Backward timelike Compton scattering to decipher the photon content of the nucleon

Abstract

The exclusive photoproduction off nucleon of a large invariant mass lepton pair in the backward region specified by the small Mandelstam variable −u is discussed in the framework of collinear QCD factorization. The amplitude is factorized in terms of photon-to-nucleon Transition Distribution Amplitudes (TDAs) which encode the photon content of the nucleon. Model estimates of these new non-perturbative objects are described and the corresponding cross sections calculated.The background due to the electromagnetic Bethe-Heitler process is shown to be negligible in the kinematical regime of interest.

u-Channel Color Transparency Observables

“…In a similar line of thought to this proposal, one may study nuclear transparency in various experiments (see [43]), where TDAs appear as the collinear factorized hadronic matrix element, while a hard scattering process should be accompanied by the appearance of color transparency. This is the case for timelike Compton scattering with a quasi-real photon beam [44], but also in the antiproton nucleus electromagnetic processes at PANDA experiment [45] and the π-nucleus program at J-PARC [46]. In these three cases, color transparency should act as a decrease in the initial (rather than final) state interactions.…”