We present the first measurement of pseudorapidity densities of primary charged particles near midrapidity in Au+Au collisions at sqrt[s(NN)] = 56 and 130 GeV. For the most central collisions, we find the charged-particle pseudorapidity density to be dN/deta|(|eta|<1) = 408+/-12(stat)+/-30(syst) at 56 GeV and 555+/-12(stat)+/-35(syst) at 130 GeV, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than 40% at the higher energy.

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Abstract. The PHOBOS experiment at RHIC has measured the charged-particle density dN/dq at mid-rapidity for cedntral Au+Au collisions at center of mass energies of&=56, and 130 GeV. We deduce that dN/dq=408*l 2(stat) k30(syst) and 555*12(stat)&35 (syst) for collision energies of 56, and 130, GeV, respectively. These numbers suggest energy densities that are some 7090 higher than have been achieved in any heavy-ion collisions previously studied, and also 25-40% higher than nucleon-nucleon collisions at comparable center of mass energies.