Geometric scaling violations in the central rapidity region of d+Au collisions at RHIC

Abstract

We show that geometric scaling is satisfied to good accuracy in the forward region of d+Au collisions at RHIC. Scaling violations do show up, however, at mid-rapidity, and the anomalous dimension of the small-x gluon distribution evolves to near its DGLAP limit for transverse momenta of a few GeV. This represents a first consistency check of RHIC deuteron-nucleus and HERA DIS phenomenology, and of the universality of the underlying Color Glass Condensate (CGC) theory, which describes both phenomena. It also reconciles successful leading-twist LO and NLO perturbative QCD computations of mid-rapidity particle production with small-x evolution. Finally, we introduce a new parameterization for the anomalous dimension of the small-x gluon distribution which properly reproduces known theoretical limits at large rapidity, at large virtuality, and on the saturation boundary, and still fits the available data from d+Au collisions at RHIC. We find indications that sub-asymptotic terms in the rapidity-evolution of the anomalous dimension are large.

Comments: v2 to appear in NPA; 17 pages, 5 figures; section 2 has been revised following useful comments by the referee, main results and conclusions unchanged; codes for computing final-state pt-distributions from hadron collisions with a CGC can be downloaded at http://www.th.physik.uni-frankfurt.de/~dumitru/CGC/CGC.html