Two-photon-exchange effects in the electroexcitation of the delta resonance

We examine the two-photon exchange contribution to the eN --> edelta(1232) --> epiN process with the aim of a precision study of the ratios of electric quadrupole (E2) and Coulomb quadrupole (C2) to the magnetic dipole (M1) gamma*Ndelta transitions. We relate the two-photon exchange amplitude to the N --> delta generalized parton distributions and obtain a quantitative estimate of the two-photon effects. The two-photon exchange corrections to the C2/M1 ratio depend strongly on whether this quantity is obtained from an interference cross section or from the Rosenbluth-type cross sections, in similarity with the elastic, eN --> eN, process.     

Two-photon physics

A brief discussion is given on the status of two-photon exchange contributions to elastic electron-nucleon scattering. The apparent discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments is discussed, as well as the understanding of this puzzle in terms of two-photon exchange corrections. It is also discussed how the imaginary part of the two-photon exchange amplitude can be accessed from the beam normal spin asymmetry in elastic electron-nucleon scattering. Correspondence: M. Vanderhaeghen, Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA     

A parton shower model for hadronic two-photon process in e+e− scattering

A new model of QCD parton shower is proposed which is dedicated to two-photon processes in e⁺e^? scattering. When hadron jets are produced, the photon may resolve into quark-antiquark pairs so that the structure functions of the photon should be introduced. Based on the Altarelli-Parisi equation for these functions, an algorithm is formulated that allows us to construct a model for parton showers for the photon. Our model consists of two parts, one of which describes the deep inelastic scattering of the photon and the other one the scattering of two quasi-real photons. Using the model some results are presented on parton distributions and jet production.     

Two-Boson Exchange Physics: A Brief Review

Current status of the two-boson exchange contributions to the elastic electron–proton scattering, both for parity conserving and parity-violating, is briefly reviewed. How the discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments can be understood, in large part, by the two-photon exchange corrections is discussed. We also illustrate how the measurement of the ratio between positron–proton and electron–proton scattering can be used to differentiate different models of two-photon exchange. For the parity-violating electron–proton scattering, the interest is on how the two-boson exchange (TBE), γ Z-exchange in particular, could affect the extraction of the long-sought strangeness form factors. Various calculations all indicate that the magnitudes of effect of TBE on the extraction of strangeness form factors is small, though can be large percentage-wise in certain kinematics.     

Delta resonance contribution to two-photon exchange in electron-proton scattering

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Delta resonance. The Delta two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Delta two-photon exchange corrections has an angular dependence compatible with both the polarization-transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.     

Compilation and analysis of charge asymmetry measurements from electron and positron scattering on nucleon and nuclei

World data on the lepton-charge asymmetry in the elastic and inelastic lepton scattering off the proton and nuclei are compiled and discussed. After reviewing the published results, we compare the elastic data to a model calculation of the two-photon-exchange mechanism. We show that the existing data do not provide any evidence for enhancement of the two-photon contribution. At confidence level 95% the data allow to exclude the two-photon exchange as an explanation for the difference between Rosenbluth and polarization transfer measurements of proton electromagnetic form factors.     

Dispersion relation formalism for the two-photon exchange correction to elastic muon–proton scattering: elastic intermediate state

We evaluate the two-photon exchange correction to the unpolarized cross section in the elastic muon–proton scattering within dispersion relations. One of the six independent invariant amplitudes requires a subtraction. We fix the subtraction function to the model estimate of the full two-photon exchange at one of three MUSE beam energies and make a prediction for the two other energies. Additionally, we present single and double polarization observables accounting for the lepton mass.     

Pair production of J/ψ as a probe of double parton scattering at LHCb

We argue that the recent LHCb observation of J/ψ-pair production indicates a significant contribution from double parton scattering, in addition to the standard single parton scattering component. We propose a method to measure the double parton scattering at LHCb using leptonic final states from the decay of two prompt J/ψ mesons.     

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     

Two-photon exchange in deep inelastic scattering

We have measured the contribution of the two-photon exchange amplitude in deep inelastic muon-nucleon scattering. We find, with 95% confidence, that the real part of this amplitude is less than 0.017 of the one photon-exchange amplitude. The range of the 4-momentum transfer covered is 0.5 < Q² < 2.1 (GeV/c)².     

