  Viscous diffusion can broaden the rapidity dependence of two-particle
transverse momentum fluctuations. Surprisingly, measurements at RHIC by the
STAR collaboration demonstrate that this broadening is accompanied by the
appearance of unanticipated structure in the rapidity distribution of these
fluctuations in the most central collisions. Although a first order classical
Navier-Stokes theory can roughly explain the rapidity broadening, it cannot
explain the additional structure. We propose that the rapidity structure can be
explained using the second order causal Israel-Stewart hydrodynamics with
stochastic noise.
