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.

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...

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

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We present charged-particle multiplicities as a function of pseudorapidity and collision centrality for the 197Au+197Au reaction at square root[s(NN)] = 200 GeV. For the 5% most central events we obtain dN(ch)/deta/(eta = 0) = 625+/-55 and N(ch)/(-4.7< or =eta < or =4.7) = 4630 +/- 370, i.e., 14% and 21% increases, respectively, relative to square root[s(NN)] = 130 GeV collisions. Charged-particle production per pair of participant nucleons is found to increase from peripheral to central collisions around midrapidity. These results constrain current models of particle production at the highest RHIC energy.

We present spectra of charged hadrons from Au+Au and d+Au collisions at sqrt[s(NN)]=200 GeV measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from p+(-)p collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central Au+Au collisions at eta=0 and eta=2.2 evidence a strong suppression in the high p(T) region (>2 GeV/c). In contrast, the d+Au nuclear modification factor (at eta=0) exhibits an enhancement of the high p(T) yields. These measurements indicate a high energy loss of the high p(T) particles in the medium created in the central Au+Au collisions. The lack of suppression in d+Au collisions makes it unlikely that initial state effects can explain the suppression in the central Au+Au collisions.

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