Elliptic flow and shear viscosity from a beam energy scan

Plumari, S. and Greco, V. and Csernai, László Pál (2014) Elliptic flow and shear viscosity from a beam energy scan. NUOVO CIMENTO C-COLLOQUIA AND COMMUNICATIONS IN PHYSICS, 37 (1). pp. 68-72. ISSN 2037-4909

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Abstract

We employ a relativistic transport theory to describe the fireball expansion of the matter created in ultra-relativistic heavy-ion collisions (uRHICs). Developing an approach to fix locally the shear viscosity to entropy density η/s, we study the impact of a temperature dependent η/s(T ) on the build-up of the elliptic flow, v2, a measure of the angular anisotropy in the particle production. Beam Energy Scan from √sNN = 62.4GeV at RHIC up to 2.76 TeV at LHC has shown that the v2(pT ) as a function of the transverse momentum pT appears to be nearly invariant with energy. We show that such a surprising behavior is determined by a rise and fall of η/s(T ) with a minimum at T ∼ Tc, as one would expect if the matter undergoes a phase transition or a cross-over. This provides an evidence for phase transition occurring in the uRHIC’s and a first constraint on the temperature dependence of η/s. In particular, a constant η/s at all temperatures or a too strong T-dependence would cause a breaking of the scaling of v2(pT ) with the energy.

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