  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.
