  The PHOBOS experiment at RHIC has measured large samples of Au+Au and Cu+Cu
collisions using a detector with uniquely large angular acceptance. These data
enable studies of particle production over a very wide pseudorapidity interval
which reveal unexpected features. In the analysis of correlations with a
high-$p_{_{T}}$ trigger particle ($p_{_{T}}>2.5$ GeV/c) a ridge extending at
least 4 units of pseudorapidity was found. The results on forward-backward and
two-particle correlations suggest that particles are produced in very large
clusters which are wider in pseudorapidity than is expected for isotropic
decays. Explanation of these experimental results requires models in which both
short-range and long-range correlations are present.
