应用光核反应计算μ原子中的核极化效应

近年来，瑞士保罗谢勒研究所的CREMA合作组通过测量$\mu$氢原子兰姆位移显著提升了质子半径的测量精确度。然而这一新实验结果与已知质子半径标准值(CODATA)相差5.6个标准差，被称为质子半径之谜，受到众多物理学家的关注。受此启发，CREMA合作组在不同的$\mu$原子中展开了一系列兰姆位移光谱的测量实验。他们计划从这些$\mu$原子的测量中得到轻核(包括$^{2,3}{\rm{H}}$,$^{3,4}{\rm{He}}$)的电荷半径。除了对光谱测量精度的要求外，轻核半径的实验精度当前仍被一项理论输入量限制:核极化效应对$\mu$原子光谱的修正。核极化效应体现了$\mu$子与原子核进行双光子交换中对核的虚激发，进而对$\mu$原子能谱产生高阶修正。因此，这一效应与光核反应以及康普顿散射直接相关。核极化效应对兰姆位移的修正可通过计算光核吸收截面以及虚光子康普顿振幅的求和规则而得到。本工作运用第一性原理的核结构计算方法，研究了$\mu$原子中的核极化效应。通过结合现代核力模型与超球简谐基展开多体方法，计算了一系列与核极化相关的光核反应及康普顿散射求和规则。这一理论研究为从$\mu$原子光谱测量中对核半径的精确提取提供了关键性的理论输入。     

The size of the proton

The root-mean-square (rms) charge radius r _p of the proton has so far been known only with a surprisingly low precision of about 1% from both electron scattering and precision spectroscopy of hydrogen. We have recently determined r _p by means of laser spectroscopy of the Lamb shift in the exotic “muonic hydrogen” atom. Here, the muon, which is the 200 times heavier cousin of the electron, orbits the proton with a 200 times smaller Bohr radius. This enhances the sensitivity to the proton’s finite size tremendously. Our new value r _p?=?0.84184 (67) fm is ten times more precise than the generally accepted CODATA-value, but it differs by 5 standard deviations from it. A lively discussion about possible solutions to the “proton size puzzle” has started. Our measurement, together with precise measurements of the 1S–2S transition in regular hydrogen and deuterium, also yields improved values of the Rydberg constant, R _?∞??=?10,973,731.568160 (16) m^???1.     

Laser spectroscopy of hydrogen and antihydrogen

Recent advances in high resolution laser spectroscopy of atomic hydrogen are reviewed. Two-photon spectroscopy of the 1S–2S transition in a cold atomic beam has reached a resolution of better than 1 part in 10¹¹. This narrow resonance has already led to order-of-magnitude advances in precision measurements of the Rydberg constant, the 1S ground state Lamb shift, and the hydrogen deuterium isotope shift. A precise spectroscopic comparison of hydrogen and antihydrogen could provide stringent tests of basic symmetry laws. Possible scenarios for ultrahigh resolution laser spectroscopy of antihydrogen are discussed.     

The Lamb shift in muonic hydrogen and the proton radius puzzle

The recent measurement of the Lamb shift in muonic hydrogen resulted in a tenfold improved value of the rms charge radius of the proton. The value is, however, 7 standard deviations discrepant from the world average of this quantitiy which is obtained from elastic electron-proton scattering and precision spectroscopy of hydrogen and deuterium. New input from both theory and experiment is needed to resolve this so-called “proton radius puzzle”.     

Theory of the 2S–2P Lamb shift and 2S hyperfine splitting in muonic hydrogen

The 7σ$7\sigma$ discrepancy between the proton rms charge radius from muonic hydrogen and the CODATA-2010 value from hydrogen spectroscopy and electron-scattering has caused considerable discussions. Here, we review the theory of the 2S–2P Lamb shift and 2S hyperfine splitting in muonic hydrogen combining the published contributions and theoretical approaches. The prediction of these quantities is necessary for the determination of both proton charge and Zemach radii from the two 2S–2P transition frequencies measured in muonic hydrogen; see Pohl et al. (2010) [9] and Antognini et al. (2013) [71].     

Muonic molecules in superintense laser fields

We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd mu) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I > or =10(21) W/cm(2). Ionization of the muonic atomic fragment occurs at much higher intensity I > or =6 x 10(22) W/cm(2). Since the field controls the ion trajectory after dissociation, it forces recollision of a approximately 10(5)-10(6) eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.     

用于氢原子1S-2S光谱研究的超窄线宽243nm半导体激光

近年来,随着激光稳频技术的发展,人们可以把972nm的外腔半导体激光器产生的激光进行放大和四倍频,并将它锁定到超稳定的光学法布里-珀罗腔上,从而可以获得Hz量级超窄线宽的243nm激光,使其成为研究氢原子1S-2S双光子跃迁的有力工具.文章主要介绍了超稳腔、四倍频243nm半导体激光器的研究进展及在氢原子1S-2S双光子跃迁精密光谱研究中的应用.     

