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A bstract Exclusive photoproduction of a γπ ± pair in the kinematics where the pair has a large invariant mass and the final nucleon has a small transverse momentum is described in the collinear factorization framework. The scattering amplitude is calculated at leading order in α s and the differential cross sections for the process are estimated in the kinematics of the JLab 12-GeV experiments. The order of magnitude of the predicted cross-sections seems sufficient for a dedicated experiment to be performed. The process turns out to be very sensitive to the axial generalized parton distribution combination H ˜ u − H ˜ d .

In the framework of collinear QCD factorization, the leading twist scattering amplitudes for deeply virtual Compton scattering (DVCS) and timelike Compton scattering (TCS) are intimately related thanks to analytic properties of leading and next-to-leading order amplitudes. We exploit this welcome feature to make data-driven predictions for TCS observables to be measured in near future experiments. Using a recent extraction of DVCS Compton form factors from most of the existing experimental data for that process, we derive TCS amplitudes and calculate TCS observables only assuming leading-twist dominance. Artificial neural network techniques are used for an essential reduction of model dependency, while a careful propagation of experimental uncertainties is achieved with replica methods. Our analysis allows for stringent tests of the leading twist dominance of DVCS and TCS amplitudes. Moreover, this study helps to understand quantitatively the complementarity of DVCS and TCS measurements to test the universality of generalized parton distributions, which is crucial e.g. to perform the nucleon tomography.

An unfortunate misprint in our numerical code has led to an overestimate of the cross section for the exclusive photoproduction of a γ ρ L pair in the kinematics where the pair has a large invariant mass and the final nucleon has a small transverse momentum, described in the collinear factorization framework. The rates are thus expected to be smaller but the possibility to disentangle the transversity GPDs through the process where the ρ −meson is transversely polarized is enhanced.

A bstract We calculate the quark coefficient function T q ( x, ξ ) that enters the factorized amplitude for deeply virtual Compton scattering (DVCS) at all order in a soft and collinear gluon approximation, focusing on the leading double logarithmic behavior in ( x ± ξ ), where x ± ξ is the light cone momentum fraction of the incoming/outgoing quarks. We show that the dominant part of the known one loop result can be understood in an axial gauge as the result of a semi-eikonal approximation to the box diagram. We then derive an all order result for the leading contribution of the ladder diagrams and deduce a resummation formula valid in the vicinity of the boundaries of the regions defining the energy flows of the incoming/outcoming quarks, i.e. x = ± ξ . The resummed series results in a simple closed expression.

The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photoproduction or electroproduction of two vector mesons on a polarized nucleon target, γ(*)N→ρ1ρ2N’, where ρ1 is produced at large transverse momentum, ρ2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal ρ2 production. To this end, we propose a model for the transversity GPD HT(x,ξ,t) and give an estimate of the relative sizes of the transverse and longitudinal ρ2 cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.

We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, \\(\\gamma p\\to \\ell^+\\!\\ell^- p\\). This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in kinematics where the Compton process should be dominated by quark exchange.

We calculate the leading order BFKL amplitude for the exclusive diffractive process in the forward direction, which can be studied in future high energy e + e- linear colliders. The resummation effects are very large compared to the fixed-order calculation. We also estimate the next-to-leading logarithmic corrections to the amplitude by using a specific resummation of higher order effects and find a substantial growth with energy, but smaller than in the leading logarithmic approximation.

In specific kinematics, hard exclusive amplitudes may be factorized into a short distance dominated part computable in a perturbative way on the one hand, and universal, confinement related hadronic matrix elements on the other hand. The extension of this description to processes such as backward meson electroproduction and forward meson production in antiproton-nucleon scattering leads to define new hadronic matrix elements of three quark operators on the light cone, the nucleon-to-meson transition distribution amplitudes, which shed a new light on the nucleon structure.

Including O(mc) terms in the coefficient functions and/or O(mD) twist 3 contributions in the heavy meson distribution amplitudes leads to a non-zero transverse amplitude for exclusive neutrino production of a D pseudoscalar charmed meson on an unpolarized target. We work in the framework of the collinear QCD approach where chiral-odd transversity generalized parton distributions (GPDs) factorize from perturbatively calculable coefficient functions.

A bstract Exclusive photoproduction of a γ ρ pair in the kinematics where the pair has a large invariant mass and the final nucleon has a small transverse momentum is described in the collinear factorization framework. The scattering amplitude is calculated at leading order in α s and the differential cross sections for the process where the ρ −meson is either longitudinally or transversely polarized are estimated in the kinematics of the JLab 12-GeV experiments.