Generalized parton distributions

We discuss how generalized parton distributions (GPDs) enter in a variety of hard exclusive processes such as deeply virtual Compton scattering (DVCS) and hard meson electroproduction reactions on the nucleon. We show some key observables which are sensitive to the various hadron structure aspects of the GPDs, and discuss their experimental status.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Fz Elastic and Compton scattering - 13.60.Le Meson production - 12.38.Bx Perturbative calculations     

Compton energy-absorption scattering cross-sections for H,C, N,O, P,Ca and assessment of Doppler broadening

Total Compton, individual shell and Compton energy-absorption scattering cross-sections are evaluated in the energy region 0.005 to 10 MeV for H, C, N, O, P and Ca. Compton energy absorption cross-sections deviate numerically with available values. The cause of the numerical discrepancies are not fully understood but can be attributed to Doppler broadening of the Compton scattered photons through a given angle. Received 25 June 2002 / Received in final form 27 September 2002 Published online 3rd April 2003 RID="a" ID="a"e-mail: dvrao@ssmain.uniss.it     

Spin polarizabilities of the nucleon from polarized low-energy Compton scattering

As a guideline for forthcoming experiments, we present predictions from Chiral Effective Field Theory for polarized cross-sections in low-energy Compton scattering for photon energies below 170 MeV, both on the proton and on the neutron. Special interest is put on the role of the nucleon spin polarizabilities which can be examined especially well in polarized Compton scattering. We present a model-independent way to extract their energy dependence and static values from experiment, interpreting our findings also in terms of the low-energy effective degrees of freedom inside the nucleon: The polarizabilities are dominated by chiral dynamics from the pion cloud, except for resonant multipoles, where contributions of the -resonance turn out to be crucial. We therefore include it as an explicit degree of freedom. We also identify some experimental settings which are particularly sensitive to the spin polarizabilities.Received: 19 August 2003, Revised: 30 October 2003, Published online: 27 April 2004PACS: 13.40.-f Electromagnetic processes and properties - 13.60.Fz Elastic and Compton scattering - 14.20.Dh Protons and neutronsH.W. Grießhammer: Permanent address: Technische Universität MünchenT.R. Hemmert: Permanent address: Technische Universität München     

HERMES impact for the access of Compton form factors

We utilize the DVCS asymmetry measurements of the HERMES collaboration for access to Compton form factors in the deeply virtual regime and to generalized parton distributions. In particular, the (almost) complete measurement of DVCS observables allows us to map various asymmetries into the space of Compton form factors, where we still rely in this analysis on dominance of twist-two associated Compton form factors. We compare this one-to-one map with local Compton form factor fits and a model dependent global fit.     

Black-Hole Thermodynamics and Electromagnetism

We show a strong parallel between the Hawking-Bekenstein black-hole thermodynamics and electromagnetism: When the gravitational coupling constant transforms into the electromagnetic coupling constant, the Schwarzchild radius, the Bekenstein temperature, the Bekenstein decay time and the Planck mass transform to respectively the Compton wavelength, the Hagedorn temperature, the Compton time and a typical elementary particle mass. The reasons underlying this parallalism are then discussed in detail.     

Nucleon polarizabilities in real and virtual Compton scattering: Recent theoretical issues

We review recent developments in the theoretical investigation of the nucleon polarizabilities. We first report on the static polarizabilities as measured in real Compton scattering, comparing and interpreting the results from various theoretical approaches. In a second step, we extend the discussion to the generalized polarizabilities which can be accessed in virtual Compton scattering, showing how the information encoded in these quantities can provide a spatial interpretation of the induced polarization densities in the nucleon.     

Virtual versus real nuclear Compton scattering in the Δ(1232) region

In this paper we calculate the cross section for Virtual Compton Scattering off nuclei in the delta resonance region. We also calculate the background for the process from Coherent Bremsstrahlung in nuclei and explore the regions where the Virtual Compton Scattering cross section dominates. The study also shows that it is possible to extract the cross section for Real Compton Scattering from the Virtual Compton one in a wide range of scattering angles.     

