Trapped Antihydrogen - Towards Spectroscopy

反氢，一个正电子和一个反质子的结合态，最近在实验室的受控条件下被创造出来。该提案试图通过使用磁梯度阱来制造和捕获非常冷的反氢来建立这一点。这是一种磁场排列，作用于反原子的小磁矩，产生一种捕获力。然而，这样的陷阱是浅的，并且目前仅能够保持具有相当于低于1开尔文的温度的动能的中性物质。为了有效地捕获反氢，它必须在这个温度或更低的温度下产生。这需要对我们的设备进行几次重大的改变和升级。其中包括一个全新的磁体和低温系统，中性阱的八极线圈安排，一个新的反氢湮灭探测器和升级的正电子蓄电池的性能。这些修改是非常技术性的，详细的支持理由旨在解释和证明这些修改是合理的。然而，所有的反氢实验都是困难的，所以我们在这里要解决的问题是：为什么要这么麻烦呢？我们将用对称性的例子来解释这一点。一段时间以来，基本的不对称性显然隐藏在自然界的深处。例如，在20世纪50年代，人们发现弱核相互作用违反了宇称守恒。然而，有缺陷的宇称镜可以通过增加所谓的电荷共轭来修复，广义地说，这意味着当每个粒子被其反粒子取代时，相互作用不受影响。有一段时间，人们相信自然定律将服从宇称反转和电荷共轭的组合。但到了20世纪60年代中期，人们发现这对于一小类涉及不寻常的、转瞬即逝的、称为K介子的粒子的反应来说是不正确的。从那时起，人们就认为电荷共轭/宇称反转镜的小缺陷可以通过时间反转来修正。然而，这种三路开关不同于三种离散对称，或者它们的任何两路组合，因为电荷/宇称/时间组合是一个定理，可以用量子场论的基本假设来证明。这些理论是我们目前对宇宙理解的基石，但被广泛认为是不完整的。因此，测试这种独特的三向开关将成为我们理解自然的核心。我们目前对宇宙起源的看法涉及到大爆炸，这被认为是一个充满活力的事件，创造了等量的物质和反物质。为什么他们不互相毁灭，留下一个没有物质的宇宙呢？对于宇宙中大量残余反物质的研究，未能找到任何痕迹。目前，人们认为我们的宇宙是物质主导的;换句话说，不对称。另一个需要补充的事实是，我们目前通过对转瞬即逝的稀有粒子的大量研究所确定的不对称性的数量不足以解释物质宇宙的存在。因此，宇宙的演化并没有完全被理解，这使得测试自然界的对称性非常重要。冷反氢的产生量适合于研究，打开了一扇新的对称之门;希望这将允许精确的激光光谱与氢的光谱线进行比较。最近，氢的光谱学已经达到了惊人的精度，特别是双光子1 S-2S跃迁，它已经被确定为一亿分之二。令人惊讶的是，由于质子性质的不确定性，这种精度远远超出了理论所能达到的水平。氢和反氢的比较将不受这种影响。我们的建议将有助于使这些比较成为现实。

Antihydrogen, the bound state of a positron and an antiproton, has recently been created under controlled conditions in the laboratory. The proposal seeks to build upon this by creating and trapping very cold antihydrogen using a magnetic gradient trap. This is an arrangement of magnetic fields that acts upon the small magnetic moment of the antiatom to produce a trapping force. However, such traps are shallow, and are currently only capable of holding neutral species with kinetic energies equivalent to a temperature below 1 Kelvin. To trap antihydrogen efficiently it must be produced at this temperature, or below. This requires several major changes and upgrades to our apparatus. These include a complete new magnet and cryogenic system, an octupole coil arrangement for the neutral trap, a new antihydrogen annihilation detector and upgrades to the performance of the positron accumulator. These changes are very technical in nature and the detailed case for support seeks to explain and justify them. However, all experiments with antihydrogen are difficult, so the question we address here is; why bother? We will explain this using the example of symmetry.It has been apparent for a while that fundamental asymmetries are hidden deep within nature. For example, in the 1950's it was discovered that the weak nuclear interaction violates parity conservation. However, the defective parity mirror can be mostly repaired by adding so-called charge conjugation, which, loosely speaking, means that interactions are unaffected when every particle is substituted by its antiparticle. For a while it was believed that the laws of nature would obey the combination of parity reversal and charge conjugation. But by the mid-1960's this was found to be untrue for a small class of reactions involving unusual, fleeting, particles called K-mesons. Since then it has been assumed that the small blemish in the combined charge conjugation/parity reversal mirror can be corrected by the application of time-reversal.However, this 3-way switch differs from the three discrete symmetries, or any 2-way combination of them because the charge/parity/time combination exists as a theorem that can be proved using the basic postulates of quantum field theory. Such theories are the cornerstone of our current understanding of the Universe, but are widely recognised as being incomplete. So testing this unique 3-way switch is going to the heart of our understanding of nature. Our current picture of the beginning of the Universe involves the Big Bang, which is thought to have been an energetic event that created equal amounts of matter and antimatter. Why then did they not all annihilate one another and leave a Universe devoid of matter? Searches for large amounts of remnant antimatter in the Universe, have failed to find any trace. Currently it is thought that our Universe is matter dominant; in other words asymmetric. The other fact to add to this is that the amount of asymmetry we can currently identify via numerous studies of fleeting and rare particles isn't enough to explain the existence of the material Universe.Thus, the evolution of the Universe is not fully understood and this makes testing the symmetries of nature of great importance. The creation of cold antihydrogen in amounts suitable for study, has opened a new door on symmetry; hopefully one which will allow precision laser spectroscopic comparisons with the spectral lines of hydrogen. Spectroscopy of hydrogen has recently reached fantastic precision for one line in particular, the two-photon 1S-2S transition, which has been determined to about 2 parts in a hundred million million. Amazingly, due to uncertainties in the properties of the proton, this level of precision is way beyond that achieved by theory. Comparisons of hydrogen and antihydrogen would be free of this effect. Our proposal will help make these comparisons a reality.