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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.

Results are presented of a two-pion interferometry (HBT) analysis in Pb+Au collisions at 40, 80, and 158 AGeV. A detailed study of the Bertsch-Pratt HBT radius Preprint submitted to Elsevier Science 26 January 2018parameters has been performed as function of the mean pair transverse momentum k t and in bins of the centrality of the collision. From these results we extract model dependent information about the space-time evolution of the reaction. An investigation of the effective volume of the pion emitting system provides an important tool to study the properties of thermal pion freeze-out.

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Based on an evaluation of data on pion interferometry and on particle yields at mid-rapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions λ f reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from AGS to RHIC. A systematic study of the space-time extent and the dynamical behavior of the pion source in relativistic heavy ion collisions at thermal freeze-out can be obtained from analysis of pion interferometry (HBT) data. Understanding these aspects is vital for interpretation of the data in terms of formation of the quark-gluon plasma. Indeed recent HBT results from RHIC and how they fit into the systematics have been noted as a major puzzle [1]. In this letter we present an investigation of the freezeout conditions at beam energies from AGS to RHIC. In particular, the recently published CERES HBT data at 40, 80, and 158 A GeV [2] provide an important link between the existing results from AGS, SPS, and RHIC, thereby shedding light on the RHIC puzzle.Thermal freeze-out of pions and its connection to the mean free path has been discussed previously (see e.g. [3,4,5,6,7,8]). The mean free path of pions at freeze-out is defined aswhere σ is the total cross section of pions with the surrounding medium and ρ f is the freeze-out density which can be replaced by the number of particles N contained in the freeze-out volume V f . The pion freeze-out volume V f can be accessed experimentally by pion interferometry. Mid-rapidity pion HBT data have been published from central collisions of lead and gold nuclei over a wide range of beam energies. Here, we focus on recent HBT results from three experiments which have kinematical access to the region of low transverse pair momentum k t = 1 2 | p t,1 + p t,2 |: Experiment E895 at the AGS [9], the CERES/NA45 experiment at the SPS [2,10], and the STAR experiment at RHIC [11]. All three experiments employ large volume Time Projection Chambers (TPCs), thereby applying comparable analysis methods with similar sources of systematic uncertainties.For the calculation of the freeze-out volume V f we use the following expression:assuming a density distribution of Gaussian shape in all three dimensions. The longitudinal and sideward radius parameters R long and R side are measured in the longitudinal co-moving system of the pion pair, using the cartesian