Long range two-particle rapidity correlations in [formula omitted] collisions from high energy QCD evolution

Abstract

Long range rapidity correlations in A + A collisions are sensitive to strong color field dynamics at early times after the collision. These can be computed in a factorization formalism (Gelis, Lappi and Venugopalan (2009) [1]) which expresses the n-gluon inclusive spectrum at arbitrary rapidity separations in terms of the multi-parton correlations in the nuclear wavefunctions. This formalism includes all radiative and rescattering contributions, to leading accuracy in α s Δ Y , where Δ Y is the rapidity separation between either one of the measured gluons and a projectile, or between the measured gluons themselves. In this paper, we use a mean field approximation for the evolution of the nuclear wavefunctions to obtain a compact result for inclusive two gluon correlations in terms of the unintegrated gluon distributions in the nuclear projectiles. The unintegrated gluon distributions satisfy the Balitsky–Kovchegov equation, which we solve with running coupling and with initial conditions constrained by existing data on electron–nucleus collisions. Our results are valid for arbitrary rapidity separations between measured gluons having transverse momenta p ⊥ , q ⊥ ≳ Q s , where Q s is the saturation scale in the nuclear wavefunctions. We compare our results to data on long range rapidity correlations observed in the near-side ridge at RHIC and make predictions for similar long range rapidity correlations at the LHC.