Importance of the Bulk Viscosity of QCD in Ultrarelativistic Heavy-Ion Collisions

We investigate the consequences of a nonzero bulk viscosity coefficient on the transverse momentum spectra, azimuthal momentum anisotropy, and multiplicity of charged hadrons produced in heavy ion collisions at LHC energies. The agreement between a realistic 3D hybrid simulation and the experimentally measured data considerably improves with the addition of a bulk viscosity coefficient for strongly interacting matter. This paves the way for an eventual quantitative determination of several QCD transport coefficients from the experimental heavy ion and hadron-nucleus collision programs.

Figure 1

The bulk viscosity over entropy density parametrization used in our simulations as a function of $T/T_c$.

Figure 2

Multiplicity (a), average transverse momentum (b), and flow harmonic coefficients (c) as a function of centrality. The bands around the dashed lines show the effect of $T_{\text{switch}}$ on the observables. The points correspond to measurements by the ALICE Collaboration [49, 50], with bars denoting the experimental uncertainty.

Figure 3

Transverse momentum spectra (upper panels) of pions, kaons, and protons and harmonic flow coefficients (lower panels) as a function of the transverse momentum. Two centrality classes are considered: 0%–5% (left panels) and 30%–40% (right panels). The bands denote the statistical uncertainty of the calculation. The full and open symbols correspond to measurements by the ALICE Collaboration [49] and CMS Collaboration [53, 54], respectively, with bars denoting the experimental uncertainty.