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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
by transferring to the target a large momentum transfer .
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
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.**

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Nucleus as a **Up:** New
Physics Opportunities **Previous:** Semi-inclusive
Deep Inelastic Scattering

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