Multiparticle Azimuthal Correlation Measurements in Lead-Lead and Proton-Lead Collisions at LHC with the CMS Detector

Abstract

The azimuthal correlations of charged particles produced in lead-lead collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV and proton-lead collisions at 5.02 TeV have been measured using multiparticle correlations. In PbPb collisions, the second, fourth and sixth Fourier coefficients of the charged-particle azimuthal distributions are studied as a function of the particle transverse momentum, pseudorapidity, and collision centrality. The results from several experimental methods that have different sensitivity to the fluctuations in the initial-state of ultra-relativistic heavy ion collisions are compared. The combined results provide access to the properties of the quark-gluon plasma through comparisons with the predictions of viscous hydrodynamic calculations. A near-perfect fluid behavior is observed in PbPb collisions at $sqrt{s_{_{NN}}}$ = 2.76 TeV at the LHC, similar to previous findings at lower center-of-mass energies from RHIC. The system-size dependence of the collective flow is studied in peripheral PbPb collisions and in high-multiplicity pPb collisions. Remarkable similarities are found when the two systems are compared for collisions with the same final-state particle multiplicity. Four-particle correlations emerge in pPb collisions with more than 40 particles detected in the final state. The elliptic anisotropies derived from four-particle or all-particle correlations are found to agree within 10\%, which indicates a high degree of collectivity in high-multiplicity pPb collisions and might be an evidence for the smallest liquid droplet ever produced in the laboratory.