Theoretical Predictions for Proton Color Transparency II: Advanced Theoretical Approaches*

The strong interaction between hadrons and nuclei leads to the phenomenon of shadowing. However, in the special situation of high-momentun-trasnfer coherent processes, these interactions can be turned off, causing the nucleus to become quantum mechanically transparent. In more technical language: color transparency is the vanishing of  iniitial- and final-state interactions, predicted by QCD to occur in high-momentum-trasnsfer quasi-elastic nuclear reactions. These are coherent reactions in which one adds different contributions to obtain the scattering amplitude. Under such conditions the effects of gluons emitted by small color-singlet objects   may vanish.  Thus color transparency is known as color coherence.

The  name 'color transparency' is rather unusual.  One might think that it concerns transparency objects that have color, but it is really about how a medium can be transparent to objects that have no color.

The key dynamical question that controls the possiblity of observing color transparency  is whether  or not the high momentum trasnfer reactions occur on a single quark (Feynman mechanism)  or on two or three closely separated quarks as in perturbtive QCD. Hints of color transparency have been seen in  nuclear (p,pp) reactions, electroproduction of pions and rho mesons from nuclei. A strong signal has been obsered in high enrgy pion-nucleus reactions leading to two jets. No evidence for color transparency has been oserved in (e,e',p) reactions, including the most recent experiment at  the highest momentum transfer of about 13 GeV2. The experimental evidence is breifly reviewed, followed by a discussiom of  the different theoretical approaches. The only recent theoretical developments involve the use of holographic ideas to generate models of hadronic wave functions and masses. The present evidence seems to be that the Feynman mechanism is dominant for reactions involving protons. Processes involving mesons may involve different mechanisms. Prospects for future observations are discussed.