The bulk viscosity of QCD is expected to vary considerably with temperature, from very small values at high temperatures to a possible large peak in the confinement region. Heavy ion collisions present a unique opportunity to establish experimental constraints on this transport coefficient, providing guidance to better understand the response of QCD systems to deviations from equilibrium.
In this work, we undertake a systematic analysis of hadronic observables within a state-of-the-art hydrodynamic model of the quark-gluon plasma, and establish a set of measurements that can constrain the temperature dependence of bulk viscosity. We explicitly show the resilience of these constraints to experimental uncertainties. The temperature dependence of shear viscosity is investigated side-by-side with bulk viscosity for completeness.
We highlight the impact of bulk viscosity on the hadronic chemistry. We further look at different models of initial conditions of the plasma to clarify their effect on phenomenological constraints of bulk viscosity. These results are used to reconcile recent investigations of bulk viscosity in heavy ion collisions [1,2].
 J. E. Bernhard, J. S. Moreland, S. A. Bass, J. Liu and U. Heinz, Phys.Rev. C 94, no. 2, 024907 (2016)
 S. Ryu, J.-F. Paquet, C. Shen, G. S. Denicol, B. Schenke, S. Jeon and C. Gale, Phys. Rev. Lett. 115, no. 13, 132301 (2015)
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