  In order to describe large transverse momentum ($p_T$) distributions observed
in high energy nucleus-nucleus collisions, a stochastic model in the three
dimensional rapidity space is introduced.
  The fundamental solution of the radial symmetric diffusion equation is
Gaussian-like in radial rapidity.
  We can also derive a $p_T$ or radial rapidity distribution function, where a
distribution of emission center is taken into account. It is applied to the
analysis of observed large $p_T$ distributions of charged particles. It is
shown that our model approaches to a power function of $p_T$ in the high
transverse momentum limit.
