Results from the BRAHMS experiment at RHIC

Results from the BRAHMS Collaboration for the inaugural physics run with the relativistic heavy ion collider (RHIC), Au + Au collisions at √s _NN = 130 GeV in the late summer of 2000, and some results of the very first data from the summer of 2001 at √s_NN = 200 GeV are presented. Here the main focus is on the determination of the pseudorapidity distribution of charged particles as a function of collision centrality and on the measurement of the antiproton to proton and the K^- to K ⁺ ratios as a function of rapidity. The pseudorapidity density at midrapidity reaches dN(ch)/dη = 553 ± 36 for the most central collisions (0-5%) at √s_NN = 130 GeV; the corresponding number for a higher energy is dN(ch)/dη = 632 ± 36, demonstrating an increase in multiplicity of about 14%. The measured antiproton/proton ratios peak at midrapidity around N(p̄)/N(p) = 0.64 ± 0.06 and drop to 0. 41 ± 0.04 at y ≈ 2 for √s_NN = 130 GeV reactions; the K^- to K⁺ ratios are 0.90 ± 0.06 (y ≈ 0) and 0. 83 ± 0.1 (y ≈ 2.5).