Generalized Parton Distributions offer a new way to access the quark and
gluon nucleon structure. We review recent progress in this domain, emphasizing
the need to supplement the experimental study of DVCS by its crossed version,
timelike Compton scattering (TCS), where data at high energy should appear
thanks to the study of ultraperipheral collisions at the LHC. This will open
the access to very low skewness quark and gluon GPDs. Our leading order
estimates show that the factorization scale dependence of the amplitudes is
quite high. This fact demands the understanding of higher order contributions
with the hope that they will stabilize this scale dependence. The magnitudes of
the NLO coefficient functions are not small and neither is the difference of
the coefficient functions appearing respectively in the DVCS and TCS
amplitudes. The conclusion is that extracting the universal GPDs from both TCS
and DVCS reactions requires much care. We also describe the extension of the
GPD concept to three quark operators and the relevance of their nucleon to
meson matrix elements, namely the transition distribution amplitudes (TDAs)
which factorize in hard exclusive pion electroproduction off a nucleon in the
backward region and baryon-antibaryon annihilation into a pion and a lepton
pair. We discuss the main properties of the TDAs.Comment: 4 pages, to be published in the proceedings of the 2011 Europhysics
Conference on High Energy Physics-HEP 2011, July 21-27, 2011, Grenoble,
Rhone-Alpes, Franc
