Baryon Stopping and the Valence Quark Distribution at Small x

Abstract

We argue that the amount of baryon stopping observed in the central rapidity region of heavy ion collisions at RHIC is proportional to the nuclear valence quark distributions at small x. By generalizing Mueller's dipole model to describe Reggeons we construct a non-linear evolution equation for the valence quark distributions at small x in the leading double-logarithmic approximation. The equation includes the effects of gluon saturation in it. The solution of the evolution equation gives a valence quark distribution function $d{n}_{val}/dy\sim e^{-(0.4÷0.5)y}$. We show that this y-dependence as well as the predictions of Regge theory are consistent with the net-proton rapidity distribution reported by BRAHMS.

Comments: 25 pages, 10 figures, Sects.II and VI are improved. Many references added, typos corrected, Fig.4 replaced, etc