Charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at S[sub NN] = 2.76 TeV with ALICE at the LHC

Abstract

The multiplicity of charged particles in the central rapidity region has been measured by the NA57 experiment in Pb-Pb collisions at the CERN SPS at two beam momenta: 158 A GeV/c and 40 A GeV/c. The value of dN ch /dη at the maximum has been determined and its behaviour as a function of centrality has been studied in the centrality range covered by NA57 (about 50% of the inelastic cross section). The multiplicity increases approximately logarithmically with the centre of mass energy.

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“…This was done using the ratio between the yield of Ξ and Ω hyperons in Pb-Pb and p-p after normalizing to the number of participant nucleons. An enhancement of the production of Ξ and Ω in heavy-ion collisions with respect to p-p is observed also at LHC energies [20]. The enhancement is lower than that observed at SPS and RHIC energies, confirming its decreasing trend with increasing √ s. This is a consequence of the smaller effect of canonical suppression for strangeness production in p-p reactions at higher collision energies.…”

“…Charged pions, kaons, protons are identified via their dE/dx and time-of-flight. K 0 S mesons, Λ, Ξ and Ω hyperons are reconstructed from their decay topologies [18,19,20].…”

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“…Uncertainties on the pp references (or pBe for the SPS data) are noted with rectangles on the left hand of the dotted line located at unity. Figure 3 summarizes the enhancement factors obtained for different hyperon species as a function of the mean number of nucleons participating in the collision for WA97/NA57 at the SPS [15,16], STAR at RHIC [17] and ALICE at the LHC [20]. The corresponding colliding energies are √ s N N = 17.2 GeV, 200 GeV and 2.76 TeV respectively.…”

“…The STAR and ALICE yields for Au-Au and Pb-Pb respectively were also extracted as a function of centrality and translated to a mean number of participants, then normalized to pp collisions (in the case of ALICE, the pp yields are interpolated between √ s = 0.9 TeV and 7 TeV). Details on the signal extraction and efficiency corrections at √ s N N = 2.76 TeV with the ALICE experiment can be found in [20].…”

“…The observed effect matches one of the earliest predictions made for the QGP formation, as it was reiterated at this conference (dedicated discussions can be found in [19] and references therein). [20]. The quadratic sum of statistical and systematics uncertainties are represented with vertical errors on data points.…”