Electromagnetic polarizabilities of the nucleon and properties of the σ-meson pole contribution

The t-channel contribution to the difference of electromagnetic polarizabilities of the nucleon, (α - β)^t, can be quantitatively understood in terms of a σ-meson pole in the complex t-plane of the invariant scattering amplitude A ₁(s, t) with properties of the σ-meson as given by the quark-level Nambu-Jona-Lasinio model (NJL). Equivalently, this quantity may be understood in terms of a cut in the complex t-plane where the properties of the σ-meson are taken from the ππ → σ → ππ, γγ → σ → ππ and Nˉ → σ → ππ reactions. This equivalence may be understood as a sum rule where the properties of the σ-meson as predicted by the NJL model are related to the f ₀(600) particle observed in the three reactions. In the following, we describe details of the derivation of (α - β)^t making use of predictions of the quark-level NJL model for the σ-meson mass. An erratum to this article is available at .     

COMPASS and HERMES contributions to our understanding of the nucleon spin

An update is given on the ongoing experimental investigation of the spin structure of the nucleon, with particular emphasis on the results from the COMPASS and HERMES experiments. Both longitudinal and transverse spin phenomena are covered. In the first case, the hot topic is the direct measurement of the gluon polarization. Evidence is presented for ΔG/G being small around x_g?0.1, and the first moment of ΔG should not be larger than 0.2-0.3. About transverse spin effects, evidence is given for new phenomena, associated with transverse-momentum-dependent distribution and fragmentation functions, which might explain the transverse spin phenomena observed for a long time in pp scattering.     

QCD spin physics: Status and prospects for relativistic heavy-ion collider

We review some of the recent developments in QCD spin physics and highlight the spin physics program now underway at RHIC.     

Electric and magnetic polarizabilities of nucleon and QCD quark models

The experimental and theoretical work carried out on electric and magnetic polarizabilities of a nucleon is reviewed. The results of an exactly solvable one-dimensional chiral bag model predicts correct signs and order of magnitudes of polarizabilities supporting the approximations used in their realistic calculations.     

Challenges in uncovering the origin of the proton's spin

One of the most fascinating challenges facing modern strong interaction physics is to understand the origin of the spin of the nucleon in terms of the spin and orbital angular momentum of the quarks and gluons. We review recent progress on this problem as well as some of the uncertainties associated with state of the art lattice QCD simulations. In particular, we explain the importance of the corrections associated with chiral extrapolation and finite volume corrections, especially for the term B(0) extracted from the appropriate low moment of the deeply virtual Compton scattering amplitude.     

Challenges in uncovering the origin of the proton's spin

One of the most fascinating challenges facing modern strong interaction physics is to understand the origin of the spin of the nucleon in terms of the spin and orbital angular momentum of the quarks and gluons.We review recent progress on this problem as well as some of the uncertainties associated with state of the art lattice QCD simulations.In particular,we explain the importance of the corrections associated with chiral extrapolation and finite volume corrections,especially for the term B（0） extracted from the appropriate low moment of the deeply virtual Compton scattering amplitude.     

Contributions of qqqq(q) components to nucleon spin structure

The spin structure of nucleons is presented in the framework of an extended quark model which in addition to the conventional qqq structure also takes into account qqqq(q) admixtures in the nucleon wave functions, where the qqqq(q) components are in colored quark cluster configurations. The axial vector weak coupling constant and spin distributions for polarized nucleons as well as spin content are obtained for the lowest positive parity qqqq(q) configurations in flavor-spin dependent interaction. In particular, the contributions of the down and strange quarks to the proton spin and the sum rule for polarized neutron are negative, in agreement with recent experiments.     

