Modification factor, high pT suppression and new states of the nuclear matter. BRAHMS experiment results

The BRAHMS (Broad RAnge Hadron Magnetic Spectrometers) Experiment is placed at two o'clock on the ring of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (USA). This experimental setup was designed to gather information on the interesting physical quantities characterising various emitted particles in ultrarelativistic nuclear collisions as functions of transverse momentum, pT, and rapidity, y. The largest range in rapidity can be investigated with BRAHMS. In the four running years many experimental data have been collected in Au-Au collisions at √SNN = 62.4 GeV, √SNN = 130 GeV and √SNN 200 GeV, as well as in d-Au collisions at √SNN = 200 GeV. The experimental data obtained in p-p collisions at the same energies are used as reference data, too. The experimental results include information on charged particle multiplicity, pseudorapidity and rapidity distributions, participants, energy density, momentum spectra, nuclear temperatures and radial flow, antiparticle to particle ratios, Coulomb momentum, chemical potentials and entropy per baryon. Nuclear modification factors have been studied in Au-Au and d-Au collisions at √SNN = 200 GeV by measurements of different transverse spectra at y ∼ 0.0 and high rapidities, too. For understanding the initial partonic state the high pT hadrons have been investigated over a large range of rapidities. The study of the evolution of the high pT components of the hadronic spectra evidenced the high pT suppression. This is an important tool in the investigation of the high density nuclear matter formed in nucleus-nucleus collisions at RHIC. The high pT suppression may result from the energy loss of the quarks and gluons passing through a dense partonic system. Interesting information on the longitudinal extent of this dense system, initial scattering effects (Cronin effect), gluon shadowing effects and jet quenching can be obtained. At very small x gluons may fuse together. Therefore, it is expected that at RHIC energies the soft gluons from different nuclei may fuse to form a Color Glass Condensate (CGC). This state can be considered as a 'smoke gun' of the quark gluon plasma. All quantities offer information on the behaviour of the highly excited and dense nuclear matter formed in the overlapping region of the two colliding nuclei, as well as on the possible formation of some new forms of the nuclear matter. Some comments and comparisons with different assumptions are included, too. A careful analysis of the experimental results evidenced some new states of the high excited and dense nuclear matter. (authors)