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Abstract
The first measurement of the charged-particle multiplicity density at mid-rapidity
in Pb-Pb collisions at a centre-of-mass energy per nucleon pair sqrt(sNN) = 2.76 TeV
is presented. For an event sample corresponding to the most central 5% of the hadronic
cross section the pseudo-rapidity density of primary charged particles at mid-rapidity
is 1584 +- 4 (stat) +- 76 (sys.), which corresponds to 8.3 +- 0.4 (sys.) 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 sqrt(sNN) = 0.2 TeV. This measurement provides the first experimental constraint
for models of nucleus-nucleus collisions at LHC energies.