ROOT is an object-oriented framework aimed at solving the data analysis challenges of high-energy physics. Here we discuss the main components of the framework. We begin with an overview describing the framework's organization, the interpreter CINT, its automatic interface to the compiler and linker ACLiC, and an example of a first interactive session. The subsequent sections cover histogramming and fitting. Then, ROOT's solution to storing and retrieving HEP data, building and managing of ROOT files, and designing ROOT trees. Followed by a description of the collection classes, the GUI classes, how to add your own classes to ROOT, and PROOF, ROOT's parallel processing facility.

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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.

This publication describes the methods used to measure the centrality of inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding nucleon pair with ALICE. The centrality is a key parameter in the study of the properties of QCD matter at extreme temperature and energy density, because it is directly related to the initial overlap region of the colliding nuclei. Geometrical properties of the collision, such as the number of participating nucleons and the number of binary nucleon-nucleon collisions, are deduced from a Glauber model with a sharp impact parameter selection and shown to be consistent with those extracted from the data. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.