Color-glass condensate

Abstract: Deep inelastic scattering, large pt particle production in hadron collisions and e+e− annihilation into hadrons are studied in perturbative QCD in the semihard region of very small x (x ⪡ 1) and large Q2(pt2). At small x the parton densities increase rapidly, so that the effects of parton-parton interactions (screenings), that stop the increase of the cross sections near their unitarity limit, and the coherent emission of soft gluons by the global colour charge of whole groups of harder particles, become essential. These effects are studied in detail. It is shown that at very small x ∼ xb(Q2) (In(1/xb) ∼ αs−2(Q2)) semihard processes acquire some features essential of soft processes. Their cross sections reach values comparable to the geometrical dimensions of hadrons. At very high energies gluon jets with large pt(pt ∼ Λ exp(1.78√In √s)), (x ∼ xb) are emitted in abundance and become the main source of secondary hadrons. In e+e− annihilation the coherence of soft gluon emission leads to a dip in the inclusive spectra at small x. The colour correlations in jets and the preconfinement effects are discussed. A convenient technique for calculating and summing leading log diagrams with two logarithmically large parameters (In 1/x and In Q2) is presented. A reggeon-type diagram technique is developed to take into account parton-parton interactions. A special chapter is devoted to the phenomenology of semihard processes, in particular to large pt production at the SPS Collider energy and to large Et processes. The possible applications of perturbative QCD to soft processes are sketched.

Abstract: We review the main results obtained by the BRAHMS Collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so-called quark–gluon plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e., the proposed color glass condensate.

Abstract: We argue that the distribution functions for quarks and gluons are computable at small $x$ for sufficiently large nuclei, perhaps larger than can be physically realized. For such nuclei, we argue that weak coupling methods may be used. We show that the computation of the distribution functions can be recast as a many-body problem with a modified propagator, a coupling constant which depends on the multiplicity of particles per unit rapidity per unit area, and for non-Abelian gauge theories, some extra media-dependent vertices. We explicitly compute the distribution function for gluons to lowest order, and argue how they may be computed in higher order.

Abstract: We determine the recombination probabilities for gluons to go into gluons or into quarks in a low-density limit. A modified Altarelli-Parisi equation expressing this recombination is given. We find recombination is very small compared to normal evolution in all interesting circumstances except that of nuclear shadowing. Thus, for evolution above Q0 ≈ 2 GeV, the gluon density remains far below saturation except at unusually small values of x.

Abstract: We consider a nonlinear evolution equation recently proposed to describe the small-$x$ hadronic physics in the regime of very high gluon density. This is a functional Fokker-Planck equation in terms of a classical random color source, which represents the color charge density of the partons with large $x$. In the saturation regime of interest, the coefficients of this equation must be known to all orders in the source strength. In this first paper of a series of two, we carefully derive the evolution equation, via a matching between classical and quantum correlations, and set up the framework for the exact background source calculation of its coefficients. We address and clarify many of the subtleties and ambiguities which have plagued past attempts at an explicit construction of this equation. We also introduce the physical interpretation of the saturation regime at small $x$ as a Color Glass Condensate. In the second paper we shall evaluate the expressions derived here, and compare them to known results in various limits.