The Nucleus as a  New Physics Opportunities  Semi-inclusive Deep Inelastic Scattering

Hard Exclusive Reactions

To understand how simple quark configurations are controlled by confining mechanisms, one needs a different type of data sensitive to the time evolution of a system of correlated quarks. This is the domain of exclusive reactions where scattered particles emitted in a specific channel are observed in coincidence. Exclusive reactions are processes in which the final state is completely resolved. These reactions probe hadronic structure at very small distances $\lambda$ by transferring to the target a large momentum transfer $Q=1/\lambda$. Hadrons behave as a collection of quasi-free objects, the partons, sharing each a fraction 0<x_i<1 of the infinite momentum and moving closely parallel to it. They are bound by the strong colour force, but their binding energy being small compared to their momentum, they behave almost freely. The parton model is for photon-hadron interactions what the impulse approximation is for nuclei. The major difference is that due to the existence of confinement, partons cannot be directly observed. In hard exclusive reactions, the wave function of a composite state such as the proton can be written as a simple expansion of states with a fixed number of quarks and gluons. The valence component turns out to be the dominant one in hard exclusive reactions. The valence wave functions depend on the light-cone momentum fraction x_i, transverse momentum p_T and helicities. They contain important information on quark confinement dynamics. The experimental strategy of ELFE physics is to sort out the hadron distribution amplitudes from various exclusive reactions, to learn about the dynamics of confinement. This is possible thanks to the fact that, within perturbative QCD, one derives a factorisation property of exclusive scattering amplitudes. The hard scattering amplitude is calculable perturbatively as an expansion in $\alpha _{s}(Q^{2})$ free of large logarithmic corrections. Exclusive reactions at high momentum transfer (form factors, real or virtual Compton scattering, photo- or electroproduction of pseudoscalar and vector mesons) will give access to the simplest partonic configurations of the hadrons. This requires higher luminosities, available only at Jefferson Lab in the near future. A significant increase of the energy beyond that available at Jefferson Lab is needed. However, accelerators designed for studying electroweak physics or QCD in inclusive reactions do not give us an access to exclusive scattering, because of the smallness of exclusive amplitudes at large transfers. The only possibility is to use a dedicated high intensity continuous beam electron accelerator to study exclusive reactions at large transfer. A new dedicated European facility such as ELFE will provide a unique window here.