  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.