Proposal for X-ray spectroscopy of muonic atoms formed from implanted ions in solid hydrogen films

A new method is proposed to extend muonic atom X-ray spectroscopy to the study of nuclear beams, including radioactive beams, by stopping both muon and nuclear beams in a solid hydrogen film. The muon transfer reaction to higher Z nuclei is used then to form muonic atoms. This method would allow studies of the nuclear charge distribution of unstable atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experiment to measure the Lamb shift in muonic hydrogen

The contribution of the root mean square (RMS) proton charge radius to the Lamb shift (2S–2P energy difference) in muonic hydrogen (μp) amounts to 2%. Apart from the uncertainty on this charge radius, theory predicts the Lamb shift with a precision on the ppm level. We are going to measure ΔE (2 S_1/2(F=1)–2 P_3/2(F=2)) in a laser resonance experiment to a precision of 30 ppm (i.e., 10% of the natural linewidth) and to deduce the RMS proton charge radius with 10⁻³ relative accuracy, 20 times more precise than presently known. The most important requirement for the feasibility of such an experiment, namely the availability of a sufficient amount of long lived metastable μp atoms in the 2S state, has been investigated in a recent experiment at PSI. Our analysis shows that in the order of one percent of all muons stopped in low pressure hydrogen gas form a long lived μp(2S) with a lifetime of the order of 1 μs. The technical realization of our experiment involves a new high intensity low energy muon beam, an efficient low energy muon entrance detector, a randomly triggered 3 stage laser system providing the 0.5 mJ, 7 ns laser pulses at 6.02 μm wavelength, and a combination of a xenon gas proportional scintillation chamber (GPSC) and a microstrip gas chamber (MSGC) with a CsI coated surface to detect the 2 keV X rays from theμp(2P → 1S) transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Einstein Lecture – Passion for precision

Optical frequency combs from mode‐locked femtosecond lasers have link optical and microwave frequencies in a single step, and they provide the long missing clockwork for optical atomic clocks. By extending the limits of time and frequency metrology, they enable new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clock are establishing sensitive limits for possible slow variations of fundamental constants. Optical high harmonic generation is extending frequency comb techniques into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy. Frequency comb techniques are also providing a key to attosecond science by offering control of the electric field of ultrafast laser pulses. In our laboratories at Stanford and Garching, the development of new instruments and techniques for precision laser spectroscopy has long been motivated by the goal of ever higher resolution and measurement accuracy in optical spectroscopy of the simple hydrogen atom which permits unique confrontations between experiment and fundamental theory. This lecture recounts these adventures and the evolution of laser frequency comb techniques from my personal perspective.     

First measurement of the temperature dependence of muon transfer rate from muonic hydrogen atoms to oxygen

We report the first measurement of the temperature dependence of muon transfer rate from muonic hydrogen atoms to oxygen between 100 and 300 K. Data were obtained from the X-ray spectra of delayed events in a gaseous target, made of a H₂/O₂ mixture, exposed to a muon beam. This work sets constraints on theoretical models of muon transfer and is of fundamental importance for the measurement of the hyperfine splitting of muonic hydrogen ground state as proposed by the FAMU collaboration.     

Present status of the investigation on muon transfer in gaseous mixtures of hydrogen and oxygen

To investigate the energy dependence of muon transfer to oxygen, we performed measurements in gaseous mixtures of hydrogen and oxygen. The time distributions of the muonic oxygen X-rays showed the same structure as the one observed earlier in H₂+SO₂ mixtures. In the delayed part of these distributions, one can distinguish a short-time and a long-time component. From the latter, we deduced the transfer rates from thermalized muonic protium, respectively deuterium, to oxygen. The short-time component can be interpreted as being due to muon transfer from epithermal muonic hydrogen atoms. The time parameters are characteristic for the deceleration process as well as for energy-dependent transfer rates. With results of recent research on the formation and the thermalization of muonic hydrogen, we performed Monte Carlo simulations in order to test the hypothesis of an energy-dependent transfer rate.     

高压氘氚靶球的制备与拉曼光谱研究

氢同位素的定量分析与监测在能源与环境领域都有着重要的意义。激光拉曼光谱由于其可以无损分析氢同位素分子,已经成为一种重要的方法,在国际热核聚变实验反应堆(ITER)和美国萨凡纳河工厂得到了广泛应用。利用高压充气装置得到了惯性约束聚变(ICF)高压靶丸,并对靶丸内气体进行原位拉曼光谱测量,通过对高压下氘氚混合气体的拉曼光谱进行分析得到了靶丸内气体的成分比例,验证了靶丸充气工艺参数。实验表明,在CCD的积分时间延长到1 min时,氘(DD),氘氚(DT)和氚(TT)的测量精度可以达到1%,同时对不同时刻靶丸内气体组分的拉曼光谱进行测量,实验结果表明在氘氚渗透和氚衰变两者共同作用下,靶丸内总气体压力随时间不断下降,但是气体组成基本不发生变化。     

