This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au+Au collisions at the highest beam energy, evidence is found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au+Au at different centralities, the dependence of the yield on the number of participants at higher p T (∼4 GeV/c) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as "extended longitudinal scaling". Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.

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Large-angle pp elastic and quasielastic (p,2p) scattering have been simultaneously observed in hydrogen and each of several nuclear targets (Li, C, Al, Cu, Pb) at incident proton momenta of 6, 10, and 12 GeV/c. The nuclear transparency is the ratio of such a cross section in a nucleus to the free pp cross section. The transparency of aluminum increases with incident momentum by more than a factor of 2 from 6 to 9.5 GeV/c and falls significantly between 9.5 and 12 GeV/c. This occurs in a region where the free-proton nucleon-absorption cross section exhibits little energy dependence. QCD predicts an increase in transparency with energy.PACS numbers: 13.75. Cs, 12.38.Qk, 13.85.Dz, 25.40.Ve This Letter describes the first results from a program of study at the Brookhaven National Laboratory Alternating-Gradient Synchrotron which investigates the effects of "color transparency." Quasielastic pp scattering from each of several nuclei is compared to pp elastic scattering in hydrogen at three energies. These data are analyzed with a simple model in which the quasielastic cross section is assumed to factor into the product of three terms, a single-particle nuclear momentum distribution, a free pp cross section, and a factor T which we refer to as the transparency of the nucleus. In the absence of Fermi motion the transparency would beData are presented for pp elastic and quasielastic scattering near 90° cm. (center of mass) at incident proton energies of 6, 10, and 12 GeV/c, corresponding to t [(four-momentum transfer) 2 ] of -4.8, -8.5, and -10.4 GeV 2 .The cross section (da/dt) for pp elastic scattering at large transverse momentum and at fixed cm. angle is characterized by an s [(center-of-mass energy) 2 ] dependence which oscillates around the nominal s~] 0 form predicted by the dimensional scaling law of Brodsky and Farrar. l The form of this energy dependence can be related to the probability of finding protons with all of their quarks confined to a region of space which is proportional to 1A/7. This implies that for large s these initial-and final-state protons are very small.It has been pointed out by Mueller 2 and others that small protons which participate in such processes are characterized by color-charge and color-field distributions confined to ever smaller dimensions as s increases. In high-/ quasielastic scattering this implies that the cross section for soft initial-and final-state interactions with other nucleons in the nucleus will vanish as the energy scale increases. It has thus been predicted that at high energy the transparency of nuclei should approach unity. This is in sharp contrast to a more conventional Glauber picture 3 of absorption in which the transparency would be expected to be energy independent.The apparatus consists of a large-angle magnetic spectrometer with a 4.5° aperture. 4 Large proportional chambers measure the trajectories of recoil tracks opposite the spectrometer. When configured for incident momentum of 10 GeV/c, the spectrometer has Ap/p = 1% and A0 = 1 mr and the recoil-...