The Large Hadron Collider (LHC) soon to be commissioned at CERN opens a new energy frontier and a new era of quantitative understanding of the phase transition between hadronic nuclear matter and the Quark-Gluon Plasma. The primary and initial question for the relativistic heavy ion (RHI) program at the LHC is to establish whether the QGP formed at the LHC behaves like a strongly coupled perfect liquid as discovered at RHIC or whether it enters a new regime approaching the ideal QGP gas of weakly interacting quarks and gluons. The ATLAS detector has excellent capabilities to make a significant contribution to the LHC RHI program. This proposal deals with measurements of the centrality dependence of global characteristics of RHI collisions at LHC using the ATLAS detector. It focuses in particular on the measurements of charged particles and transverse energy pseudorapidity density, the inclusive particle spectra and the amount of the elliptic flow. These measurements can be carried out using the relatively small data samples expected in the first heavy ion runs at the LHC and they are likely to provide first answers on the nature of the matter formed in nuclear collisions at the LHC. A crucial pre-requisite for these measurements is to establish the methodology to define centrality and categorize the collisions according to the number of participating nucleons or the number of elementary collisions. Special effort will be devoted to determine centrality using the most reliable and universal approach based on the Glauber-inspired model of HI collisions and the statistics of the Negative Binomial Distribution.
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