PHOBOS at RHIC: some global observations

Particle production in Au + Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies for a large span of pseudorapidities, η < 5.4. Three empirical observations have emerged from this data set which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au + Au collisions, when expressed as dN/dη' (η' ≡ η - ybeam), becomes energy independent at high energy for a broad region of η' around η'= 0. This energy-independent region grows with energy, allowing only a limited region (if any) of longitudinal boost-invariance. Second, there is a striking similarity between particle production in e+e- and Au + Au collision (scaled by the number of participating nucleon pairs). Both the total number of produced particles and the longitudinal distribution of produced particles are approximately the same in e+e- and in scaled Au + Au. This observation was not predicted and has not been explained. Finally, particle production has been found to scale approximation with the number of participating nucleon pairs for (Npart) > 65. This scaling occurs both for the total multiplicity and for high ρT particles (3 < ρT < 4.5 GeV/c). (author)