Recent results from the BRAHMS experiment

We present recent results obtained by the BRAHMS experiment at the Relativistic Heavy Ion Collider (RHIC) for the systems of Au+Au and Cu+Cu at root s(NN) = 200 GeV and at 62.4 GeV, and p+p at root s(NN) = 200 GeV. BRAHMS explores reaction dynamics and properties of the hot and high energy density matter produced in ultra-relativistic heavyion collisions versus its longitudinal expansion. Overall charged-particle production and particle spectra over a large rapidity interval and PT range are presented. Nuclear modification factors for Au+Au and Cu+Cu collisions are discussed. The observed number of charged particles produced per unit of rapidity in the central rapidity region indicates that a high energy density system is produced at the initial stage of the Au+Au reaction. Analysis of anti-particle to particle ratios as a function of rapidity and collision energy reveal that, particle populations at the chemical freeze-out stage for heavy-ion reactions at and above SPS energies are controlled by the baryon chemical potential. From the particle spectra we deduce significant radial expansion (beta approximate to 0.75), as expected for systems created with a large initial energy density. We also measure the elliptic flow parameter nu(2) versus rapidity and p(T). A weak dependence on rapidity of the p(T) differential nu(2) is observed. We present rapidity dependent p/pi ratios within 0 < y < 3 for Au+Au and Cu+Cu at root s(NN) 200 GeV. The ratios are enhanced in nucleus-nucleus collisions as compared to p+p collisions. The particle ratios are discussed in terms of their system size and rapidity dependence. We compare R-AA for Au+Au at root s(NN) = 200 GeV and at 62.4 GeV, and for Au+Au and Cu+Cu at root s(NN) = 62.4 GeV. R-AA is found to increase with decreasing collision energy, decreasing system size, and when going towards more peripheral collisions. However, R-AA shows only a very weak dependence on rapidity (for 0 < y < 3.2), both for pions and protons.