High energy pA collisions in the color glass condensate approach I: gluon production and the Cronin effect

Abstract

We study gluon production in high energy proton-nucleus collisions in the semi-classical framework of the Color Glass Condensate. We develop a general formalism to compute gluon fields in covariant gauge to lowest order in the classical field of the proton and to all orders in the classical field of the nucleus. The use of the covariant gauge makes the diagrammatic interpretation of the solution more transparnt. k ⊥ -factorization holds to this order for gluon production -Our results for the gluon distribution are equivalent to the prior diagrammatic analysis of Kovchegov and Mueller. We also show that these results are equivalent to the computation of gluon production by Dumitru and McLerran in the Fock-Schwinger gauge. We demonstrate how the Cronin effect arises in this approach, and examine its behavior in the two extreme limits of a) no small-x quantum evolution, and b) fully saturated quantum evolution. In both cases, the formalism reduces to Glauber's formalism of multiple scatterings. We comment on the possible implications of this study for the interpretation of the recent results on Deuteron-Gold collisions at the Relativistic Heavy Ion Collider (RHIC).

Particle production in field theories coupled to strong external sources, I: Formalism and main results

“…Summing over the + or − indices of the "leaves" (or "blobs") of the tree, the outer layer of propagators is transformed into retarded propagators, thanks to eqs. (59). These then give factors of d 4 yG R (· · · , y)j(y) that are independent of the indices of the vertices immediately below.…”

Classical initial conditions in high energy nucleus-nucleus collisions

“…The spectrum of produced gluon is defined as follows 11) where λ is the gluon polarization. The symbol ... stands for the average over the color sources ρ A and ρ B [21].…”

“…To probe the saturation regime, proton(deuteron)-nucleus collisions have been studied at RHIC since they represent a clean probe of the nuclear wave function at high energy. The gluon production cross-section has been calculated analytically [7,8,9,10,11,12]. In this case, the proton is a dilute object, whereas the nucleus is saturated.…”

High density QCD through eletromagnetic probes

“…The simplest model to this function should be a local Gaussian, however, it was verified that this consideration implies in a nonrealistic description of the RHIC experimental results concerning the hadron transverse momentum distribution [31]. In this work we have evaluated the cross section using the function C(l T , x, A) from the mean-field asymptotic solution for the JIMWLK evolution equation [32], introducing an x dependence through the nuclear saturation scale [13], which is parametrized in the form Q 2 s (x, A) = A 1/3 Q 2 s (x).…”

“…In this work we have evaluated the cross section using the function C(l T , x, A) from the mean-field asymptotic solution for the JIMWLK evolution equation [32], introducing an x dependence through the nuclear saturation scale [13], which is parametrized in the form Q 2 s (x, A) = A 1/3 Q 2 s (x). These considerations imply for the correlator the following form [31,33] …”