Correlations and fluctuations measured by the CMS experiment in pp and PbPb

TL;DR

This paper reports on measurements of dihadron angular correlations in pp and PbPb collisions at the LHC, revealing a ridge-like structure in high-multiplicity pp events and analyzing flow coefficients in PbPb collisions.

Contribution

It presents the first observation of a ridge-like correlation in high-multiplicity pp collisions and provides a detailed Fourier analysis of correlations in PbPb collisions.

Findings

Ridge-like structure observed in high-multiplicity pp collisions at 7 TeV.

Long-range dihadron correlations analyzed in PbPb collisions at 2.76 TeV.

Higher order flow coefficients extracted from Fourier analysis.

Abstract

Measurements of charged dihadron angular correlations are presented in proton-proton (pp) and Lead-Lead (PbPb) collisions, over a broad range of pseudorapidity and azimuthal angle, using the CMS detector at the LHC. In very high multiplicity pp events at center-of-mass energy of 7 TeV, a striking "ridge"-like structure emerges in the two-dimensional correlation function for particle pairs with intermediate pt of 1-3 GeVc, in the kinematic region 2.0<|\Delta\eta|<4.8 and small \Delta\phi, which is similar to observations in heavy-ion collisions. Studies of this new effect as a function of particle transverse momentum are discussed. The long-range and short-range dihadron correlations are also studied in PbPb collision at a nucleon-nucleon center-of-mass energy of 2.76 TeV, as a function of transverse momentum and collision centrality. A Fourier analysis of the long-range dihadron correlations is presented and discussed in the context of CMS measurements of higher order flow coefficients.