核子白旋结构的实验研究进展

本文主要介绍对核子纵向自旋结构函数的实验研究进展。首先简要介绍核子自旋结构的部分理论模型,包括朴素的部份子模型和QCD中的夸克一部份子模型,同时简要介绍标度律和标度律的破坏及其原因;接着介绍实验研究的理论基础,包括弱作用和轻子一核子的深度非弹散射中轴流的作用和几个关于核子自旋结构的求和规则和它们的QCD修正;且简要介绍深度非弹散射实验的研究方法,包括单举测量、半单举和遍举测量。最后,详细介绍实验研究进展,包括对质子和。中子的纵向自旋结构函数的测量,国际上几个主要实验室在不同的能量下,在用不同的靶、不同的束流对不同范围的Bjorken变量x和不同的四动量转移范围Q2下的实验、特点及其结果。最后简略介绍了我国实验物理工作者在该领域的国际合作组的部分工作。     

Strangeness in the nucleon: Newest results from Happex and G0

We present an introduction to accurate measurements of the weak neutral current in the elastic scattering of longitudinally polarized electrons off hadronic targets, and show how they can provide new and stringent limits on the contribution of strange quarks to the nucleon form factors. Such a contribution is of particular interest for the study of nucleon structure, since strange quarks do not contribute to the valence state and give us a direct insight into the dynamics of quark-antiquark fluctuations. We discuss the latest published results of Happex and G0 experiments as well as the perspectives of high precision measurements of parity violating asymmetries.     

Quark-hadron duality of nucleon spin structure function

Bloom-Gilman quark-hadron duality of nuclear spin structure function is studied by comparing the integral of g₁ from perturbative QCD prediction in the scaling region to the moment of g₁ in the resonance region. The spin structure function in the resonance region is estimated by the parametrization forms of non-resonance background and of resonance contributions. The uncertainties of our calculations due to those parametrization forms are discussed. Moreover, the effect of the (1232)-resonance in the first resonance region and the role of the resonances in the second resonance region are explicitly shown. Elastic peak contribution to the duality is also analyzed.     

Low-energy Compton scattering and the polarizabilities of the proton

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θ^lab _γ = 59^°-155^° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5_stat.±1.3_syst.±0.3_mod.] ^. 10^-4 fm ³ and = [1.2±0.7_stat.±0.3_syst.±0.4_mod.)] ^. 10^-4 fm ³. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] ^. 10^-4 fm ³. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γ_π beyond the t-channel π⁰-exchange. Received: 9 January 2001 / Accepted: 13 February 2001     

Measurement of the generalized polarizabilities of the proton in virtual Compton scattering at MAMI

Virtual Compton scattering off the proton has been studied at Q ² = 0.33 (GeV/c)² at the MAMI accelerator in Mainz (Germany). The goal of the experiment is to measure the generalized polarizabilities (GPs) of the proton using the double polarized e p→e' p'γ reaction. This paper reports the measurement of the unpolarized photon electroproduction cross-section for the new data and the extraction of two linear combinations of GPs, which are essential for the analysis of the double spin asymmetry.     

Quasi-free Compton scattering and the polarizabilities of the neutron

Differential cross-sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz [48]cm ;SPMOslash; × [64]cm NaI(Tl) photon detector and the G?ttingen SENECA recoil detector. The data cover photon energies ranging from [200]MeV to [400]MeV at θ^LAB _γ = 136.2^°. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction p(γ,π⁺ n). The “free” proton Compton scattering cross-sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross-section for free scattering from quasi-free data. Differential cross-sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron has been determined to be α_n - β_n = 9.8±3.6(stat)^+2.1 _-1.1(syst)±2.2(model) in units of [10^-4]fm ³. In combination with the polarizability sum α_n + β_n = 15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, α_n = 12.5±1.8(stat)^+1.1 _-0.6(syst)±1.1(model) and β_n = 2.7±1.8(stat)^+0.6 _-1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ⁽ⁿ⁾ _π = (58.6±4.0)×10^-4 fm ⁴. Received: 21 August 2002 / Accepted: 16 October 2002 / Published online: 11 February 2003 RID="a" ID="a"Part of the Doctoral Thesis. RID="b" ID="b"Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig. RID="a" ID="a"Part of the Doctoral Thesis. RID="b" ID="b"Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig. RID="c" ID="c"Part of the Habilitation Thesis. RID="d" ID="d"e-mail: schumacher@physik2.uni-goettingen.de Communicated by Th. Walcher     

