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Colour Transparency

 
Table: European electron and photon facilities operating in 2000 and beyond. 
 
Facility Projectile Energy (GeV) $\mathcal{L}$ (cm-2s-1)
MAMI e, $ \gamma $ 0.9 $\rightarrow$ 1.5 1038
ELSA e, $ \gamma $ 3.5 1035
HERMES e 27.5 1032
Graal $ \gamma $ 1.5 1035
COMPASS $\mu ,$Hadrons 200 1032
 
 

Colour transparency illustrates the power of hard exclusive reactions to isolate simple elementary quark configurations. Large Q2 experiments select very simple quark configurations where connected quarks are close together, and form small size hadrons. These mini-hadrons are not stationary states but evolve in time to build up normal hadrons. Such colour singlet systems cannot emit or absorb soft gluons, which carry energy or momentum smaller than Q. This is because gluon radiation is a coherent process and there is thus destructive interference between gluon emission amplitudes by quarks with ``opposite'' colour. Even without knowing exactly how exchanges of soft gluons and other constituents create strong interactions, we know that these interactions must be turned off for small colour singlet objects. Letting the mini-state evolve during its travel through different nuclei of various sizes allows an indirect but unique way to test how the squeezed mini-state goes back to its full size and complexity, i.e. how quarks inside the proton re-arrange themselves spatially to ``reconstruct'' a normal size hadron. In this respect the observation of baryonic resonance production as well as detailed spin studies are mandatory. The results on proton scattering on nuclei and $\rho $ meson lepto-production have to be confirmed. The study of the A(e,e'p) reactions at SLAC does not show any significant effect. It is likely that the values of Q2 are too low to observe colour transparency in the quasi free kinematics channel. An alternate way is to study reactions induced by electrons in few body systems. The kinematics should be chosen such that the interactions of the emerging hadron with a second nucleon are maximal. This maximum occurs when the produced hadron propagates on-shell. A clear signal for colour transparency would be the suppression of final state interactions when the momentum transfer increases. The study of colour transparency with hadronic probes has been proposed by measuring $J\psi$ production in $\bar{p}$ - nuclei collisions. Existing high energy electron accelerators designed to study electroweak physics have intensities too low to study such exclusive reactions. Jefferson Lab provides an intense continuous beam of electrons, but its energy is too low. One needs a dedicated high energy and high intensity continuous beam electron facility such as ELFE. 


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