Scaling tests of the cross section for deeply virtual Compton scattering

We present the first measurements of the e[over -->]p-->epgamma cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q(2) dependence (from 1.5 to 2.3 GeV(2)) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q(2). The helicity-independent cross section is also measured at Q(2)=2.3 GeV(2). We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.     

Radiation transport in a spherically symmetric hot plasma

An analytical solution is obtained for the problem of radiation transport in a spherically symmetric plasma. The ions are assumed to be in a complete steady state with constant ion density and electron temperature. The radiation density is assumed to be small so that the rate of the radiative processes in the plasma is small relative to that of the collisional processes, but not negligible. The effect of the plasma on the radiation density, as well as the influence of the radiation on the population probabilities, are properly accounted for. Under these conditions explicit expressions are given, valid to the first order in the plasma dimensions, for the radiation density and the population probability of the ionic states.     

Perturbative calculation of the cross section in double ionization by high-energy Compton scattering

In this paper we investigate double ionization in high-energy Compton scattering from the He-atom including both the shake-off mechanism and a perturbative correction to that mechanism. The correction is calculated in second-order perturbation theory and includes Coulomb electron-electron interaction in addition to the correlation in the ground state of the He-atom. Our calculations for the ratio of double to single cross section cover the range from 30 to 300 keV of impact photon energy and explain the slow convergence of the ratio towards the asymptotic value.     

The total virtual photoabsorption cross section,deeply virtual Compton scattering and vector-meson production

Based on the generic two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low , we stress the intimate direct connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA. Received: 16 February 2004, Revised: 16 July 2004, Published online: 2 September 2004 Supported by Deutsche Forschungsgemeinschaft, contract number schi 189/6-2. An erratum to this article is available at .     

Influence of quantum wire cross section fluctuations on transport phenomena

For all currently available methods of quantum wire (QWI) fabrication the cross section of the QWI varies along the length, producing fluctuations in the energy position of the subband energy along the QWI. This fluctuation leads to a smearing of the peak-like structure of the dependence of the average density of states on carrier energy. This, in turn, results in a smearing of the peak-like structure of the dependence of the resistivity of a QWI on the Fermi energy and/or total electron concentration, and in an increase of resistivity. In previous attempts to obtain agreement between theory and experiment the subband energy fluctuation was not taken into account, that is why the importance of other factors, such as impurity scattering, have been overestimated.     

Compton散射下等离子体球太赫兹散射特性

为了研究Compton散射对等离子体球太赫兹(THz)散射特性的影响,将多光子非线性Compton散射模型引入到该研究中,提出了将Compton散射作为影响等离子体球太赫兹散射的重要因素,给出了各向异性等离子体球散射场修正方程及介电常数张量、观测方位、极化状态等因素对THz波散射的影响,并进行了仿真实验。结果表明:微分散射截面随THz波频率变化近乎呈准谐振规律变化,较散射前增大了0.001;微分散射截面随观测方位和极化状态的变化较散射前增大了0.002。     

Relationship between the cross section for deep-inelastic scattering and the cross section for inclusive annihilation

Arguments have been presented that suggest that available experimental data on the structure functions for the processes ep → eX and e ⁺ e ^? → p( $\bar p$ )X do not confirm the so-called reciprocity relation, which was obtained in the leading-logarithm approximation of perturbation theory. It has also been shown that the asymptotic relationship between the above processes that was obtained on the basis of more general considerations is compatible with existing data.     

Reflection coefficient of low-energy ions as a universal function of the scaled transport cross section

Multiple collision theory of heavy ion ranges in an infinite medium has been used to calculate the reflection coefficient from the penetration profile, by a method first described by Bottiger et al. Interaction with target electrons has been neglected and nuclear collisions have been described by the power cross sections. Calculations are restricted to low reduced ion energies )₀А.1 and to target/ion mass ratios 7⢪, when electronic stopping is much less than the nuclear stopping. The Gaussian approximation of the penetration profile and the reflection coefficient are found in a form of simple analytic formulas. Furthermore, the penetration profile was constructed by using Edgeworth expansion and moments of the distribution up to the fourth order, and the reflection coefficient was determined from the profile. Good agreement between the analytical results and the reflection coefficient obtained from Edgeworth expansion was found, for target atom/ion mass ratios 7̿.3. It is shown that the reflection coefficient is a universal function of the scaled transport cross section. The scaling is fulfilled for v̀, when ion reflection is determined by large-angle multiple collisions, and breaks down for v<1 i.e. 7<1, when scattering angles are small. Results are compared with TRIM computer simulation data.     

