  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 thermalization and transverse
radial expansion of the system create strong position-momentum correlations and
lead to characteristic rapidity, transverse momentum, and azimuthal
correlations among the produced particles.
