We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate.

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Data from a number of different experimental measurements have been used to construct caloric curves for five different regions of nuclear mass. These curves are qualitatively similar and exhibit plateaus at the higher excitation energies. The limiting temperatures represented by the plateaus decrease with increasing nuclear mass and are in very good agreement with results of recent calculations employing either a chiral symmetry model or the Gogny interaction. This agreement strongly favors a soft equation of state. Evidence is presented that critical excitation energies and critical temperatures for nuclei can be determined over a large mass range when the mass variations inherent in many caloric curve measurements are taken into account.Comment: In response to referees comments we have improved the discussion of the figures and added a new figure showing the relationship between the effective level density and the excitation energy. The discussion has been reordered and comments are made on recent data which support the hypothesis of a mass dependence of caloric curve

We report on a study of the transverse momentum dependence of nuclear modification factors R dAu for charged hadrons produced in deuteron + gold collisions at √ sNN = 200 GeV, as a function of collision centrality and of the pseudorapidity (η = 0, 1, 2.2, 3.2) of the produced hadrons. We find a significant and systematic decrease of R dAu with increasing rapidity. The mid-rapidity enhancement and the forward rapidity suppression are more pronounced in central collisions relative to peripheral collisions. These results are relevant to the study of the possible onset of gluon saturation at energies reached at BNL RHIC.

We have measured rapidity densities dN/dy of π ± and K ± over a broad rapidity range (−0.1 < y < 3.5) for central Au+Au collisions at √ sNN = 200 GeV. These data have significant implications for the chemistry and dynamics of the dense system that is initially created in the collisions. The full phase-space yields are 1660±15±133 (πThe systematics of the strange to non-strange meson ratios are found to track the variation of the baryo-chemical potential with rapidity and energy. Landau-Carruthers hydrodynamic is found to describe the bulk transport of the pions in the longitudinal direction.In ultra-relativistic heavy ion collisions at RHIC energies, charged pions and kaons are produced copiously. The yields of these light mesons are indicators of the entropy and strangeness created in the reactions, sensitive observables to the possible existence of an early color deconfined phase, the so-called quark gluon plasma. In such collisions, the large number of produced particles and their subsequent reinteractions, either at the partonic or hadronic level, motivate the application of concepts of gas or fluid dynamics in their interpretation. Hydrodynamical properties of the expanding matter created in heavy ion reactions have been discussed by Landau [1] (full stopping) and Bjorken [2] (transparency), in theoretical pictures using different initial conditions. In both scenarios, thermal equilibrium is quickly achieved and the subsequent isentropic expansion is governed by hydrodynamics. The relative abundances and kinematic properties of particles provide an important tool for testing whether equilibrium occurs in the course of the collision. In discussing the source characteristics, it is important to measure most of the produced particles in order not to violate conservation laws (e.g. strangeness and charge conservation).In this letter, we report on the first measurements at RHIC energies of transverse momentum (p T ) spectra of π ± and K ± over the rapidity range −0.1 < y < 3.5 for the 5% most central Au+Au collisions at √ s N N = 200 GeV. The spectra are integrated to obtain yields as a function of rapidity (dN/dy), giving full phase-space (4π) yields. At RHIC energies, a low net-baryon density is observed at mid-rapidity [3], so mesons may be predominantly produced from the decay of the strong color field created initially. At forward rapidities, where primordial baryons are more abundant [4], other production mechanisms, for example associated strangeness production, play a larger role. Therefore, the observed rapidity distributions provide a sensitive test of models describing the space-time evolution of the reaction, such as Landau and Bjorken models [1,2]. In addition, integrated yields are a key input to statistical models of particle production [5,6].BRAHMS consists of two hadron spectrometers, a mid-rapidity arm (MRS) and a forward rapidity arm (FS), as well as a set of detectors for global event characterization [7]. Collision centrality is determined from charged particle multiplicities, measured by sc...

