Some results obtained at the relativistic heavy ion collider
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me Results Obtained at the Relativistic Heavy Ion Collider A. G. Litvinenko Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia e-mail: [email protected] Abstract—Some results obtained in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory are discussed. Along with observables studied at energies lower than those at RHIC (particle production ratios, soft spectra, flows, and so on), observables specific for RHIC energies related to the Jet Quenching effect and Color Glass Condensate (CGC) manifestation are discussed. Preliminary results and specifics of particular experiments at RHIC are not considered. PACS numbers: 25.75 Dw DOI: 10.1134/S1063779607020037
1. INTRODUCTION The problems of investigation of the properties of nuclear matter at extremely high densities and temperatures were put forward before quantum chromodynamics was formulated as the theory describing strong interactions (see, e.g., [1–3]). Very soon after the ideas of confinement and asymptotic freedom were accepted, it was realized that states with deconfined quarks and gluons (partons) can exist in nuclear matter under extreme conditions (strong compression and/or heating of hadron matter). Thus, it was noted in [4] that hadron matter with properties determined by deconfined quarks and gluons, rather than colorless hadrons, (strongly compressed nuclear matter) can occur in the center of a neutron star due to decreasing coupling constant at small distances. The possible existence of deconfinement at an early stage of the evolution of the Universe was pointed out in [5, 6]. The term quark– gluon plasma (QGP) was probably introduced in [7], where it was pointed out that such a state can appear in heavy ion collisions if an energy density of an order of 1 GeV/fm3 is reached. Progress in theoretical understanding of the regularities of transition from the state of nuclear matter with the properties determined by colorless hadrons to the state with the properties determined by partons is related to lattice calculations of QCD (see [8, 9]). The success of these calculations is determined to a large extent by the rapid growth of computer power during the past twenty years. Experimental studies of the formation of quark– gluon plasma in heavy ion collisions have been conducted for over twenty years. In this period, a wide range of collision energies S NN = 1 GeV at Bevalac (LBNL), 5 GeV at AGS (BNL), and 17 GeV at SPS (CERN) have been studied. No unambiguous indica-
tion of QGP formation was obtained at these energies. However, it should be noted that results which cannot be explained in the framework of nucleon–nucleon scattering and which are assumed to point to the formation of “strongly compressed nuclear matter” (see [10]) were obtained in experiments at SPS. The Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL) was put in operation in 2000, and provides a possibility of investigation of nuclear–nuclear collisions at the highest energies achieved
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