New results in exclusive hard reactions

TL;DR

This paper reviews recent advances in accessing nucleon structure via generalized parton distributions, emphasizing the importance of timelike Compton scattering and transition distribution amplitudes, and discusses theoretical challenges like scale dependence and higher order corrections.

Contribution

It highlights the potential of TCS at high energy for probing GPDs, discusses the extension to TDAs, and addresses the need for higher order calculations to reduce scale dependence.

Findings

High scale dependence of amplitudes at leading order

Significant NLO coefficient functions affecting GPD extraction

Potential of ultraperipheral collisions at LHC for TCS data

Abstract

Generalized Parton Distributions offer a new way to access the quark and gluon nucleon structure. We review recent progress in this domain, emphasizing the need to supplement the experimental study of DVCS by its crossed version, timelike Compton scattering (TCS), where data at high energy should appear thanks to the study of ultraperipheral collisions at the LHC. This will open the access to very low skewness quark and gluon GPDs. Our leading order estimates show that the factorization scale dependence of the amplitudes is quite high. This fact demands the understanding of higher order contributions with the hope that they will stabilize this scale dependence. The magnitudes of the NLO coefficient functions are not small and neither is the difference of the coefficient functions appearing respectively in the DVCS and TCS amplitudes. The conclusion is that extracting the universal GPDs from both TCS and DVCS reactions requires much care. We also describe the extension of the GPD concept to three quark operators and the relevance of their nucleon to meson matrix elements, namely the transition distribution amplitudes (TDAs) which factorize in hard exclusive pion electroproduction off a nucleon in the backward region and baryon-antibaryon annihilation into a pion and a lepton pair. We discuss the main properties of the TDAs.