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An exactly solvable model is used to investigate the assumptions behind color transparency.
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Color Transparency refers to the vanishing of the hadron-nucleon interaction for hadrons produced inside a nucleus in high momentum exclusive processes. We briefly review the concept behind this unique Quantum Chromo Dynamics phenomenon, the experimental search for its onset and the recent progress made at intermediate energies.
We suggest a simple physical picture for the diffractive parton distributions that appear in diffractive deeply inelastic scattering. In this picture, partons impinging on the proton can have any transverse separation, but only when the separation is small can they penetrate the proton without breaking it up. By comparing the predictions from this picture with the diffractive data from HERA, we determine rough values for the small separations that dominate the diffraction process.
We have measured the nuclear transparency of the incoherent diffractive $A(e,erho^0)$ process in $^{12}$C and $^{56}$Fe targets relative to $^2$H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced $rho^0$s on a nucleus relative to deuterium, which is sensitive to $rho A$ interaction, was studied as function of the coherence length ($l_c$), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared ($Q^2$). While the transparency for both $^{12}$C and $^{56}$Fe showed no $l_c$ dependence, a significant $Q^2$ dependence was measured, which is consistent with calculations that included the color transparency effects.
Ongoing experiments at JLAB investigate the nuclear transparency in exclusive rho0(770) electroproduction off nuclei. In this work we present transport model predictions for the attenuation of rho0s in nuclei and for color transparency (CT) effects as observable at CLAS with a 5 GeV electron beam energy. A full event simulation presented here permits to study the impact of actual experimental acceptance conditions and kinematical cuts. The exclusive (e,erho0) cross section off nucleons is described by diffractive and color string breaking mechanisms extended toward the onset of the deep inelastic regime. Different hadronization and CT scenarios are compared. We show that a detailed analysis of elementary cross section, nuclear effects and experimental cuts is needed to reveal the early onset of rho0-CT at present JLAB energies.
New parameter free calculations including a variety of necessary kinematic and dynamic effects show that the results of BNL $(p,2p)$ measurements are consistent with the expectations of color transparency.
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