Baryon stopping and 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 dn val /dy ∼ 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.

Rapidity and centrality dependence of azimuthal correlations in deuteron–Gold collisions at RHIC

“…In-spite of this we believe that the general structure of the correlation function as well as its * Authors of [17,21] discussed the process gqv → ggqv assuming the collinear approximation to the unintegrated valence quark distribution [51]. centrality dependence is not strongly affected by the valence quark contribution.…”

Baryon stopping and saturation physics in relativistic collisions

“…In this section, we derive a parametric formula for the region of small x 1 (away from the peak), corresponding to the midrapidity valley (y ∼ 0). In this kinematic regime the valence quark distribution behaves as xq v ∝ x ∆ , where ∆ ≃ 0.5 is the intercept of the Regge trajectory [17], which allows us to perform analytic calculations.…”

“…Here, we focus on the forward rapidity region and interpolate to midrapidity where small-x quarks are dominant, by matching the leading-order distributions [35] and the Regge trajectory, xq v ∝ x 0.5 , at x = 0.01 [17].…”

Helicity Evolution at Small $x$

“…Similar to [5], our evolution equations also resum the leading-logarithmic (LLA) powers of α s ln(1/x) by including the BK/JIMWLK evolved unpolarized dipole S-matrix…”