Recent results from the BRAHMS experiment

Abstract

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 s NN = 200 GeV and at 62.4 GeV, and p+p at s NN = 200 GeV . BRAHMS explores reaction dynamics and properties of the hot and high energy density matter produced in ultra-relativistic heavy-ion collisions versus its longitudinal expansion. Overall charged-particle production and particle spectra over a large rapidity interval and p T 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 ( β ≈ 0.75 ), as expected for systems created with a large initial energy density. We also measure the elliptic flow parameter v 2 versus rapidity and p T . A weak dependence on rapidity of the p T differential v 2 is observed. We present rapidity dependent p / π ratios within 0 y 3 for Au+Au and Cu+Cu at 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 s NN = 200 GeV and at 62.4 GeV, and for Au+Au and Cu+Cu at s NN = 200 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.