Pion and photon beam initiated backward charmonium or lepton pair production

TL;DR

This paper explores backward charmonium and lepton pair production initiated by pion and photon beams, testing the universality of Transition Distribution Amplitudes (TDAs) across different regimes and estimating cross sections for experiments at J-PARC, JLab, EIC, and EicC.

Contribution

It introduces models for pion-to-nucleon and photon-to-nucleon TDAs and provides cross section estimates for backward charmonium and lepton pair production, aiding experimental investigations.

Findings

Estimated cross sections for backward J/ψ photoproduction.

Developed a two-parameter TDA model based on preliminary data.

Demonstrated the feasibility of studying TDAs in near-backward kinematics.

Abstract

Hard exclusive reactions initiated by pion or photon beams within the near-backward kinematical regime specified by the small Mandelstam variable $-u$ can be studied to access pion-to-nucleon and photon-to-nucleon Transition Distribution Amplitudes (TDAs). Checking the validity of collinear factorized description of pion and photon induced reactions in terms of TDAs allows to test the universality of TDAs between the space-like and time-like regimes that is the indispensable feature of the QCD collinear factorization approach. In this short review we consider the exclusive pion- and photo-production off nucleon of a highly virtual lepton pair (or heavy quarkonium) in the near-backward region. We first employ a simplistic cross channel nucleon exchange model of pion-to-nucleon TDAs to estimate the magnitude of the corresponding cross sections for the kinematical conditions of J-PARC. We then illustrate the flexibility of our approach by building a two parameter model for the photon-to-nucleon TDAs based on preliminary results for near threshold $J/\psi$ photoproduction at JLab and provide our estimates for near-backward $J/\psi$ photoproduction and Timelike Compton Scattering cross sections for the kinematical conditions of JLab and of future EIC and EicC.