Two particle rapidity, transverse momentum, and azimuthal correlations   in relativistic nuclear collisions and transverse radial expansion
  At the very first stage of an ultra-relativistic nucleus-nucleus collision new particles are produced in individual nucleon-nucleon collisions. In the transverse plane, all particles from a single $NN$ collision are initially located at the same position. The subsequent transverse radial expansion of the system creates strong position-momentum correlations and leads to characteristic rapidity, transverse momentum, and azimuthal correlations among the produced particles. 
Sergei A. Voloshin