Studies of azimuthal dihadron correlations in ultra-central PbPb collisions at=2.76 TeV

Chatrchyan S., Khachatryan V., Sirunyan A. M., Tumasyan A., Adam W., Bergauer T., ...More

JOURNAL OF HIGH ENERGY PHYSICS, no.2, 2014 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: Issue: 2
  • Publication Date: 2014
  • Doi Number: 10.1007/jhep02(2014)088
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Keywords: Relativistic heavy ion physics, Heavy Ions, harmonic flow, QUARK-GLUON PLASMA, ELLIPTIC FLOW, PERSPECTIVE, SPECTRA, MATTER, MODEL
  • Middle East Technical University Affiliated: Yes


Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at = 2.76TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 mu b -aEuro parts per thousand 1. The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v (2) to v (6), are extracted as a function of particle transverse momentum. At higher transverse momentum, the v (2) harmonic becomes significantly smaller than the higher-order v (n) (n a parts per thousand yenaEuro parts per thousand 3). The p (T)-averaged v (2) and v (3) are found to be equal within 2%, while higher-order v (n) decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-by-event fluctuating initial conditions is also presented.