Perturbative and non-perturbative QCD corrections to wide-angle Compton scattering

We investigate corrections to the handbag approach for wide-angle Compton scattering off protons at moderately large momentum transfer: the photon–parton subprocess is calculated to next-to-leading order in and contributions from the generalized parton distribution E are taken into account. Photon and proton helicity flip amplitudes are non-zero due to these corrections, which leads to a wealth of polarization phenomena in Compton scattering. Thus, for instance, the incoming photon asymmetry or the transverse polarization of the proton is non-zero, although small. Received: 31 October 2001 / Revised version: 11 December 2001 / Published online: 8 February 2002     

浅析影响康普顿谱线位置的因素

康普顿散射中影响康普顿谱线位置的因素是十分复杂的,文章主要针对影响康普顿谱线位置的几个因素作了理论探讨,得出影响因素主要在于:电子具有初速度会增大散射波长的改变量;束缚能的存在要减小峰值波长的改变量;双光子散射和二次散射与谱线峰值位置的关系不大,二者对峰值波长的改变是不确定的、复杂的、在连续的范围内变化的.最后对康普顿谱线位置和康普顿轮廓的理论研究和应用前景作了展望.     

Compton scattering of 59.54 keV γ‐rays from Fe and p‐Si samples in an external magnetic field

We report on the Compton scattering of photons from samples whose surface charge density distributions are changed by an external magnetic field. We performed a Compton scattering experiment known to be particularly sensitive to the behavior of the relatively slower moving outer electrons (valence electrons) involved in bonding in condensed matter. The external magnetic field was used to change the surface charge density distributions of Fe and p‐Si samples. Samples were located in the external magnetic field of intensity 215 G and in a direction which was perpendicular both to the current and surface of the samples bombarded by 59.5 keV γ‐photons emitted from an Am‐241 point source. Currents in the ranges 0–8.5 A and 0–300 µA were applied to the Fe and p‐Si samples, respectively. The Compton scattered photons at an angle of 100° were detected by an Si(Li) detector. It was observed that the counts acquired under the Compton peaks tended to decrease linearly with increasing current in a magnetic field. The results show that positive charge carriers behave like negative charge carriers and electrons are more effective than holes in the Compton scattering of γ‐rays. Copyright © 2003 John Wiley & Sons, Ltd.     

Sensitivity of the DVCS Cross Section to the Compton form Factors in Scalar QED

Using deeply virtual Compton scattering as a tool to study the structure of hadrons in an exclusive process, one expresses the amplitudes in terms of invariant quantities: the Compton form factors. In this paper the sensitivity of the hadronic part of the cross section to the Compton form factors is determined.     

Verification of terahertz-wave spectrophotometry by Compton backscattering of coherent synchrotron radiation

We developed a continuous-spectrum light beam from Compton backscattering by using coherent synchrotron radiation in an L-band linac at the Kyoto University Research Reactor Institute. The ratio of Compton backscattered photons to background photons when coherent synchrotron radiation was used was three times larger than when coherent transition radiation was used. The transmission spectrum of a polystyrene film in the terahertz-wave region was evaluated by measuring the spectrum of the Compton backscattered photons and it roughly agreed with that measured by a Martin–Puplett-type interferometer. The spectrophotometry using Compton backscattering shows promise as a new tool for investigations in terahertz-wave science.     

