Generalized Parton Distributions and Generalized Distribution Amplitudes : New Tools for Hadronic Physics 1

The generalized parton distributions and the generalized distribution amplitudes give access to a deeper understanding of the quark and gluon content of hadrons. In this short review, we select some new developments of their interesting connections with the physics information that one can extract from exclusive reactions at medium and high energies. A considerable amount of theoretical and experimental work is currently being devoted to the study of generalized parton distributions, which are defined as Fourier transforms of matrix elements between different hadron states, such as: dλe iλx(p+p ′).n N ′ (p ′ , s ′)| ¯ ψ(−λn)γ.nψ(λn)|N(p, s) Their measurements are expected to yield important contributions to our understanding of how quarks and gluons assemble themselves into hadrons [1]. The simplest and cleanest exclusive processes where these distributions occur are deeply virtual Compton scattering (DVCS), i.e., γ * N → γN ′ and meson electroproduction i.e., γ * N → MN ′ in kinematics where the γ * has large spacelike virtuality Q 2 while the invariant momentum transfer t to the proton is small (for recent reviews, see [2]). Their " inverse " processes, γN → γ * N ′ and πN → γ * N ′ at small t and large timelike virtuality of the final state photon [3] are quite similar. DVCS combines features of the inelastic processes with those of an elastic process. A relativistic charged lepton is scattered from a target nucleon or nucleus. A real photon of