The Lamb Shift Experiment in Muonic Hydrogen
The Lamb Shift Experiment in Muonic Hydrogen
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μ子氢的兰姆位移实验
DOI：
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发表时间：
2005
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影响因子：
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通讯作者：
A. Antognini
中科院分区：
文献类型：
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作者：
A. Antognini
本论文的主题是瑞士保罗·谢勒研究所正在进行的μ子氢兰姆位移实验。它的目标是通过激光光谱学测量µp原子中的2S-2 P能量差，并以10-3的精度推导出质子均方根（rms）电荷半径，比
目前已知的。这将使得在10-7的相对精度水平上测试氢中的束缚态量子电动力学（QED）成为可能，并将导致里德伯常数的测定提高7倍以上。此外，它将代表QCD理论的基准。
该实验是基于使用可调谐脉冲激光器测量µp原子中2S（F=1）和2 P（F=2）能级之间的能量差，精度为30 ppm。
波长在6微米左右。负μ子从一个独特的低能量μ子束停止在70 s-1的速率在0.6 hPa的氢气。高激发的µp原子形成，其中大多数在100 ns内迅速退激发到基态。然而，大约有1%的概率会形成寿命为1.3 µs的长寿命µp（2S）原子。
进入的μ子触发脉冲，多级激光系统，以55 s-1的重复率在6 µm处提供0.2 mJ/脉冲。它由两个XeCl准分子激光器和一个染料激光器组成，染料激光器泵浦一个振荡放大器频率控制的Ti：Sa激光器。其在708 nm处的6 ns长脉冲然后通过充满氢的拉曼池中的第三斯托克斯产生频移到6 µm。激光脉冲相对于瞬发μ子级联有大约1.5 µs的延迟。
如果激光处于共振状态，它会引起2S-2 P跃迁。随后的去激发到1 S状态发射出1.9 keV的莱曼-α X射线，该射线由大面积雪崩光电二极管检测。共振频率以及兰姆位移和质子半径是通过测量这些x射线的强度作为激光波长的函数来确定的。
2003年11月，当扫描宽范围的激光频率（49.7409 - 49.8757 THz）时，对2S-2 P共振线进行了搜索，对应于0.844和0.905 fm之间的质子半径。数据分析的结果是没有观察到显著的2S-2 P共振。由于低统计而不是不正确的搜索区域，否定结果具有高概率。
本文第一部分介绍了兰姆位移理论在微p中的研究现状。以下是对装置和数据分析的详细描述。对目前和未来的激光诱导事件率进行了估计，并对目前和未来的背景进行了研究。在附录中讨论了：氢的能级，质子半径的定义，这个实验的相关性，2S态的布居和寿命，以及2S-2 P跃迁的光谱性质。
The subject of this thesis is the muonic hydrogen (µp) Lamb shift experiment being performed at the Paul Scherrer Institute, Switzerland. Its goal is to measure the 2S-2P energy difference in µp atoms by laser spectroscopy and to deduce the proton root mean square (rms) charge radius with 10-3 precision, an order of magnitude better than
presently known. This would make it possible to test bound-state quantum electrodynamics (QED) in hydrogen at the relative accuracy level of 10-7, and will lead to an improvement in the determination of the Rydberg constant by more than a factor of seven. Moreover it will represent a benchmark for QCD theories.
The experiment is based on the measurement of the energy difference between the 2S(F=1) and 2P(F=2) levels in µp atoms to a precision of 30 ppm, using a pulsed laser tunable
at wavelengths around 6 µm. Negative muons from a unique low energy muon beam are stopped at a rate of 70 s-1 in 0.6 hPa of hydrogen gas. Highly excited µp atoms are formed, and most of them promptly deexcite to the ground state within 100 ns. However, there is a roughly 1% probability that longlive µp(2S) atoms with a lifetime of 1.3 µs are formed.
An incoming muon triggers a pulsed, multistage laser system which delivers 0.2 mJ per pulse at 6 µm with 55 s-1 repetition rate. It consists of two XeCl excimer lasers followed by dye lasers which pump an oscillator amplifier frequency controlled Ti:Sa laser. Its 6 ns long pulse at 708 nm is then frequency shifted to 6 µm via third Stokes production in a Raman cell filled with hydrogen. The laser pulse has a delay of about 1.5 µs with respect to the prompt muon cascade.
If the laser is on resonance, it induces 2S-2P transitions. The subsequent deexcitation to the 1S state emits a 1.9 keV Lyman-alpha x ray which is detected by large area avalanche photo diodes. The resonance frequency, and hence the Lamb shift and the proton radius, are determined by measuring the intensity of these x rays as a function of the laser wavelength.
A search for the 2S-2P resonance line was performed in November 2003 when a broad range of laser frequencies was scanned (49.7409 - 49.8757 THz), corresponding to proton radii between 0.844 and 0.905 fm. The result of the data analysis is that no significant 2S-2P resonance was observed. The negative result is with high probability due to the low statistics and not to an incorrect search region.
The first part of this thesis reports on the present status of the Lamb shift theory in µp. Following, there is a detailed description of the apparatus and analysis of the data. An estimate of the present and future laser-induced event rates are given, together with a study of the present and future background. In the Appendices are discussed: the energy levels in hydrogen, the proton radius definition, the relevance of this experiment, the 2S state population and lifetime, and the spectroscopic properties of the 2S-2P transition.