碱金属原子多极极化率的解析计算及其应用

通过考虑碱金属原子中的价电子在模型势中的运动, 给出了碱金属原子激发态(包括分立谱和连续谱)的波函数, 导出了碱金属原子的多极动态极化率的计算式中所涉及的矩阵元的解析表达式, 实现了碱金属原子多极动态极化率的解析计算. 作为应用, 利用范德瓦尔斯相互作用系数与动态极化率之间的积分关系, 计算了异核基态碱金属原子间的三体相互作用系数, 将计算结果与此前用稳定变分方法得到的结果进行了比较, 结果显示两者是一致的. 此工作为后续研究激发态碱金属原子间的相互作用奠定了基础.     

Dispersion relation formalism for virtual Compton scattering off the proton

We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into the Δ(1232)-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the Δ(1232)-resonance show an enhanced sensitivity to the generalized polarizabilities. Received: 14 March 2001 / Accepted: 18 June 2001     

Calculations of the dynamic dipole polarizabilities for the Li~+ ion

The B-spline configuration-interaction method is applied to the investigations of dynamic dipole polarizabilities for the four lowest triplet states(2~3S,3~3S,2~3P,and 3~3P) of the Li~+ ion.The accurate energies for the triplet states of n~3S,n~3P,and n~3D,the dipole oscillator strengths for 2~3S(3~3S)→n~3P,2~3P(3~3P)→n~3S,and 2~3P(3~3P)→n~3D transitions,with the main quantum number n up to 10 are tabulated for references.The dynamic dipole polarizabilities for the four triplet states under a wide range of photon energy are also listed,which provide input data for analyzing the Stark shift of the Li~+ ion.Furthermore,the tune-out wavelengths in the range from 100 nm to 1.2 μm for the four triplet states,and the magic wavelengths in the range from 100 nm to 600 nm for the 2~3S→3~3S,2~3S→2~3P,and 2~3S→3~3P transitions are determined accurately for the experimental design of the Li~+ ion.     

Cold plasma dispersion relations in the vicinity of a Schwarzschild black hole horizon

We apply the ADM 3 + 1 formalism to derive the general relativistic magnetohydrodynamic equations for cold plasma in spatially flat Schwarzschild metric. Respective perturbed equations are linearized for non-magnetized and magnetized plasmas both in non-rotating and rotating backgrounds. These are then Fourier analyzed and the corresponding dispersion relations are obtained. These relations are discussed for the existence of waves with positive angular frequency in the region near the horizon. Our results support the fact that no information can be extracted from the Schwarzschild black hole. It is concluded that negative phase velocity propagates in the rotating background whether the black hole is rotating or non-rotating.     

Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions,δs(x) = s(x) ˉs(x),and the combination of light quark sea and strange quark sea,Δ(x) =ˉ d(x) + uˉ(x) s(x) ˉs(x),are originated from non-perturbative processes and can be calculated using non-perturbative models of the nucleon.We report calculations of δs(x) and Δ(x) using the meson cloud model.Combining our calculations of Δ(x) with relatively well known light antiquark distributions obtained from global analysis of available experimental data,we estimate the total strange sea distributions of the nucleon.     

Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions,δs（x） = s（x） ˉs（x）,and the combination of light quark sea and strange quark sea,Δ（x） =ˉ d（x） ＋ uˉ（x） s（x） ˉs（x）,are originated from non-perturbative processes and can be calculated using non-perturbative models of the nucleon.We report calculations of δs（x） and Δ（x） using the meson cloud model.Combining our calculations of Δ（x） with relatively well known light antiquark distributions obtained from global analysis of available experimental data,we estimate the total strange sea distributions of the nucleon.