A first look at Au+Au collisions at RHIC energies using the PHOBOS detector

The PHOBOS detector has been used to study Au + Au collisions at √SNN = 56, 130, and 200 GeV. Several global observables have been measured and the results are compared with theoretical models. These observables include the charged-particle multiplicity measured as a function of beam energy, pseudo-rapidity, and centrality of the collision. A unique feature of the PHOBOS detector is its almost complete angular coverage such that these quantities can be studied over a pseudo-rapidity interval of |η| ≤ 5.4. This allows for an almost complete integration of the total charged particle yield, which is found to be about Nchtot = 4200 ± 470 at √SNN = 130 GeV and Nchtot = 5300 ± 530 at √SNN = 200 GeV. The ratio of anti-particles to particles emitted in the mid-rapidity region has also been measured using the PHOBOS magnetic spectrometer. Of particular interest is the ratio of anti-protons to protons in the mid-rapidity region, which was found to be p̄/p = 0.6 ± 0.04(stat) ± 0.06(syst) at √SNN = 130 GeV. This high value suggests that an almost baryon-free region has been produced in the collisions.