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 nuclear modification factor, R AA , of the prompt charmed mesons D 0 , D + and D * + , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy √ s NN = 2.76 TeV in two transverse momentum intervals, 5 < p T < 8 GeV/c and 8 < p T < 16 GeV/c, and in six collision centrality classes. The R AA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the R AA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.Keywords: Charm physics, Heavy Ions, Heavy-ion collision The ALICE collaboration 17 IntroductionWhen heavy nuclei collide at high energy, a state of strongly-interacting matter with high energy density is expected to form. According to Quantum Chromodynamics (QCD) calculations on the lattice, this state of matter, the so-called Quark-Gluon Plasma (QGP) is characterised by the deconfinement of the colour charge (see e.g. [1][2][3][4]). High-momentum partons, produced at the early stage of the nuclear collision, lose energy as they interact with the QGP constituents. This energy loss is expected to proceed via both inelastic (gluon radiation) [5,6] and elastic (collisional) processes [7][8][9]. The nuclear modification factor R AA is used to characterise parton energy loss by comparing particle production yields in nucleus-nucleus collisions to a scaled proton-proton (pp) reference, that corresponds to a superposition of independent nucleon-nucleon collisions. R AA is defined aswhere dσ pp /dp T and dN AA /dp T are the transverse momentum (p T ) differential cross section and yield in proton-proton and nucleus-nucleus (AA) collisions, respectively. T AA is the average nuclear overlap function, estimated within the Glauber model of the nucleusnucleus collision geometry, and proportional to the average number of nucleon-nucleon (binary) collisions [10,11]. Energy loss shifts the momentum of quarks and gluons, and thus hadrons, towards lower values, leading to a suppression of hadron yields with respect to binary scaling at p T larger than few GeV/c (R AA < 1). Energy loss is expected to be smaller for quarks than for gluons because the colour charge factor of quarks is smaller than that of gluons [5,6]. In the energy regime of the Large Hadron Collider (LHC), light-flavour hadrons with p T ranging from 5 to 20 GeV/c originate predominantly from gluon fragmentation (see e.g. As discussed in ref.[15], this should be the case also for charm and beauty quarks produced in gluon splitting processes, if their transverse momentum is lower than about 50 GeV/c. Therefore, the comparison of the heavy-flavour hadron R AA with that of pions allows the colour-charge dependence of parton energy loss to be tested. The softer fragmentation of gluons than that of charm quarks, and the observed increase of the charged hadr...

A brief review of the string percolation model and its results are presented together with the comparison to experimental data. First, it is done an introduction to the quark-gluon phase diagram and the lattice results concerning the connement and the percolation of center domains. It is studied the interaction of the strings produced in nucleus-nucleus and proton-proton collisions showing how the string percolation arises. The main consequences of the string percolation, concerning the dependence on the energy and centrality, on the multiplicities and the mean transverse momentum are obtained comparing with experimental data. It is emphasized the non-abelian character of the color eld of the strings forming the cluster to reproduce the rise of the transverse momentum with multiplicity and the relative suppression of multiplicities. It is also studied dierent observables like multiplicity and transverse momentum distributions, dependence with multiplicity and transverse momentum correlations, forward-backward correlations, the strength of the Bose-Einstein correlations, dependence on the multiplicity of J/ψ  production and its possible suppression in pp collisions at high multiplicity, strangeness enhancement, elliptic ow, and ridge structure. The comparison with the data shows an overall agreement. The thermodynamical properties of the extended cluster formed in the collision are discussed computingits energy and entropy density, shear viscosity over entropy density ratio, bulk viscosity, sound speed and trace anomaly as a function of temperature, showing a remarkable agreement with lattice QCD evaluations. The string percolation can be regarded as the initial frame able to describe the collective behavior produced in AA and pp collisions.

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