Bernard Pire

Ultraperipheral collisions in hadron colliders give new opportunities to investigate the hadron structure through exclusive photoproduction processes. We describe the possibility of measuring the Generalized Parton Distributions in the Timelike Compton Scattering process and in the production of heavy vector meson.

We study the timelike virtual Compton scattering process with a linearly polarized photon beam, in the generalized Bjorken scaling region and in the medium energy range, which will be studied intensely at JLab12 experiments. We define new observables and show how they should help to access the polarized quark and gluon generalized parton distributions.

EPJ Web of Conferences, open-access proceedings in physics and related sciences

We define in a systematic way, based on the light-cone collinear factorization method, the chiral-odd generalized parton distributions (GPDs) of a pseudoscalar hadron (such as the pi0) up to twist 5. For that, we introduce the relevant matrix elements for 2-parton non-local operators, as well as matrix elements for 3-parton non-local correlators. Their detailed parametrization is fixed based on parity, charge conjugation and time reversal invariance. This leads to the introduction of 20 real GPDs, whose symmetry properties are explicitely given. The reduction of these GPDs to a minimal set is performed by the use of constraints provided by QCD equations of motion and rotation on the light-cone. We show that these 20 GPDs can be expressed through 8 GPDs which satisfy 4 integral sum rules. A surprising outcome of this analysis is the fact that, when assuming the vanishing of 3-parton correlators, as in the so-called Wandzura-Wilczek approximation, the whole set of GPDs vanishes.

Using twist 3 contributions and the relations, which emanate from the QCD equations of motion, we derive the gauge invariant amplitude for the deeply virtual Compton scattering (DVCS) off hadron with an arbitrary spin. We emphasize that our derivation is not based on a certain parametrization of the relevant hadron matrix elements of the quark(gluon) operators. As an application, we present the gauge invariant amplitude for the DVCS off spin-one hadron. Using the derived gauge invariant amplitude, the single spin asymmetry is discussed.

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.