ALICE is the heavy-ion experiment at the CERN Large Hadron Collider. The experiment continuously took data during the first physics campaign of the machine from fall 2009 until early 2013, using proton and lead-ion beams. In this paper we describe the running environment and the data handling procedures, and discuss the performance of the ALICE detectors and analysis methods for various physics observables.

ALICE is an LHC experiment devoted to the study of strongly interacting matter in proton-proton, proton-nucleus and nucleus-nucleus collisions at ultra-relativistic energies. The ALICE VZERO system, made of two scintillator arrays at asymmetric positions, one on each side of the interaction point, plays a central role in ALICE. In addition to its core function as a trigger source, the VZERO system is used to monitor LHC beam conditions, to reject beam-induced backgrounds and to measure basic physics quantities such as luminosity, particle multiplicity, centrality and event plane direction in nucleus-nucleus collisions. After describing the VZERO system, this publication presents its performance over more than four years of operation at the LHC. * See Appendix A for the list of collaboration members The VZERO systemThe detailed description of the VZERO system may be found in [3,4] and references therein. It is composed of two arrays, VZERO-A and VZERO-C, which cover the pseudorapidity ranges 2.8 < η < 5.1 and −3.7 < η < −1.7 for collisions at the nominal vertex (z = 0). Each of the VZERO arrays is segmented in four rings in the radial direction, and each ring is divided in eight sections in the azimuthal direction (Fig. 1). The pseudorapidity coverage of each ring is given in Tab. 1. Each channel of both arrays is made of a BC404 plastic scintillator from Bicron [5] with a thickness of 2.5 and 2.0 cm for VZERO-A and VZERO-C respectively. BCF9929A Wave-Length Shifting (WLS) fibers from Bicron are embedded in both faces of the segments (VZERO-A) or glued along their two radial edges (VZERO-C). Figure 2 shows a schematic view of an elementary cell of each array. These two different designs were mandatory to comply with the integration constraints of each array. The VZERO-A is located 329 cm from the nominal vertex (z = 0) on the side opposite to the muon spectrometer (Fig. 3). The VZERO-C is fixed on the front face of the hadronic absorber. The position of the various rings of the VZERO-C along the z direction is given in Tab. 1. The specific geometry of the VZERO-C array permits an optimized azimuthal coverage while leaving room for the passage of the WLS fibers. As shown in Fig. 2 there are no such constraints for the VZERO-A array.The light is transferred to the R5946-70 photomultiplier tube (PMT) (Hamamatsu [6]) directly for VZERO-A and through an extra 3 m of PMMA (Poly(methyl methacrylate)) clear fibers (Mitsubishi [7]) for VZERO-C (Fig. 2). Fine mesh PMTs were chosen for their capacity to operate in a magnetic field,

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The main tracking detector of the ALICE experiment at the LHC is a large time projection chamber. This detector has been fully assembled and, before being lowered to the underground experimental area, was commissioned with cosmic rays and tracks produced by an UV laser system. This paper presents the commissioning including preliminary results from the analysis of data on noise, electron diffusion, drift velocity and spatial resolution.