Experimental analyses of moderate temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon 64 Zn projectiles with 92 Mo and 197 Au target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature and density dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A ≤ 4. At densities of 0.01 to 0.05 times the ground state density of symmetric nuclear matter, the temperature and density dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean field Calculations and reflects the clusterization of low density nuclear matter. He are expected to be small and they are ignored in the calculation. In the work reported in reference [1] these virial coefficients were then used to make predictions for a variety of properties of nuclear matter over a range of density, temperature and composition. The authors view this virial equation of state, derived from experimental observables, as modelindependent, and therefore a benchmark for all nuclear equations of state at low densities. Its importance in both nuclear physics and in the physics of the neutrino sphere in supernovae is discussed in the VEOS paper [1]. A particularly important feature of the VEOS, emphasized in reference [1], is the natural inclusion of clustering which leads to large symmetry energies at low baryon density.In this paper we extend our investigations of the nucleon and light cluster emission that occurs in near-Fermi energy heavy ion collisions [2,3,4,5,6] to investigate the properties of the low density participant matter produced in such collisions. The data provide experimental evidence for a large degree of alpha clustering in this low density matter, in agreement with theoretical predictions [1,7,8,9]. Temperature and density dependent symmetry free energies and symmetry energies have been determined at densities of 0.05ρ 0 or less, where ρ 0 is the ground state density of symmetric nuclear matter, by application of an isoscaling analysis [10,11]. The symmetry energy coefficient values obtained, 9.03 to 13.6 MeV, are much larger then those derived from effective interactions in mean field models. The values are in reasonable agreement with those calculated in the VEOS treatment of reference [1]. EXPERIMENTAL PROCEDURESThe reactions of 35A MeV 64 Zn projectiles with 92 Mo and 197 Au target nuclei were studied at the K-500 SuperConducting Cyclotron at Texas A&M University, using the 4π detector array NIMROD [3]. NIMROD consists of a 166 segment charged particle array set inside a neu-

Charged particle pseudorapidity densities are presented for the 197 Au + 197 Au reaction at √ s NN = 130 GeV. These densities provide an essential characterization of the underlying reactions mechanisms for ultra-relativistic heavy-ion collisions. This talk details how the global charged particle yields are measured at BRAHMS and presents some preliminary results from the analysis of data taken during the first year of the RHIC experimental program.

Transverse momentum spectra and rapidity densities, dN/dy, of protons, anti-protons, and netprotons (p −p) from central (0-5%) Au+Au collisions at √ sNN = 200 GeV were measured with the BRAHMS experiment within the rapidity range 0 ≤ y ≤ 3. The proton and anti-proton dN/dy decrease from mid-rapidity to y = 3. The net-proton yield is roughly constant for y < 1 at dN/dy ∼ 7, and increases to dN/dy ∼ 12 at y ∼ 3. The data show that collisions at this energy exhibit a high degree of transparency and that the linear scaling of rapidity loss with rapidity observed at lower energies is broken. The energy loss per participant nucleon is estimated to be 73 ± 6 GeV. PACS numbers: 25.75 Dw.The energy loss of colliding nuclei is a fundamental quantity determining the energy available for particle production (excitation) in heavy ion collisions. This deposited energy is essential for the possible formation of a deconfined quark-gluon phase of matter (QGP). Because baryon number is conserved, and rapidity distributions are only slightly affected by rescattering in late stages of the collision, the measured net-baryon (B −B) distribution retains information about the energy loss and allows the degree of nuclear stopping to be determined. Such measurements can also distinguish between different proposed phenomenological mechanisms of initial coherent multiple interactions and baryon transport [1,2,3] .The average rapidity loss, δy = y p − y [23], is used to quantify stopping in heavy ion collisions [4,5]. Here, y p is rapidity of the incoming projectile and y is the mean net-baryon rapidity after the collision :where N part is the number of participating nucleons in the collision. The two extremes correspond to full stopping, where initial baryons lose all kinetic energy ( δy = y p ) and full transparency, where they lose no kinetic energy ( δy = 0). For fixed collision geometry (system size and centrality) at lower energy (SIS, AGS, and SPS) it was observed that δy is proportional to the projectile rapidity. For central collisions between heavy nuclei (Pb, Au), δy ∼ 0.58 · y p [5,6,7].Bjorken assumed that sufficiently high energy collisions are "transparent", thus the mid-rapidity region is approximately net-baryon free [8]. The energy density early in the collision, ǫ, can then be related in a simple way to the final particle production. At RHIC it has been estimated that ǫ ∼ 5 GeV/fm 3 , well above the lattice QCD prediction (ǫ crit ∼ 1GeV/fm 3 [9]) for the hadron gas to QGP phase transition.In this letter, results on proton and anti-proton production, and baryon stopping in Au + Au collisions at √ s N N = 200 GeV are presented. The data