Pion and photon beam initiated backward charmonium or lepton pair production

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 recent 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.