ALICE alignment challenge

Abstract

ALICE physics goals and the characteristics of the ALICE detector essential to achieve them are briefly described. The reconstruction precision for ALICE physics is presented and commented upon from the point of view of the alignment quality. The strategic choices to cope with the alignment challenges are briefly developed. 13.1 ALICE experimental programme The main goal of ALICE is to study the role of chiral symmetry in the generation of mass in composite particles (hadrons) using heavy-ion collisions, in order to attain high-energy densities over large volumes and long time scales [1]. In addition, the aim is to study the physics of parton densities close to phasespace saturation, and parton collective dynamical evolution towards hadronization (confinement) in a dense nuclear environment. In this way, one expects to gain a deeper insight into the structure of the QCD-phase diagram and the properties of the QGP (Quark Gluon Plasma) phase. The nucleon–nucleon centre-of-mass energy for collisions of the heaviest ions at the LHC (PbPb, √ s = 5.5 TeV) exceeds that available at RHIC by a factor of about 30, opening up a new physics domain. The successful completion of the heavy-ion programme requires the study of pp, pA and lighter A– A collisions in order to establish the benchmark processes under the same experimental conditions. Moreover, these measurements are interesting per se as ALICE has a unique ability to study both soft and hard aspects of such interactions. A list of the main heavy-ion observables in ALICE follows: – particle multiplicities, – particle spectra, – particle correlations (flow, HBT), – event-by-event fluctuations, – jets, – direct photons, – dileptons, – heavy-quarks and quarkonia. These observables, studied as a function of collision centrality, form a basis for accessing the physics goals of ALICE. The possibility of reaching them and the number of events needed in order to obtain satisfactory significance levels will depend on the performance of the detectors and on the precision of their calibration and alignment. [2]. In general, the number of collected events depends on the nature and the energy of the beam, the luminosity, and the running time. For completeness let us mention that the overall success of the ALICE physics programme will depend, as for the other LHC experiments, on the background conditions of the LHC. ALICE will take its first data with pp collisions because the LHC will be commissioned with proton beams, and also because pp physics is an integral part of the ALICE programme. ALICE will in fact require pp running throughout its operation; during the initial years longer periods to both commission the detector and to take pp physics data, and later in the programme, shorter periods to start up and calibrate/align the detector prior to each heavyion period. The very first data will be most probably taken with the cosmics which enable initial precious information to be obtained concerning the mechanical precision of most of the ALICE detectors. Later on cosmic events will be collected in parallel with the normal data-taking in the framework of the standard running scheme, both for physics and alignment.