M. Diehl

Light-Cone Wavefunction Representation of Deeply Virtual Compton Scattering

Abstract We give a complete representation of virtual Compton scattering γ ∗ p→γp at large initial photon virtuality Q 2 and small momentum transfer squared t in terms of the light-cone wavefunctions of the target proton. We verify the identities between the skewed parton distributions H(x,ζ,t) and E(x,ζ,t) which appear in deeply virtual Compton scattering and the corresponding integrands of the Dirac and Pauli form factors F 1 (t) and F 2 (t) and the gravitational form factors A q (t) and B q (t) for each quark and anti-quark constituent. We illustrate the general formalism for the case of deeply virtual Compton scattering on the quantum fluctuations of a fermion in quantum electrodynamics at one loop.

Linking parton distributions to form-factors and Compton scattering

Abstract. We relate ordinary and skewed parton distributions to soft overlap contributions to elastic form factors and large angle Compton scattering, using light-cone wave functions in a Fock state expansion of the nucleon. With a simple ansatz for the wave functions of the three lowest Fock states we achieve a good description of unpolarised and polarised parton distributions at large x, and of the data for the Dirac form factor and for Compton scattering, both of which can be saturated with soft contributions only. Large angle Compton scattering appears as a good case to investigate the relative importance of soft and hard contributions in exclusive processes which are sensitive to the end point regions of the nucleon wave function.

Generalized parton distributions from nucleon form factor data

Abstract.We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon (“nucleon tomography”). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around

The overlap representation of skewed quark and gluon distributions

vert tvert simeq 3 {rm GeV}^2

Exclusive production of pion pairs in {gamma}*{gamma} collisions at Large Q{sup 2}

. We also evaluate the valence contributions to the energy-momentum form factors entering Ji’s sum rule.

Skewed parton distributions in real and virtual compton scattering

Within the framework of light-cone quantisation we derive the complete and exact overlap representation of skewed parton distributions for unpolarised and polarised quarks and gluons. Symmetry properties and phenomenological applications are discussed.

The perturbative limit of the two-pion light-cone distribution amplitude

The authors perform a QCD analysis of the exclusive production of two mesons in {gamma}*{gamma} collisions in the kinematical domain of large photon virtuality Q and small hadronic invariant mass W. This reaction is dominated by a scale invariant mechanism which factorizes into a perturbative subprocess, {gamma}*{gamma} {r{underscore}arrow} q{anti q} or {gamma}*{gamma} {r{underscore}arrow} gg, and a generalized two-meson distribution amplitude. The authors develop in detail the phenomenology of this process at e{sup +}e{sup {minus}} colliders. Using a simple model for the two-pion distribution amplitude, based on its general properties, the authors estimate the cross section for the kinematics accessible at BABAR, BELLE, CLEO and LEP.

Erratum to: “The overlap representation of skewed quark and gluon distributions”: [Nucl. Phys. B 596 (2001) 33–65]

The handbag contribution to Compton scattering at moderately large momentum transfer factorises into parton-photon subprocess amplitudes and new form factors representing 1/x-moments of skewed parton distributions. A detailed phenomenological study for polarised and unpolarised real and virtual Compton scattering is presentedAbstract The handbag contribution to Compton scattering at moderately large momentum transfer factorises into parton-photon subprocess amplitudes and new form factors representing 1/ x -moments of skewed parton distributions. A detailed phenomenological study for polarised and unpolarised real and virtual Compton scattering is presented.

QCD factorization in γ*γ → ππ and γ*N → ππN

The authors consider pion pair production in two-photon collisions, {gamma}*{gamma} --> {pi}{sup +}{pi}{sup {minus}}, at large photon virtuality Q{sup 2} and center-of-mass energy {radical}s in the hard-scattering picture. In the limit where s is large but s {much{underscore}lt} Q{sup 2} the authors derive an expression for the two-pion light-cone distribution in terms of the conventional pion distribution amplitudes. The result reproduces the well-known scaling behavior, helicity selection rule and symmetry relations in this regime.

Exploring Skewed Parton Distributions with Polarized Targets

Within the framework of light-cone quantisation we derive the complete and exact overlap representation of skewed parton distributions for unpolarised and polarised quarks and gluons. Symmetry properties and phenomenological applications are discussed.