ALICE is a general-purpose heavy-ion experiment designed to study the physics of strongly interacting matter and the quark–gluon plasma in nucleus–nucleus collisions at the LHC. It currently involves more than 900 physicists and senior engineers, from both the nuclear and high-energy physics sectors, from over 90 institutions in about 30 countries.The ALICE detector is designed to cope with the highest particle multiplicities above those anticipated for Pb–Pb collisions (dNch/dy up to 8000) and it will be operational at the start-up of the LHC. In addition to heavy systems, the ALICE Collaboration will study collisions of lower-mass ions, which are a means of varying the energy density, and protons (both pp and pA), which primarily provide reference data for the nucleus–nucleus collisions. In addition, the pp data will allow for a number of genuine pp physics studies.The detailed design of the different detector systems has been laid down in a number of Technical Design Reports issued between mid-1998 and the end of 2004. The experiment is currently under construction and will be ready for data taking with both proton and heavy-ion beams at the start-up of the LHC.Since the comprehensive information on detector and physics performance was last published in the ALICE Technical Proposal in 1996, the detector, as well as simulation, reconstruction and analysis software have undergone significant development. The Physics Performance Report (PPR) provides an updated and comprehensive summary of the performance of the various ALICE subsystems, including updates to the Technical Design Reports, as appropriate.The PPR is divided into two volumes. Volume I, published in 2004 (CERN/LHCC 2003-049, ALICE Collaboration 2004 J. Phys. G: Nucl. Part. Phys. 30 1517–1763), contains in four chapters a short theoretical overview and an extensive reference list concerning the physics topics of interest to ALICE, the experimental conditions at the LHC, a short summary and update of the subsystem designs, and a description of the offline framework and Monte Carlo event generators.The present volume, Volume II, contains the majority of the information relevant to the physics performance in proton–proton, proton–nucleus, and nucleus–nucleus collisions. Following an introductory overview, Chapter 5 describes the combined detector performance and the event reconstruction procedures, based on detailed simulations of the individual subsystems. Chapter 6 describes the analysis and physics reach for a representative sample of physics observables, from global event characteristics to hard processes.
A measurement of strange baryon and antibaryon production in Pb-Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 A GeV/c beam momentum. Results on Λ, Ξ and Ω hyperon yields at mid-rapidity in the most central 53% of Pb-Pb collisions at 40 A GeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range.The Λ and Ξ − yields per unit rapidity stay roughly constant while those of Ω − , Λ, Ξ + and Ω + increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in √ s N N = 130 GeVAu-Au collisions at RHIC.
We argue that strong final state rescattering among the secondary particles created in relativistic heavy ion collisions is essential to understand the measured Bose-Einstein correlations. The recently suggested "random walk models" which contain only initial state scattering are unable to reproduce the measured magnitude and K ⊥ -dependence of R ⊥ in Pb+Pb collisions and the increase of R l with increasing size of the collision system. 25.75.Gz An important aspect of understanding ultrarelativistic heavy ion collisions is the clear identification of genuine collective nuclear effects which cannot be explained in terms of a simple superposition of nucleon-nucleon collisions. An integral part of our task to look for new physics must therefore be the careful construction of models for nucleus-nucleus collisions ("A-B collisions") based on a superposition of individual nucleon-nucleon (N -N ) collisions in order to establish where they fail.Recently there have been some renewed attempts to construct such models. In the "Random Walk Model" (RWM) of [1] the single-particle transverse mass spectra measured in A-B collision have been calculated by extrapolating those from pA collisions. The LEXUS model of [2] goes even further, by simulating A-B collisions as a simple folding of independent N -N collisions.Here we demonstrate that, while these models have had some success in describing measured single-particle spectra, they fail to reproduce crucial features of the observed two-particle Bose-Einstein correlations. This is shown to be due to their lack of final state rescattering.The idea behind the RWM was to provide an alternative interpretation of the measured single-particle m ⊥ spectra, opposite in spirit to the popular hydrodynamical parametrizations [3]. The latter are based on the assumption of a locally thermalized hadron resonance gas which undergoes longitudinal and transverse hydrodynamic expansion. The transverse expansion is interpreted as a genuine nuclear collective effect with no analogue in N -N collisions; it is identified through the characteristic flattening it causes in the transverse mass spectra, especially at small M ⊥ < 2m 0 where the inverse slope parameter is found to increase linearly with the rest mass m 0 of the produced hadrons [3,4].The RWM, on the other hand, starts from the observation that such a flattening, relative to N -N collisions, happens even in pA collisions where hydrodynamic transverse expansion is not expected to occur. In the RWM an incident nucleon undergoes multiple collisions in the nuclear target, leading to a random walk pattern in the transverse momentum plane. When it collides inelastically it creates a little "fireball", identical to those formed in elementary inelastic pp collisions, but moving with a transverse rapidity ρ which is Gaussian distributed. In A-B collisions the same mechanism works for both projectile and target nucleons. The width of the ρ distribution is fixed in pA collisions and then extrapolated to A-B collisions using geometric consideration...
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