Prüfung der Quantenelektrodynamik durch Compton-Streuung und Paarvernichtung

In view of the present experimental efforts, Compton scattering and pair annihilation in flight are analysed as purely electrodynamic processes to test quantum electrodynamics at small distances. After a short discussion of the introduction of the form factors we consider the possibility of a (partial) cancellation between elctrodynamic and nucleon structure effects. There is also the possibility of a partial cancellation of the non local effects in Compton scattering and wide angle bremsstrahlung because of the opposite action of spacelike and timelike form factor arguments. We calculate the Compton and the two photon pair annihilation cross sections with form factors and illustrate the result with several numerical diagrams. Two photon pair annihilation turns out to be especially favourable for a first test of quantum electrodynamics because there is no cancellation of possible non local effects and the necessary very high energy can be circumvented by a colliding beam experiment. With a 10% experimental error the planned colliding beam experiment at Stanford with 500 MeV could test the electron propagation function down to 0.6 · 10^?14cm. After that, Compton scattering can be used to investigate the electron propagation function also for time-like arguments.     

Towards a Microscopic Understanding of Nucleon Polarizabilities

We outline a microscopic framework to calculate nucleon Compton scattering from the level of quarks and gluons within the covariant Faddeev approach. We explain the connection with hadronic expansions of the Compton scattering amplitude and discuss the obstacles in maintaining electromagnetic gauge invariance. Finally we give preliminary results for the nucleon polarizabilities.     

偏振X射线荧光分析尾矿薄层样品的散射校正方法研究

了解尾矿浆中的重金属元素含量能为矿物浮选提供决策依据,不仅可以提高矿物的利用率,还可减少环境污染。X射线荧光光谱法是一种常用的重金属元素分析技术,对于地质类样品的分析,康普顿散射内标法是一种常用的定量方法。但对于薄层沉积样品,其康普顿散射峰强度会受到支撑滤膜的散射影响。由于样品紧密附着在支撑滤膜上,难以直接获得来自样品本身的康普顿散射强度,不利于直接应用康普顿散射峰强度进行定量分析。以尾矿薄层样品为分析对象,研究了不同聚丙烯滤膜厚度对康普顿散射峰强度的影响,并对薄层样品的康普顿散射强度进行了校正。实验结果表明,在0.34～3.06 mm厚度范围内,康普顿散射峰强度随聚丙烯滤膜厚度的增加线性增加,通过建立探测器获得的总康普顿散射强度与滤膜厚度的线性关系,计算出样品的净康普顿散射峰强度。为验证该修正方法的可靠性,利用蒙特卡洛方法模拟研究了无滤膜的尾矿样品和带有不同厚度滤膜的尾矿样品,结果显示经滤膜厚度影响修正后的净康普顿散射峰强度与无滤膜样品康普顿散射峰强度基本一致,相对偏差为0.41%。同时通过实验和模拟计算了0.34 mm厚聚丙烯滤膜时修正后的净康普顿散射峰强度占总康普顿散射峰的比例,分别为91.31%和89.91%,二者基本一致。最后,利用了上述基于滤膜厚度康普顿散射影响的校正方法,建立了基于康普顿散射内标法的定量校准曲线,对两种尾矿浆中的Cu,Zn和Pb元素的定量分析结果显示,未经滤膜厚度修正的康普顿内标校正相比校正前,部分元素定量结果与ICP-OES结果相比,其相对偏差反而增加3.18%～9.00%。而经滤膜厚度修正的康普顿内标方法的定量结果与ICP-OES结果的相对误差在1.14%～11.15%之间,相比于校正之前,相对偏差减少了0.30%～8.97%。     

Renormalisation and mass singularities

We emphasize the role of renormalization in the consistent treatment and identification of mass singularities. In this connection we present the treatment of a completely massless process: Compton scattering of massless electrons, to lowest non-trivial order in perturbation theory, including the calculation of all non-leading singularities.     

Observation of ionic deformation and bonding from compton profiles

We describe a new method of interpreting Compton profile data using a function B(r) which is the autocorrelation function of the one-electron wave function Ψ(r). In any particular direction B(r) can be obtained by a one-dimensional Fourier transform of the corresponding directional Compton profile. The technique is applied to LiF and it provides a means of determining the ionic radii, as well as locating some inadequacies in the tight-binding overlap model for ionic crystals.