Control and Spectroscopy of Excited States of Positronium

正电子激发态的控制和光谱学

• 批准号: EP/R006474/1
• 负责人: David Cassidy
• 金额: $ 102.24万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FR006474%2F1
• 关键词: Control; Spectroscopy; Excited; States; Positronium

Despite the success of the Standard Model (SM) of particle physics there are still some large gaps in our knowledge. Perhaps the most striking of these are the unknown properties of Dark Matter and Energy, and the lack of antimatter in the Universe: the Big Bang should have produced equal amounts of matter and antimatter according to the SM, but we seem to live in a matter-dominated Universe. These mysteries are driving much current research in particle astrophysics and cosmology, but remain unexplained. As these cosmic problems vex us, we can at least take comfort in our mastery of the physics of ordinary matter, electrons and protons and the like, right? Well, perhaps not; there may be some mysteries there as well: using exotic muonic hydrogen atoms accurate measurements of the proton radius have been found to be in serious disagreement with values measured by hydrogen spectroscopy [R. Pohl, et al. (2010). The size of the proton. Nature. 466 (7303): 213-216]. Our experiments may help to shed some light on these seemingly disparate problems. Our goal is to produce atoms composed of electrons and positrons (known as positronium, or Ps, atoms) and perform high resolution spectroscopy on them. Before we can do this we have to control them, turning a hot gas into a cold collimated beam. We also have to use lasers to put the Ps atoms into highly-excited Rydberg states: this will prevent the positrons and electrons (which are antiparticles of each other) from annihilating. Creating Rydberg states also gives us a way to control the Ps atoms: a pair of separated charges will have a large dipole moment, and that makes it possible to use electric fields to exert a force on these long-lived atoms. We have already shown that we can produce the right long-lived Rydberg states and that we can control them using electrostatic fields. The next step is to refine what we have learned, and to produce higher quality Ps beams, after we have used time-varying fields to slow them down. Once we have these cold atoms beams we can perform two kinds of experiments: first we will irradiate the atoms with microwaves and observe transitions between states. Because the atoms will be slow and won't annihilate we can probe them for a long time in order to obtain accurate measurements of their energy levels. This lets us test basic QED theory and measure the Rydberg constant, which is needed in the proton radius measurements. This number relates atomic energy levels to the atomic structure, but it is also necessary to know the proton radius to make this connection. One possible reason for the disagreement between the normal hydrogen and muonic hydrogen experiments could be if the Rydberg constant is not known accurately enough. A more exciting reason could be to do with quantum gravity or extra dimensions, but either way we need to understand the problem. Positronium is a lot like hydrogen except it doesn't have any protons, which means that measurements of the Rydberg constant in this system are not complicated by not knowing the proton size. Of course there are other problems to be overcome, but in principle this measurement might help to understand the present discrepancy. If we can excite our Rydberg Ps atoms with lots of microwaves then we can create special states (called circular states) that have very long lifetimes, of the order of milliseconds. With atoms that live this long we can measure how they fall in the gravitational field of the earth. This will help answer the question: does antimatter fall differently to matter? If the answer is not "no" there will be profound implications for our existing physical theories. There has never been a direct test, but with very long-lived (and cold) Ps we hope to be able to do the measurement.

尽管粒子物理学的标准模型（SM）取得了成功，但我们的知识仍然存在一些巨大的差距。也许其中最引人注目的是暗物质和能量的未知性质，以及宇宙中缺乏反物质：根据SM，大爆炸应该产生等量的物质和反物质，但我们似乎生活在一个物质主导的宇宙中。这些谜团正在推动粒子天体物理学和宇宙学的许多当前研究，但仍然无法解释。当这些宇宙问题困扰着我们时，我们至少可以从我们对普通物质、电子和质子等物理学的掌握中得到安慰，对吗？嗯，也许不是;在那里也可能有一些奥秘：使用外来μ介子氢原子精确测量质子半径与氢光谱学测量的值严重不符[R. Pohl等人（2010年）。质子的大小。自然466（7303）：213-216]。我们的实验可能有助于阐明这些看似无关的问题。我们的目标是产生由电子和正电子组成的原子（称为正电子素或Ps原子），并对它们进行高分辨率光谱分析。在我们能做到这一点之前，我们必须控制它们，把热气体变成冷的准直光束。我们还必须使用激光将Ps原子置于高度激发的里德伯态：这将防止正电子和电子（它们是彼此的反粒子）湮灭。创建里德伯态也给了我们一种控制Ps原子的方法：一对分离的电荷将具有很大的偶极矩，这使得使用电场对这些长寿原子施加力成为可能。我们已经证明，我们可以产生正确的长寿命里德伯态，并且我们可以使用静电场控制它们。下一步是改进我们所学到的知识，并在我们使用时变场使其减速后产生更高质量的Ps光束。一旦我们有了这些冷原子束，我们就可以进行两种实验：首先，我们将用微波照射原子，观察状态之间的跃迁。因为原子的速度很慢，不会湮灭，所以我们可以长时间探测它们，以获得它们能级的精确测量。这使我们能够测试基本的QED理论并测量质子半径测量所需的里德伯常数。这个数字将原子能级与原子结构联系起来，但也需要知道质子半径才能建立这种联系。正常氢和μ子氢实验之间不一致的一个可能原因可能是里德伯常数不够准确。一个更令人兴奋的原因可能与量子引力或额外维度有关，但无论哪种方式，我们都需要理解这个问题。正电子素很像氢，只是它没有质子，这意味着在这个系统中测量里德伯常数并不复杂，因为不知道质子的大小。当然，还有其他问题需要克服，但原则上，这种测量可能有助于理解目前的差异。如果我们能用大量的微波激发里德伯Ps原子，那么我们就能创造出具有很长寿命的特殊状态（称为循环态），寿命在毫秒级。有了能活这么长时间的原子，我们就能测量它们在地球引力场中的下落。这将有助于回答这个问题：反物质与物质的下落不同吗？如果答案不是“不”，那么对我们现有的物理理论将产生深远的影响。从来没有一个直接的测试，但与非常长的寿命（和冷）的Ps，我们希望能够做的测量。

项目成果

Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2 S 1 3 ? 2 P J 3 ( J = 1 , 2 ) fine-structure intervals

正电子素 2 S 1 3 ? 精密微波光谱中不对称线形的观察

• DOI: 10.1103/physreva.103.042805
• 发表时间: 2021
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: Gurung L

Positronium emission from MgO smoke nanocrystals

氧化镁烟雾纳米晶体的正电子发射

• DOI: 10.1088/1361-6455/ab0f06
• 发表时间: 2019
• 期刊: Atomic, Molecular and Optical Physics
• 作者: Gurung L

State-selective electric-field ionization of Rydberg positronium

里德伯正电子素的状态选择性电场电离

• DOI: 10.1103/physreva.98.053417
• 发表时间: 2018
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: Alonso A

Velocity selection of Rydberg positronium using a curved electrostatic guide

• DOI: 10.1103/physreva.95.053409
• 发表时间: 2017-05
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: A. Alonso;B. Cooper;A. Deller;S. Hogan;L. Gurung;D. Cassidy

Line-shape modeling in microwave spectroscopy of the positronium n = 2 fine-structure intervals

正电子素 n = 2 精细结构区间的微波光谱线形建模

• DOI: 10.1103/physreva.104.062810
• 发表时间: 2021
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: Akopyan L