Laser Spectroscopy of Dense Positronium

在这个项目中，加州大学滨江分校（UCR）的科学家们产生了数百万个正电子素原子的短脉冲，并将它们冷却到远低于室温。其目的是制备适合于进行精密激光光谱测量和产生玻色-爱因斯坦凝聚态的正电子素系综。正电子素（化学符号Ps）是由一个电子和一个正电子组成，正电子是电子的反粒子，通过它们相反电荷的吸引力结合在一起。正电子素在化学上与氢相似，但不同的是，正电子素原子的重量只有氢原子的千分之一，而且正电子素原子中的电子和正电子在大约100纳秒内相互湮灭。在之前的工作中，UCR团队使用了室温及以上的正电子素原子的密集爆发来测量Ps-Ps自旋交换碰撞速率，并观察Ps2分子基态和第一激发态之间的光学跃迁。在这项新工作中，UCR科学家团队及其学生将使用NSF MRI奖#1040590开发的新ps啁啾脉冲激光仪器来研究Ps原子在绝热快速状态下的相干激发和去激发，这是NMR实验中最熟悉的。他们将在自由空间中对Ps进行激光冷却，使用这种技术来产生这些短寿命原子所需的快速冷却。在未来的实验中，该项目开发的方法有望用于产生和研究第一个由Ps原子稠密气体形成的玻色-爱因斯坦凝聚体（BEC）。该项目的实验具有广泛而引人注目的意义。纯电子-正电子物质的精确光谱学将提供重要的信息，这些信息可以与量子电动力学的基本精确理论相比较，这反过来将使对新物理学的独特探索成为可能。由于Ps原子的质量轻，物质-反物质BEC应该表现出与较高温度下相同粒子集合相关的迷人量子特性。从Ps BEC发射出的冷Ps原子的可用性对于Ps的引力自由落体的可能测量是必不可少的，这将首次为我们提供关于反物质引力性质的直接信息。BEC产生受激电子-正电子湮灭光子的可能性可能有一天会成为伽马射线激光器的基础，对国防和能源生产具有重要意义。该项目具有很强的教育成分，因为它将部分由研究生进行，他们将在各种前沿技术，数据分析以及为高影响力期刊撰写科学论文方面接受高水平培训。实验室正在进行的活动将形成针对大学预科学生的反物质项目的基础，并将成为高中游客和暑期实习高中教师的实验室图尔斯参观和讨论的中心。

In this project Scientists at the University of California, Riverside (UCR) produce short bursts of millions of positronium atoms and cool them far below room temperature. The purpose is to prepare positronium ensembles suitable for making precision laser spectroscopy measurements and producing a Bose-Einstein condensed state. Positronium (chemical symbol Ps) is composed of an electron and a positron, the electron's antiparticle, bound together by the attraction of their opposite charges. Positronium is chemically like hydrogen but different because a positronium atom weighs only one-thousandth as much as a hydrogen atom and because the electron and positron in a positronium atom annihilate each other within about 100 nanoseconds. In previous work the UCR team has used dense bursts of positronium atoms at room temperature and above to measure Ps-Ps spin exchanging collision rates and to observe optical transitions between the ground and first excited states of Ps2 molecules. In the new work the team of UCR scientists and their students will use a new ps chirped pulse laser instrument developed under NSF MRI Award #1040590 to study the coherent excitation and deexcitation of Ps atoms in the adiabatic fast regime most familiar from NMR experiments. They will implement laser cooling of Ps in free space, using this technique to produce the rapid cooling needed for these short-lived atoms. In future experiments the methods developed in this project are expected to be used to produce and study the first Bose-Einstein condensate (BEC) formed from a dense gas of Ps atoms. The experiments of this project are of wide and compelling interest. Precision spectroscopy of purely electron-positron matter will give important information that can be compared with the essentially exact theory of quantum electrodynamics, which in turn will enable a unique search for new physics. The matter-antimatter BEC should exhibit fascinating quantum properties associated with collections of identical particles at comparatively high temperatures due to the light mass of the Ps atoms. The availability of cold Ps atoms emitted from a Ps BEC would be essential to possible measurements of the gravitational free-fall of Ps that would give us direct information about the nature of antimatter gravity for the first time. The possibility of a BEC producing stimulated electron-positron annihilation photons might someday be the basis for a gamma ray laser with important implications for defense and energy production. The project has a strong educational component since it will be partly carried out by graduate students who will receive high level training in a wide variety of leading edge techniques, in data analysis, and in the writing of scientific papers for high impact journals. The ongoing activities in the laboratory will form the basis for antimatter-based programs directed toward pre-college students and will be the centerpiece for laboratory tours and discussions for high school visitors and summer intern high school teachers.