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

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.

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