Dynamics of muonic atom cascade in hydrogen-helium mixtures

The deexcitation of excited muonic protium and deuterium in the mixture of hydrogen and helium isotopes is considered. Methods of experimental determination of the probability of direct atomic muon capture by hydrogen and muon transfer rates from excited muonic hydrogen to helium are proposed. Theoretical results for the population of the muonic atoms in the ground state, , are compared with the existing experimental data. Results obtained for mixtures are of interest for investigation of nuclear fusion in muonic molecules. Received: 6 August 1998 / Revised: 1st October 1998 / Accepted: 2 October 1998     

The Muonic Hydrogen Lamb Shift Experiment at PSI

A measurement of the 2S Lamb shift in muonic hydrogen (μ⁻p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the energy difference ΔE(2⁵ P _3/2−2³ S _1/2) by laser spectroscopy (λ≈6μm) to a precision of 30 ppm and to deduce the root mean square (rms) proton charge radius with 10⁻³ relative accuracy, 20 times more precise than presently known. An important prerequisite to this experiment is the availability of long-lived μp_2S -atoms. A 2S-lifetime of ∼1 μs – sufficiently long to perform the laser experiment – at H₂ gas pressures of 1–2 hPa was deduced from recent measurements of the collisional 2S-quenching rate. A new low-energy negative muon beam yields an order of magnitude more muon stops in a small low-density gas volume than a conventional cloud muon beam. A stack of ultra-thin carbon foils is the key element of a fast detector for keV-muons. The development of a 2 keV X-ray detector and a 3-stage laser system providing 0.5 mJ laser pulses at 6 μm is on the way. This revised version was published online in August 2006 with corrections to the Cover Date.     

Proton polarizability and lamb shift in the muonic hydrogen atom

The corrections that the structure of the proton and its polarizability induce in the Lamb shift in the muonic and the conventional hydrogen atom are calculated on the basis of up-to-date experimental data on the structure functions in deep-inelastic scattering. Numerically, the contribution from proton polarizability to the 2P-2S shift in the muonic hydrogen atom is 4.4 GHz.     

Hyperfine structure effects in the collisions of the 2s metastable muonic hydrogen with hydrogen atoms

Cross sections for elastic and spin-flip scattering of metastable 2s muonic hydrogen on hydrogen atoms are calculated for the symmetric systems pμ+p, dμ+d and tμ+t. The calculations are performed in the two-level approximation of the adiabatic method with inclusion of the Lamb shift and electron screening. Due to the large polarizability of the 2s state, the obtained cross sections significantly exceed their counterparts for the ground-state muonic hydrogen.     

Advantages and Limitations of Solid Layer Experiments in Muon Catalyzed Fusion

Since the discovery that muonic deuterium at energies near a few eV could travel distances of the order of 1 mm in condensed hydrogen, and in particular that muonic tritium and muonic deuterium could emerge from the surface of a solid hydrogen layer, the advantages of solid targets have enabled the study of several processes important in muon catalyzed fusion. A review of the results is presented, emphasizing the strengths and limitations of the use of solid hydrogen layer targets. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ephemeral and/or coloured nuonic hydrogen atoms

The unexpected and, in conventional terms, yet unexplained experimental results, obtained in systematic measurements of muon transfer from muonic hydrogen to sulphur and oxygen of sulphur dioxide, seem to violate the principle of reproducibility of muon transfer data. With the hypothesis of ephemeral muonic hydrogen atoms, the number of different hydrogen atoms can be reduced from four to two. This hypothesis does, however, not help to interpret the transfer data to helium, neon and argon, where the muonic hydrogen atoms seem to wear colours.     

Differential cross sections for muonic hydrogen scattering on hydrogen molecules

The results of first calculations of the differential cross sections for muonic hydrogen scattering on hydrogen molecules are presented. They are functions of the initial and final kinetic energy of the system and the scattering angle. These calculations are based on the respective set of cross sections for muonic hydrogen scattering on hydrogen nuclei, obtained within the framework of the adiabatic method. The Fermi pseudopotential method is used to estimate the molecular binding effects. The influence of electrons on the cross sections under consideration is described in terms of the effective screening potential. Rotational and vibrational transitions are taken into account. The calculated molecular differential cross sections show a strong angular dependence. This effect is very significant for the electronic contributions to the cross sections, e.g. for collision energies above approximately 0.1 eV only the cross sections of small scattering angles are influenced considerably by the screening. Since these differential cross sections give detailed information about the final energies and complicated angular distributions of the scattered muonic atoms they are the proper basis for calculations concerning the deceleration of muonic hydrogen atoms in molecular hydrogen targets and for Monte Carlo simulations of different experiments in muonic physics.