Charged-particle multiplicity density at mid-rapidity in central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

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2.76 TeV
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The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair sNN=2.76  TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584±4(stat)±76(syst), which corresponds to 8.3±0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at sNN=0.2  TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.