The PHOBOS Perspective on Discoveries at RHIC

TL;DR

PHOBOS presents a cohesive view of RHIC heavy-ion collisions, showing that central Au+Au at top energies create a very high energy density medium whose description cannot rely on hadronic degrees of freedom alone, and that the system exhibits strong early interactions as evidenced by large elliptic flow and high-pT suppression. A notable outcome is the emergence of simple scaling laws: total multiplicity scales with the number of participant nucleons, global observables factorize into separate energy and centrality dependencies, and extended longitudinal scaling makes observables energy-independent when viewed in the appropriate rest frame. Comparisons across Au+Au, d+Au, p+p, and e+e− suggest universal aspects of bulk particle production, with the medium in central heavy-ion collisions displaying properties consistent with strong collective behavior rather than a weakly interacting gas. The results emphasize the role of initial geometry and early-time dynamics in shaping final-state observables and motivate further tests of these scaling features with lighter systems and at higher energies (e.g., LHC).

Abstract

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 pT (~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.

Figures (44)

• Figure 1: Pseudorapidity density of charged particles emitted in Au+Au collisions at three different values of the nucleon-nucleon center-of-mass energy Bac03c. Data are shown for a range of centralities, labeled by the fraction of the total inelastic cross section in each bin, with smaller numbers being more central. Grey bands shown for selected centrality bins indicate the typical systematic uncertainties (90% C.L.). Statistical errors are smaller than the symbols.
• Figure 2: (Left panel) Results of PHOBOS measurements of the charged particle density near midrapidity in central Au+Au at $\sqrt{s_{_{NN}}}$=200 GeV Bac03cBac02bBac02cBac04e (shown by the vertical line with the dashed lines denoting the systematic uncertainty) compared to theoretical predictions. This panel is adapted from Esk02. From top to bottom, the references for the models are Wan99Zha99Sor99Ble99Bas99aCas99Dre99Cap99Ran99Arm00Jeo00Esk00Bas99bSta99Kra01. See text for discussion. (Right panel) Normalized pseudorapidity density of charged particles emitted within $|\eta|\le$1 in central Au+Au (AGS Ahl98aBac02eAhl00Kla03Dun99 and PHOBOS at RHIC Bac03cBac00aBac02aBac02bBac02cBac04e) and Pb+Pb (SPS Afa02Ant04) collisions as a function of nucleon-nucleon center-of-mass energy. See text for discussion.
• Figure 3: Transverse momentum distributions of identified charged particles emitted near midrapidity in central Au+Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. Invariant yield data shown are from PHENIX at higher momenta Adl04a and PHOBOS at lower momenta Bac04c. Boxes around the PHOBOS data indicate systematic uncertainties. Fits to PHENIX measurements are shown by solid curves $(\propto 1/[e^{(m_{_T}/T_i)}+\epsilon]$, where $\epsilon=-$1 and +1 for mesons and baryons, respectively, $m_{_T}$ is the transverse mass, and $T_i$ is the fit parameter for each species). Note that the extrapolations (dashed curves) of the fit to the data at higher momenta are consistent with the low momentum yields.
• Figure 4: Ratios of identified antiparticles over particles measured near midrapidity in central collisions of Au+Au (AGS Ahl00Ahl99Ahl98b and PHOBOS at RHIC Bac01aBac03a) and Pb+Pb (SPS Bea96Bac99) as a function of nucleon-nucleon center-of-mass energy. Error bars are statistical only.
• Figure 5: The ratio of antiprotons to protons emitted in a rapidity region spanning approximately $0.0<y<0.8$ (where positive rapidity is in the direction of the deuteron projectile) for d+Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV Bac04b. Data are shown for 4 centrality ranges. The parameter $\langle\nu\rangle$ is the average number of collisions suffered by each participant in the deuteron ($N_{coll}/N^d_{part}$). Statistical and point-to-point systematic uncertainties are shown as bars and brackets, respectively. The results of several models Gyu94Sor95Lin01Zha00 are shown for comparison.