等离子体用于腔体的雷达散射截面减缩的实验研究

利用荧光灯等离子体管覆盖金属腔体内壁,测量了筒状等离子体覆盖的金属腔体对电磁波的回波衰减。测试结果表明:金属圆筒内壁的等离子体能够有效吸收0.80~1.75 GHz波段的入射波,入射电磁波的回波衰减值为5~25 dB;当入射电磁波频率接近1.75 GHz时,回波衰减最为强烈,在入射角度为10°时,吸收峰值可达26.71 dB,因此选择合适的电磁波入射角度,能够使等离子体对微波的吸收达到峰值。理论分析了影响等离子体对电磁波衰减的主要因素。结果表明：等离子体可有效缩减腔体结构的雷达散射截面面积,因此在进气道等腔体结构的隐身方面具有一定的应用前景。     

等离子体用于腔体的雷达散射截面减缩的实验研究

 利用荧光灯等离子体管覆盖金属腔体内壁,测量了筒状等离子体覆盖的金属腔体对电磁波的回波衰减。测试结果表明:金属圆筒内壁的等离子体能够有效吸收0.80~1.75 GHz波段的入射波,入射电磁波的回波衰减值为5~25 dB;当入射电磁波频率接近1.75 GHz时,回波衰减最为强烈,在入射角度为10°时,吸收峰值可达26.71 dB,因此选择合适的电磁波入射角度,能够使等离子体对微波的吸收达到峰值。理论分析了影响等离子体对电磁波衰减的主要因素。结果表明：等离子体可有效缩减腔体结构的雷达散射截面面积,因此在进气道等腔体结构的隐身方面具有一定的应用前景。     

Magnetic trap for confining a hot plasma

The magnetic field of a two-helix Tornado trap has all the properties needed to efficiently confine a hot plasma. However, its practical utilization has been restricted because of the ponderomotive interaction between the turns of the helices, which disrupts the structure of the magnetic field. A modification of a Tornado trap, which permits significant reduction of the ponderomotive interaction is considered, and arguments in favor of using a magnetic field to maintain the properties of the field in an unmodified trap are presented. Zh. Tekh. Fiz. 67, 25–28 (September 1997)     

Scattering and absorption differential cross sections for double photon Compton scattering

The scattering and absorption differential cross sections for nonlinear QED process such as double photon Compton scattering have been measured as a function of independent final photon energy. The incident gamma photons are of 0.662 MeV in energy as produced by an 8 Ci¹³⁷Cs radioactive source and thin aluminum foils are used as scatterer. The two simultaneously emitted photons in this process are detected in coincidence using two Nal(T1) scintillation detectors and a slow-fast coincidence set-up of 30 nsec resolving time. The measured values of scattering and absorption differential cross sections agree with theory within experimental estimated error.     

Steady-state differential cross section for elastic scattering

Lower limits are found on the differential cross section for elastic scattering and on its derivatives with respect to the momentum transfer t at t = 0 at high energies on the basis of completely general considerations. The results are expressed in terms of the measurable total cross sections.Translated from Izvestiya VUZ. Fizika, No. 4, pp. 103–108, April, 1970.     

