Abstract
This paper presents the first measurements of the charge independent (CI) and charge dependent (CD) two-particle transverse momentum correlators G2CI and G2CD in Pb–Pb collisions at sNN=2.76TeV by the ALICE collaboration. The two-particle transverse momentum correlator G2 was introduced as a measure of the momentum current transfer between neighboring system cells. The correlators are measured as a function of pair separation in pseudorapidity (Δη) and azimuth (Δφ) and as a function of collision centrality. From peripheral to central collisions, the correlator G2CI exhibits a longitudinal broadening while undergoing a monotonic azimuthal narrowing. By contrast, G2CD exhibits a narrowing along both dimensions. These features are not reproduced by models such as HIJING and AMPT. However, the observed narrowing of the correlators from peripheral to central collisions is expected to result from the stronger transverse flow profiles produced in more central collisions and the longitudinal broadening is predicted to be sensitive to momentum currents and the shear viscosity per unit of entropy density η/s of the matter produced in the collisions. The observed broadening is found to be consistent with the hypothesized lower bound of η/s and is in qualitative agreement with values obtained from anisotropic flow measurements.
Highlights
Measurements of particle production and their correlations performed at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) provide compelling evidence that the matter produced in heavy-ion collisions is characterized by extremely high temperatures and energy densities consistent with a deconfined, but strongly interacting Quark–Gluon Plasma (QGP) [1,2,3,4]
The results presented in this letter are based√on 1.1 × 107 selected minimum bias (MB) Pb–Pb collisions at sNN = 2.76 TeV collected during the 2010 LHC heavy-ion run by the ALICE experiment
The amplitude of the φ modulations associated with collective flow decreases but the longitudinal broadening remains
Summary
Measurements of particle production and their correlations performed at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) provide compelling evidence that the matter produced in heavy-ion collisions is characterized by extremely high temperatures and energy densities consistent with a deconfined, but strongly interacting Quark–Gluon Plasma (QGP) [1,2,3,4]. Collective flow, which manifests itself by the anisotropy of particle production in the plane transverse to the beam direction, is characterized by the harmonic coefficients of a Fourier expansion of the azimuthal distribution of particles relative to the reaction plane Comparisons of these harmonic coefficients with hydrodynamical model predictions indicate that the matter produced in those collisions has a shear viscosity per unit of entropy density, η/s, that nearly vanishes [2,5]. Novel approaches based on Bayesian parameter estimation [13,14] bring progress on a simultaneous characterization of the initial conditions and the QGP It was pointed out [15] that the strength of momentum current correlations may be sensitive to η/s.
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