Evidence for observation of color transparency in proton nucleus collisions
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We introduce a data analysis procedure for color transparency experiments which is considerably less model dependent than the transparency ratio method. The new method is based on fitting the shape of the A dependence of the nuclear cross section at fixed momentum transfer to determine the effective attenuation cross section for hadrons propagating through the nucleus. The hard scattering cross section is then determined directly from the data. We apply this procedure to the Brookhaven experiment of Carroll et al and find that it clearly shows color transparency: the effective attenuation cross section in events with momentum transfer $Q^2$ is approximately $40 mb (2.2 GeV^2/Q^2)$. The fit to the data also supports the idea that the hard scattering inside the nuclear medium is closer to perturbative QCD predictions than is the scattering of isolated protons in free space. We discuss the application of our approach to electroproduction experiments.Keywords:
Momentum transfer
Momentum (technical analysis)
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Color transparency (CT) is an effect of suppression of nuclear shadowing of hard reactions, closely related to the color screening. A brief review of theoretical development and experimental search for CT, failed and successful, are presented. A special emphasis is made on a quantum-mechanical nature of CT, as opposed to a wide spread erroneousclassical treatment of this phenomenon. The typical predictions of the classical approach, all contradicting quantum mechanics are: - factorization of cross section of hard reactions on a nucleus;\\ - "nuclear transparency", a normalized ratio of nuclear to nucleon cross sections, cannot exceed one;\\ - the larger is a radius of a hadron, the stronger it attenuates in a nucleus;\\ - the higher is the energy of hadrons participating in a hard reaction, the less is the nuclear attenuation;\\ - due to CT hard processes provide a better opportunity to study Fermi-momentum distribution, than soft reactions; etc. (Talk presented at XXVIII Rencontres de Moriond, March 1993)
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We compute cross sections for incoherent diffractive J/Psi production in lepton-nucleus deep inelastic scattering. The cross section is proportional to A in the dilute limit and to A^(1/3) in the black disc limit, with a large nuclear suppression due to saturation effects. The t-dependence of the cross section, if it can be measured accurately enough, is sensitive to the impact parameter profile of the gluons in the nucleus and their fluctuations, a quantity that determines the initial conditions of a relativistic heavy ion collision. The nuclear suppression in incoherent diffraction shows how the transverse spatial distribution of the gluons in the nucleus gradually becomes smoother at high energy. Since the values of the momentum transfer |t| involved are relatively large, this process should be easier to measure in future nuclear DIS experiments than coherent diffraction.
Saturation (graph theory)
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We show that particle production in proton-nucleus (pA) collisions in the color glass condensate model can be related to deep inelastic scattering (DIS) of leptons on protons or nuclei. The common building block is the quark-antiquark (or gluon-gluon) dipole cross section which is present in both DIS and pA processes. This correspondence in a sense generalizes the standard leading twist approach to pA collisions based on collinear factorization and perturbative QCD, and allows one to express the pA cross sections in terms of a universal quantity (dipole cross section) which, in principle, can be measured in DIS or other processes. Therefore, using the parametrization of the dipole cross section at DESY HERA, one can calculate particle production cross sections in proton-nucleus collisions at high energies. Alternatively, one could use proton-nucleus experiments to further constrain models of the dipole cross section. We show that the McLerran-Venugopalan model predicts an enhancement of the cross sections at large ${p}_{\ensuremath{\perp}}$ (Cronin effect) and a suppression of the cross sections at low ${p}_{\ensuremath{\perp}}.$ The crossover depends on rapidity and moves to higher ${p}_{\ensuremath{\perp}}$ as one goes to more forward rapidities.
Color Glass Condensate
Parametrization (atmospheric modeling)
DESY
Discrete dipole approximation
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Nuclear Structure
EMC effect
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Structure function
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Structure function
EMC effect
Neutral current
Nuclear cross section
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Momentum (technical analysis)
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Semi-inclusive deep inelastic scattering off the Deuteron with production of a slow nucleon in recoil kinematics is studied in the virtual nucleon approximation, in which the final state interaction (FSI) is calculated within general eikonal approximation. The cross section is derived in a factorized approach, with a factor describing the virtual photon interaction with the off-shell nucleon and a distorted spectral function accounting for the final-state interactions. One of the main goals of the study is to understand how much the general features of the diffractive high energy soft rescattering accounts for the observed features of FSI in deep inelastic scattering(DIS). Comparison with the Jefferson Lab data shows good agreement in the covered range of kinematics. Most importantly, our calculation correctly reproduces the rise of the FSI in the forward direction of the slow nucleon production angle. By fitting our calculation to the data we extracted the $W$ and $Q^2$ dependences of the total cross section and slope factor of the interaction of DIS products, $X$, off the spectator nucleon. This analysis shows the $XN$ scattering cross section rising with $W$ and decreasing with an increase of $Q^2$. Finally, our analysis points at a largely suppressed off-shell part of the rescattering amplitude.
Recoil
Eikonal approximation
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