The initial gluon multiplicity in heavy ion collisions

Abstract

The initial gluon multiplicity per unit area per unit rapidity, dN/L^2/d\eta, in high energy nuclear collisions, is equal to f_N (g^2\mu L) (g^2\mu)^2/g^2, with \mu^2 proportional to the gluon density per unit area of the colliding nuclei. For an SU(2) gauge theory, we compute f_N (g^2\mu L)=0.14\pm 0.01 for a wide range in g^2\mu L. Extrapolating to SU(3), we predict dN/L^2/d\eta for values of g^2\mu L in the range relevant to the Relativistic Heavy Ion Collider and the Large Hadron Collider. We compute the initial gluon transverse momentum distribution, dN/L^2/d^2 k_\perp, and show it to be well behaved at low k_\perp.