Saturation and Pion Production in Proton-Nucleus Collisions

Abstract

We study the effects of gluon saturation on pion production in high energy proton-nucleus collisions using the color glass condensate model. At high p_perp, we show that the p_perp-distribution of gluons behaves as ~ 1/(p_perp^7) in accordance with both conventional perturbative QCD calculations and experiment. Fragmentation of gluons into pions leads to a rapidity dependent depletion of pions relative to the conventional perturbative QCD predictions. We argue that these clear and systematic differences provide a signal for the onset of gluon saturation which is accessible in upcoming experiments.

Figures (3)

• Figure 1: Transverse momentum distributions of gluons produced in a proton--gold collision computed from conventional pQCD (dotted) and from the CGC calculation (solid). The uppermost set of curves is for $y=0$, while the lower ones correspond to $y=3$. The dashed curve shows the CGC result with purely gluonic source.
• Figure 2: Transverse momentum distributions of pions produced in a proton--gold collision computed by fragmenting gluons from conventional pQCD (dotted) and from CGC (solid) calculations. The topmost two curves are for $y=0$ and the other pairs are for $y=2$ and $y=3$, respectively. The dashed line shows the suggested fit of the pion distribution.
• Figure 3: The rapidity densities at fixed $p_\perp$. The dotted curves show the spectrum of gluons in the CGC model at $p_\perp=1.4$ GeV (upper) and $p_\perp=3$ GeV (lower). The solid curves give the pion spectra from CGC model at $p_\perp=0.4, 0.6, 0.8, 1.0, 1.7$ GeV (top to bottom), and the dashed ones show the pion spectra from pQCD at $p_\perp=0.4$ (upper) and 1.0 GeV (lower).