Measurement of the Pseudorapidity and Centrality Dependence of the Transverse Energy Density in Pb-Pb Collisions atsNN=2.76  TeV

Abstract

The transverse energy (E T ) in Pb-Pb collisions at 2.76 TeV nucleon-nucleon center-of-mass energy ( ffiffiffiffiffiffiffiffi s NN p ) has been measured over a broad range of pseudorapidity () and collision centrality by using the CMS detector at the LHC. The transverse energy density per unit pseudorapidity (dE T =d) increases faster with collision energy than the charged particle multiplicity. This implies that the mean energy per particle is increasing with collision energy. At all pseudorapidities, the transverse energy per participating nucleon increases with the centrality of the collision. The ratio of transverse energy per unit pseudorapidity in peripheral to central collisions varies significantly as the pseudorapidity increases from ¼ 0 to jj ¼ 5:0. For the 5% most central collisions, the energy density per unit volume is estimated to be about 14 GeV=fm 3 at a time of 1 fm=c after the collision. This is about 100 times larger than normal nuclear matter density and a factor of 2.6 times higher than the energy density reported at ffiffiffiffiffiffiffiffi s NN p ¼

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

“…± 0.50(sys.) GeV, two standard deviations below the value observed by CMS [30]. All methods resulted in a lower E T than that reported by CMS, although the systematic errors on the measurements are significantly correlated.…”

“…All methods resulted in a lower E T than that reported by CMS, although the systematic errors on the measurements are significantly correlated. One possible explanation of the differences is that the corrections for the CMS calorimetry measurement are determined by Monte Carlo [30], while the corrections for the ALICE measurement are mainly data driven. PHENIX [28] reported that while dE T /dη scaled by N part has a pronounced centrality dependence, as seen in Fig.…”

“…To be compatible with the E T of a calorimetry measurement, the energy E j of Eq. (1) must be replaced with the single-particle energies This definition of E T was used in the measurements of the transverse energy by CMS [30] (based on calorimetry), PHENIX [26] (based on electromagnetic calorimetry), and STAR [29] (based on a combination of electromagnetic calorimetry and charged-particle tracking). To facilitate comparison between the various data sets the definition of E T given by Eqs.…”

Kenneth Wilson and Lattice QCD

“…We observe very good agreement up to Let us see how lattice QCD predictions compare with heavy ion experiments at RHIC and the LHC. Phenomenological estimates on the energy density reached at the initial stage of the collision range from e ≈ 5.6 GeV/fm 3 for Au-Au collisions at RHIC with √ s N N = 200 GeV [131], to e ≈ 15 GeV/fm 3 for Pb-Pb collisions at the LHC with √ s N N = 2.76 TeV [132]. Looking at the right panel of Fig.…”

Characteristics of hadronic interactions in the forward direction

“…Using minimum bias PbPb collision events collected by CMS at nominal magnetic field at center-of-mass energy of 2.76 TeV per nucleon it is possible to extend the absolute measurement of the energy flow, dE/dη, into the region close to the beam rapidity (y beam = 7.99) [4,5]. The CASTOR energy scale is set via a cross-calibration procedure according to energy pseudorapidity density measured in the HF calorimeter using an independent data set of minimum bias pp collision events at 7 TeV center-of-mass energy.…”