Notes on number density fluctuations and the scattering cross section for electromagnetic waves scattered from a thermal nonequilibrium plasma

The scattering cross section and the Doppler spectrum for electromagnetic waves scattered by the electron density fluctuations of a plasma, where the mean kinetic temperature of the electronsT _e may differ from that of the ionsT _i , has been obtained among others byFejer, Buneman, Renau, Camnitz andFlood, andSalpeter. These authors use different methods of approach to calculate the autocovariance of the electron number-density fluctuations (from the mean) and then obtain the scattering cross section. Because of the differing results, the methods, concepts, and derivations of the scattering cross section are carefully examined in this paper. It is shown that the short-time dynamical considerations incorporated in the formulation of the statistical theory of the electron number-density fluctuations of the plasma as used by several authors (for instanceFejer, Buneman, Salpeter,) leads to results of limited validity. In addition, a fundamental error in calculating the electron density fluctuations leads these latter authors to an incorrect scattering cross section. The theory of scattering of electromagnetic waves from a plasma, where the electrons arenot in thermal equilibrium with the ions but statistical equilibrium exists, is developed in a general way. The covariance of the number-density fluctuations from the mean of the charged species of the plasma and the scattering cross sectionσ(q) are obtained. In particular it is shown that for a wavelength λ much greater than the effective Debye lengthd, the backscattering cross section increases and approaches complete incoherent scattering asT _e /T _i increases. This result is explained by noting that in the case of thermal equilibrium, the predicted value of the back-scattering cross section is smaller than that of the backscattering cross section from completely uncorrelated electron density fluctuations because the electrostatic interaction between the charged particles of the plasma, which is a function ofT _e andT _i , introduces a certain amount of organization in otherwise completely uncorrelated electron density fluctuations. When the mean temperature of the electrons increases relative to that of the ions, the organization introduced in the fluctuations diminishes because of the increasing thermal agitation of the electrons relative to that of the ions, and the backscattering process approaches that of incoherent backscattering (Thomson-type scattering). The spectrum function of incoherent scattering of electromagnetic waves from a nonequilibrium plasma is obtained and some cases of current interest are plotted.     

Backscattering cross section of a rigid biconic reflector

Plane wave incidence on a rigid biconic target is considered. A biconic reflector consists of two cones truncated by planes perpendicular to their axes and joined at their smaller flat faces. The cone angles are allowed to be variable, provided their sum is equal to 90 degrees. The backscattering cross section is expressed in terms of a surface integral of the geometrical acoustics field, which results from incident singly and doubly reflected rays. A saddle-point calculation gives a first-order high-frequency approximation in which the backscattering cross section is proportional to the incident wave number and a function of the angle of incidence, cone angles, and inner and outer radii of the truncated cones. This expression is algebraically complex but easy to implement numerically. Results are presented that exercise the parameters of the problem. An interesting result of the solution is that for fixed outer radii there is a nonzero optimum inner radius for backscattering strength. For broadside incidence on 45 degrees cones with equal outer radii, this optimum value is approximately equal to 11% of the outer radius.     

电极位置和截面尺寸对分子器件输运性质的调控

研究表明分子器件的性能受器件结构搭建精度影响,分子与电极接触构型的微弱变化可能引起电输运特性较大差异.本文运用密度泛函理论和非平衡格林函数相结合的方法,研究了由金纳米线与benzene-1,4-dithiol（BDT）形成的分子结的电输运性质.通过对不同的Au-BDT接触构型输运性质的研究,发现当两电极处于对位构型时,有较好的电荷输运行为,而且比较符合制备工艺要求;当电极偏离轴线的角度不大于5°,且电极散射截面尺寸不小于4×4时,该分子结体系的电导和透射谱均比较稳定.电极截面尺寸小于4×4或者电极偏离轴线的夹角大于5°时,透射谱在费米能级附近出现不连续现象,导致体系电导降低.较小电极截面尺寸或者电极以较大角度偏离轴线将导致该分子结体系电导降低和透射谱连续性降低,主要是组成电极的金原子轨道与苯基分子轨道耦合缺失造成的.该研究为Au-BDT-Au体系设计和制备过程中电极的位置及电极截面尺寸做了科学的界定.     

Partial cross sections in a plasma

The collision integral for a plasma obtained in the ring approximation with a preliminary ladder renormalization is used to determine the first two partial cross sections. Numerical calculations show that the partial cross sections display a very nonlinear behavior (in logarithmic scale) and do not diverge at small values of the plasma parameter.