Hard exclusive processes in the backward region

TL;DR

This paper reviews the study of backward exclusive processes involving large Q2 photons and baryonic exchanges, highlighting the role of Transition Distribution Amplitudes in understanding hadron structure and discussing recent experimental and theoretical developments.

Contribution

It introduces the concept of Transition Distribution Amplitudes and discusses their application in describing backward exclusive processes, including recent phenomenological studies and future outlooks.

Findings

Potential of backward processes to reveal hadron structure

Initial phenomenological results at JLAB, HERMES, and GSI-FAIR

Theoretical approaches like the pion-cloud model are promising

Abstract

We review the potentialities offered by the study of backward exclusive processes in a new scaling regime, i.e. involving a large -timelike or spacelike- Q2 photon and a baryonic exchange in the t-channel. We recall the concept of Transition Distribution Amplitudes (TDAs) containing unique information on the hadron structure, then discuss how they enter the description of processes such as backward electroproduction of a pion, antiproton-proton annihilations into a dilepton + meson as well as into J/psi + meson. We then discuss first phenomenological studies for processes that are being analysed at JLAB and HERMES or that will be measured by Panda at GSI-FAIR. Finally we present outlooks for their theoretical studies based on approaches such as the pion-cloud model.