Two-photon processes in the parton model

We study here some two-photon processes in colliding e^±e^? beam reactions, with the aim of testing the parton model. The parton contributions should dominate in certain well defined kinematical limits, and provide an essential difference between photonic and hadronic processes.     

Spin asymmetries for large-pt jet production in two-photon processes

Relying on the hard scattering formalism we estimate cross sections and spin asymmetries for large-p_t jet production from two-photon processes in collisions of electrons and positrons of definite helicity states. We use distribution functions of polarized partons in polarized electrons obtained in a modified leading logarithmic approximation as well as spin-dependent parton cross sections according to lowest-order QCD calculations. Detecting at least one forward or backward hadronic jet, topologies of jets can be observed different from the 2 large p_t production.     

Coordinate space structure in the parton model

We study the relation between the parton model and light cone analyses for highly inelastic leptonic processes. Results are displayed which follow from scaling laws in general, independently of specific parton model predictions. We conclude that the assumptions of the light cone analysis of inelastic electron scattering are supported by the parton model. However, the parton model matrix element for massive muon pair production is not light-cone dominated, nor does it have the same light cone singularity as inelastic electron scattering.     

Spin distributions in the quark parton model

Spin-dependent parton distributions are described in a broken SU(6) quark parton model. The model predicts definite forms for the spin-dependent structure functions in deep inelastic lepton-nucleon scattering and leads to several relations between Regge intercepts and couplings. Resonance electroproduction at large momentum transfer is explored via Bloom-Gilman duality.     

Model-independent properties of two-photon exchange in elastic electron-proton scattering

We derive from general symmetry properties of the hadron electromagnetic interaction, such as C-invariance and crossing symmetry, the general characteristics of two-photon exchange in electron-proton elastic scattering. We show that the presence of this mechanism destroys the linearity of the Rosenbluth separation.PACS: 25.30.Bf Elastic electron scattering - 13.40.-f Electromagnetic processes and properties - 13.60.-r Photon and charged-lepton interactions with hadrons - 13.88. + e Polarization in interactions and scattering     

Lepton-pair production in hadronic processes via a two-photon exchange mechanism

We give some details of the calculation of the cross-section for muon-pair production in hadron-hadron scattering via a two-photon exchange mechanism, together with numerical results. This two-photon process is found to become significant compared to the familiar single-photon (Drell-Yan) process at very high (e.g. ISABELLE) energies, although in certain restricted kinematic regions it has a measurable cross-section itself under currently attainable experimental conditions. Some suggestions as to how this process can be observed separately from the single-photon background in present and future experiments are also given.     

Transverse beam spin asymmetries at backward angles in elastic electron-proton and quasielastic electron-deuteron scattering

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasielastic scattering on the deuteron, at backward angles (lab scattering angle of 108°) for Q2 = 0.22 GeV2/c2 and 0.63 GeV2/c2 at beam energies of 362 and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single-photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (πN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasistatic deuterium approximation, and is also in agreement with theory.     

Rapidity spectra in proton-proton and proton-antiproton scattering up to 540 GeV in a dual parton model

Some recent data at the ISR and SPS colliders are discussed in terms of a multi-pomeron exchange model formulated in the dual parton framework. Charged particle rapidity distributions, energy dependence of the central plateau, ratio of particle densities in proton-proton and antiproton-proton scattering and non-diffractive multiplicities are successfully accounted for in the model.     

Three jet cross sections in photoproduction at HERA

We calculate three jet cross sections in photoproduction using exact matrix elements for the direct and resolved contributions. Numerical distributions are presented in a generic, irreducible set of variables that allows to disentangle the dynamics of partonic QCD subprocesses from each other and from pure phase space distributions. The results are compared to preliminary data from the ZEUS collaboration at HERA. It is found that the largest contribution comes from photon-gluon fusion in the mass range 36 GeV 80 GeV. The measured leading jet scattering angle distribution is consistent with the -channel exchange of a massless fermion in scattering, where the third parton is assumed to arise from soft bremsstrahlung. The data are inconsistent with pure phase space and Rutherford scattering distributions. Received: 5 August 1998 / Published online: 19 October 1998