First results from the PHOBOS experiment at RHIC

The PHOBOS experiment at RHIC has measured the charged-particle density dN/dη at mid-rapidity for central Au+Au collisions at center of mass energies of sNN=56, and 130 GeV. We deduce that dN/dη=408±12(stat)±30(syst) and 555±12(stat)±35(syst) for collision energies of 56 GeV and 130 GeV, respectively. These numbers suggest energy densities that are some 70% 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.