Measurement of transverse energy at midrapidity in Pb-Pb collisions atsNN=2.76TeV

Abstract

We report the transverse energy (E T ) measured with ALICE at midrapidity in Pb-Pb collisions at √ s NN = 2.76 TeV as a function of centrality. The transverse energy was measured using identified single-particle tracks. The measurement was cross checked using the electromagnetic calorimeters and the transverse momentum distributions of identified particles previously reported by ALICE. The results are compared to theoretical models as well as to results from other experiments. The mean E T per unit pseudorapidity (η), dE T /dη , in 0%-5% central collisions is 1737 ± 6(stat.) ± 97(sys.) GeV. We find a similar centrality dependence of the shape of dE T /dη as a function of the number of participating nucleons to that seen at lower energies. The growth in dE T /dη at the LHC energies exceeds extrapolations of low-energy data. We observe a nearly linear scaling of dE T /dη with the number of quark participants. With the canonical assumption of a 1 fm/c formation time, we estimate that the energy density in 0%-5% central Pb-Pb collisions at √ s NN = 2.76 TeV is 12.3 ± 1.0 GeV/fm 3 and that the energy density at the most central 80 fm 2 of the collision is at least 21.5 ± 1.7 GeV/fm 3 . This is roughly 2.3 times that observed in 0%-5% central Au-Au collisions at √ s NN = 200 GeV.

Centrality and pseudorapidity dependence of the transverse energy density inpPbcollisions atsNN=5.02TeV

“…√ s NN ≈ 10 GeV the scaled transverse energy density increases as a power law according to s γ NN . Such an energy dependence has been previously observed for the charged-particle multiplicity density, dN ± /dη, near η = 0 [3,19,36]. Table II lists the results of fitting the energy dependence of the scaled dN ± /dη and dE T /dη for central events to a function of the form s γ NN .…”

“…Figure 2 also shows a compilation of results for central AA collisions from Ref. [19] with the addition of a recent ALICE PbPb data point [3]. Although the geometries and lifetimes of pA and AA collisions are very different, it is interesting to note that the pPb minimum bias value of 5.33 ± 0.25 GeV per participant pair is higher than the central AuAu result at √ s NN = 200 GeV [19] and consistent with the peripheral PbPb result at 2.76 TeV [4].…”

“…The uncertainties are generally smaller than the size of the data points. Also shown are the corresponding results for central AuAu and PbPb collisions, as well as simulation for minimum bias pPb collisions from three event generators [3,4,18,[37][38][39][40][41][42][43][44][45][46].…”

“…The time τ 0 at which it is first appropriate to speak about an energy density is a model assumption. Some collaborations have chosen to report the product of energy density and proper time BJ τ 0 [2,3] while others have used τ 0 = 1 fm/c as a reference value [4,5]. For the top 5% most central lead-lead collisions at √ s NN = 2.76 TeV, this formula gives energy densities up to 14 GeV/fm 3 at a time τ 0 = 1 fm/c [4].…”

Energy density and path-length dependence of the fractional momentum loss in heavy-ion collisions at sNN from 62.4 to 5020 GeV

“…Since no transverse energy data are available for the top LHC energy ( √ s NN = 5.02 TeV), the corresponding values were extrapolated using the fact that, within 25%, dE T /dη / N ch /dη vs. N part is nearly energy independent. This has been reported by the ALICE Collaboration, where such a scaling holds for measurements at RHIC and run I LHC energies [29]. Due to this assumption, 15% was assigned as systematic uncertainty to dE T /dη for Pb-Pb collisions at √ s NN = 5.02 TeV.…”