Overview of results on flow and correlations from the CMS collaboration

S.J Sanders

doi:10.1016/j.nuclphysa.2013.01.050

Abstract

Abstract Highlights of the results obtained by the CMS experiment at the LHC on collective flow and dihadron correlations in PbPb collisions are presented. At lower transverse momenta of the outgoing particles and for a wide range of collision centralities, the 2 nd - through 6 th -order Fourier components of the azimuthal distribution have been found using different analysis techniques with different sensitivities to non-flow and flow fluctuation effects. An ultra-central collision trigger allows the Fourier components to be extended to a region of centrality where initial-state effects are expected to be small. A high- p T single-track trigger has allowed the elliptic anisotropy ( ν 2 ) and higher-orderharmonics to be explored up to ≈ 60 GeV/c , as a function of pseudorapidity and collision centrality. The CMS results provide new information on both the hydrodynamic properties of the medium at low p T and the path-length dependence of in-medium parton energy loss at high p T . By subtracting the long-range in pseudorapidity anisotropy components arising from hydrodynamic flow from dihadron correlation results, it is possible to gain further insight into the medium effect on dijets. This is done with a study of the “near-side” and “away-side” yields of dijets measured in PbPb collisions compared to those found in pp collisions.

Highlights

One of the most significant results of the RHIC program of ultra relativistic heavy-ion collisions was obtained early on with the discovery of hydrodynamic flow behavior suggesting the formation of a quark-gluon plasma that behaves as an almost perfect, strongly-interacting fluid
Highlights of the results obtained by the CMS experiment at the LHC on collective flow and dihadron correlations in PbPb collisions are presented
By subtracting the long-range in pseudorapidity anisotropy components arising from hydrodynamic flow from dihadron correlation results it is possible to gain further insight into the medium effect on dijets

Summary

Introduction

One of the most significant results of the RHIC program of ultra relativistic heavy-ion collisions was obtained early on with the discovery of hydrodynamic flow behavior suggesting the formation of a quark-gluon plasma that behaves as an almost perfect, strongly-interacting fluid. A “day-one” goal of the LHC heavy-ion program, which was achieved by the time of the Quark Matter 2011 conference [1], was to confirm that√this fluid behavior persists with th√e almost 14-times increase in energy going from RHIC ( sNN = 200 GeV) to the LHC ( sNN = 2.76 TeV) This past year has seen a considerable extension of the type and range of measurements studying hydrodynamic behavior at LHC energies, including a CMS measurement of the elliptic anisotropy of π◦ particles [2]. The study of the azimuthal behavior of charged particles with pT < 20 GeV/c was done using 22.6 million minimum-bias events from the 2010 run, corresponding to an integrated luminosity of 3 μb−1 after a selection of tracks within 10 cm of the geometric center of the detector. Using the high-pT trigger it was possible to analyze 20 times more high-pT tracks than available in the 2010 minimum-bias dataset

Elliptic flow

Ultra-central collisions

Findings

Summary