Abstract
Measurements of the pseudorapidity distributions of charged hadrons produced in xenon-xenon collisions at a nucleon-nucleon centre-of-mass energy of sNN=5.44 TeV are presented. The measurements are based on data collected by the CMS experiment at the LHC. The yield of primary charged hadrons produced in xenon-xenon collisions in the pseudorapidity range |η|<3.2 is determined using the silicon pixel detector in the CMS tracking system. For the 5% most central collisions, the charged-hadron pseudorapidity density in the midrapidity region |η|<0.5 is found to be 1187±36 (syst), with a negligible statistical uncertainty. The rapidity distribution of charged hadrons is also presented in the range |y|<3.2 and is found to be independent of rapidity around y=0. Existing Monte-Carlo event generators are unable to simultaneously describe both results. Comparisons of charged-hadron multiplicities between xenon-xenon and lead-lead collisions at similar collision energies show that particle production at midrapidity is strongly dependent on the collision geometry in addition to the system size and collision energy.
Highlights
Collisions between ultra-relativistic heavy ions are the only known way of experimentally studying quantum chromodynamics (QCD) matter at high temperatures and energy densities
The shapes of the distributions, where the overall normalisations are factored out, are consistent with those predicted by the EPOS LHC event generator within the total systematic uncertainties
The results are compared to predictions from the EPOS LHC v3400, HYDJET 1.9, and AMPT 1.26t5 event generators
Summary
Collisions between ultra-relativistic heavy ions are the only known way of experimentally studying quantum chromodynamics (QCD) matter at high temperatures and energy densities. The dependence of the charged-particle multiplicity on the colliding system, centreof-mass energy, and collision geometry can provide information about nuclear shadowing and gluon saturation effects [3], as well as the relative contributions to particle production from hard scattering and soft processes [4]. These observables provide input for models of the particle production process [5], from which information about the formation and properties of the QGP can be extracted. The AMPT generator combines the HIJING event generator [22] with Zhang’s parton cascade procedure [23] and the ART model [24] for the last stage of parton hadronisation
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