。本项目旨在通过对反氢形成过程的计算机模拟来帮助这些努力。我们的模拟包括对整个过程的逼真表示。我们从正电子等离子体开始，它被电场和磁场俘获。反质子被注入陷阱，并通过正电子等离子体来回传递，由于与等离子体的相互作用，每一次传递都会损失一些动能。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，形成的大部分反氢将再次被摧毁，要么是被新的碰撞，要么是被陷阱中的电场摧毁。只有在极少数情况下，反氢才能成功地通过等离子体和陷阱进入并被检测到。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形成如何依赖于温度和密度。在什么条件下，我们最有可能形成能被捕获的反氢原子？哪些过程是重要的，只有碰撞还是辐射形成？从未形成反氢，但不知何故丢失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.