Characterizing the medium created in Pb–Pb collisions at by means of the evolution of two-particle transverse momentum correlations

Víctor González

doi:10.1088/1742-6596/1602/1/012010

Víctor González

Open Access

https://doi.org/10.1088/1742-6596/1602/1/012010

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Abstract

Two-particle correlations are powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision centrality, which is related to the system lifetime, provides information about the shear viscosity, ɳ/S, and the system relaxation time, . We report on measurements of two-particle transverse momentum correlations as a function of centrality in Pb-Pb collisions using the ALICE detector at the LHC. The centrality dependence of the near-side peak of the correlation function, particularly in the longitudinal dimension, provides information about the shear viscosity and the system relaxation time of the produced medium. The data are compared to predictions from selected Monte Carlo models. The charge independent momentum correlator exhibits a longitudinal broadening from peripheral to central collisions that is qualitatively consistent with expectations from a model with viscous effects. We will discuss an interpretation of the observed broadening in the context of this model.

Highlights

Properties of the medium formed in heavy-ion collisions can be extracted measuring the Fourier harmonic coefficients of the particles produced azimuthal distribution
GC2 I broadens by about 25% from peripheral to semi-central collisions, where its width saturates, while GC2 D narrows by about 10% from peripheral to central collisions
The longitudinal broadening observed by STAR, which saturates, reaches 70% [9] pointing to a strong dependence on the beam energy

Summary

Introduction

Properties of the medium formed in heavy-ion collisions can be extracted measuring the Fourier harmonic coefficients of the particles produced azimuthal distribution. Differential charge independent (CI) and charge dependent (CD) two-particle transverse momentum correlators, GC2 I and GC2 D, respectively, are measured as a function of pair pseudorapidity and azimuthal angle differences, ∆η and ∆φ, respectively, for selected ranges of Pb–Pb collision centrality. The dilution on the away side of the long range correlation shown in Fig. 1 is visible in the progress of the azimuthal projections from semi-central to peripheral collisions. The focus now is on the evolution with centrality of the width of the the two-particle transverse momentum correlation on the near-side peak To extract such widths, along the azimuthal and longitudinal dimensions, the two-dimensional correlations are parametrized with a two-component model defined as.

Results and discussion

Tc η s

Conclusion