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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.

The ratio of the yields of antiprotons to protons in pp collisions has been measured by the ALICE experiment at sqrt[s]=0.9 and 7 TeV during the initial running periods of the Large Hadron Collider. The measurement covers the transverse momentum interval 0.45

The production of π + , π − , K + , K − , p, and p at mid-rapidity has been measured in proton-proton collisions at √ s = 900 GeV with the ALICE detector. Particle identification is performed using the specific energy loss in the inner tracking silicon detector and the time projection chamber. In addition, time-of-flight information is used to identify hadrons at higher momenta. Finally, the distinctive kink topology of the weak decay of charged kaons is used for an alternative measurement of the kaon transverse momentum (p t ) spectra. Since these various particle identification tools give the best separation capabilities over different momentum ranges, the results are combined to extract spectra from p t = 100 MeV/c to 2.5 GeV/c. The measured spectra are further compared with QCD-inspired models which yield a poor description. The total yields and the mean p t are compared with previous measurements, and the trends as a function of collision energy are discussed.t Deceased.

The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy √ s = 7 TeV, were measured in the central pseudorapidity region |η| < 1. Comparisons are made with previous measurements at √ s = 0.9 TeV and 2.36 TeV. At √ s = 7 TeV, for events with at least one charged particle in |η| < 1, we obtain dN ch /dη = 6.01 ± 0.01(stat.) +0.20 −0.12 (syst.). This corresponds to an increase of 57.6% ± 0.4%(stat.) +3.6 −1.8 %(syst.) relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV is described fairly well by the negative binomial distribution.