  Longitudinal density correlations of produced matter in Au+Au collisions at
sqrt(s_NN)=200 GeV have been measured from the inclusive charged particle
distributions as a function of pseudorapidity window sizes. The extracted
\alpha \xi parameter, related to the susceptibility of the density fluctuations
in the long wavelength limit, exhibits a non-monotonic behavior as a function
of the number of participant nucleons, N_part. A local maximum is seen at
N_part ~ 90, with corresponding energy density based on the Bjorken picture of
\epsilon_Bj \tau ~ 2.4 GeV/(fm^2 c) with a transverse area size of 60 fm^2.
This behavior may suggest a critical phase boundary based on the
Ginzburg-Landau framework.
