Dependence on pseudorapidity and on centrality of charged hadron production in PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV

Abstract

A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density, dN ch /dη| η=0 , for head-on collisions is found to be 1612 ± 55, where the uncertainty is dominated by systematic effects. Comparisons of these results to previous measurements and to various models are also presented. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the CMS collaboration doi:10.1007/JHEP08(2011)141 The CMS collaboration 21 JHEP08(2011)141 IntroductionQuantum chromodynamics (QCD), the theory of strong interactions, predicts a phase transition at high temperature between hadronic and deconfined matter [1]. Strongly interacting matter under extreme conditions can be studied experimentally using ultrarelativistic collisions of heavy nuclei. The field entered a new era in November 2010 when the Large Hadron Collider (LHC) produced the first PbPb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. This represents an increase of more than one order of magnitude over the highest-energy nuclear collisions previously achieved in the laboratory. The multiplicity of charged particles produced in the central-rapidity region is a key observable characterising the properties of the quark-gluon matter created in these collisions [2]. Nuclei are extended objects, and their collisions occur at various impact parameters, referred to as "centralities". The studies of the dependence of the charged particle density on the type of colliding nuclei, on the centre-of-mass energy, and on the collision geometry are important for understanding the relative contributions of hard scattering and soft processes to particle production and provide insight into the partonic structure of the nuclei.-1 - JHEP08(2011)141In this paper we report measurements of the multiplicity density dN ch /dη of primary charged hadrons. The analysis is based on the 2.76 TeV-per-nucleon PbPb collision data recorded by the Compact Muon Solenoid (CMS) detector in December 2010, in runs without magnetic field. The pseudorapidity is defined as η = − ln[tan(θ/2)] with θ the polar angle with respect to the counterclockwise beam direction (the z axis). The number of primary charged hadrons N ch is defined as all charged hadrons produced in an event including decay products of particles with proper lifetimes less than 1 cm.A detailed description of the CMS experiment can be found in ref. [3]. The pixel tracker used for the analysis covers the region |η| < 2.5 and a full 2π in azimuth, with 66M detector channels out of which 97.5% were functional during data...

Scaling Properties of the Mean Multiplicity and Pseudorapidity Density in e−+e+, e±+p, p( p ¯ )+p, p+A and A+A(B) Collisions

“…Measurements of particle yields and kinematic distributions in electron-positron (e − +e + ), electron-proton (e ± +p), proton-proton (p(p)+p), proton-nucleus (p+A) and nucleus-nucleus (A+A(B)) collisions are essential for characterizing their global properties and to develop a good understanding of the mechanism(s) for particle production [1][2][3][4][5][6][7][8][9]. The p+p measurements also provide crucial reference data for studies of nuclear-medium effects in A+A(B) and p+A collisions, as well as improved constraints to differentiate between particle production models and to fine-tune event generators.…”

Suppression of non-prompt J/ψ, prompt J/ψ, and $ \Upsilon $(1S) in PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV

“…The centrality variable is defined as the fraction of the total cross section, starting at 0% for the most central collisions. This fraction is determined from the distribution of total energy measured in both HF calorimeters [38]. Using a Glauber-model calculation as described in ref.…”

Elliptic flow of identified hadrons in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

“…The existence of the QGP has been stipulated by observations at the Relativistic Heavy Ion Collider (RHIC) [7][8][9][10]. The first experimental results from the heavy-ion program at the LHC [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] have also provided evidence of the existence of the QGP in this new energy regime.-1 - …”