First results from the ALICE experiment
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ELEMENTARY PARTICLES AND FIELDS Experiment
First Results from the ALICE Experiment* I. Belikov** (for the ALICE Collaboration) IPHC, Universite´ de Strasbourg, CNRS-IN2P3, France Received March 31, 2011
Abstract—The results from first series of measurements performed by the ALICE experiment at the LHC are presented. These measurements include the charged-particle pseudorapidity densities, multiplicity distributions and transverse momentum spectra obtained by analyzing the data collected in 2009 and 2010 in proton–proton collisions at three different center-of-mass energies of 0.9, 2.36, and 7 TeV. The results are compared to previous proton–antiproton data and to model predictions. DOI: 10.1134/S1063778812050031
1. INTRODUCTION The energy density expected to be reached in PbPb collisions at the LHC will be of the order of a few tens of GeV/fm3 . Under these conditions, a deconfined state of quarks and gluons, the quark–gluon plasma, is expected to be formed. A Large Ion Collider Experiment (ALICE) [1] at CERN is a general-purpose heavy-ion experiment designed to study the physics of this new state of matter. However, these studies cannot be effectively performed without having a reliable “hadronic reference”. All the measurements that will be done by ALICE in heavy-ion collisions will have to be compared with the corresponding, properly scaled, proton–proton results. Also, it is important to check if the models that successfully described the particle production in “elementary” collisions at lower energies still do so when extrapolated to the LHC energy domain. Finally, one might expect something completely new (like some kind of collective effects at the partonic level) to happen in pp collisions at these new LHC energies. For this, we would need to study all the observables as the function of multiplicity. Which, in turn, requires good multiplicity measurements. The description of the ALICE detectors can be found in [1]. As for the first pp runs in 2009 and 2010, the most important detectors are the Inner Tracking System (ITS), the Time Projection Chamber (TPC), the Time-Of-Flight detector (TOF) (all covering the pseudo-rapidity range |η| < 0.9), and the muon spectrometer (MUON) (with the pseudorapidity coverage −4 < η < −2.5). All these detectors are fully installed and operational. ∗ **
The text was submitted by the author in English. E-mail: [email protected]
The triggering for the first collisions is essentially minimum bias. At least one charged particle is required in about 8 units of pseudorapidity covered by the two innermost pixel layers of the ITS (with the pseudorapidity windows of |η| < 2.0 and |η| < 1.4, respectively) and the two scintillating rings of the V0A and the V0C detectors (with the coverage of 2.8 < η < 5.1 and −3.7 < η < −1.7). A special single-muon trigger is also implemented for triggering the muon spectrometer, and it is read out in coincidence with the general minimum-bias trigger. The coincidence with the beam pickup counters, which can be done in several logical combinations, allows
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