Bulk viscosity, particle spectra, and flow in heavy-ion collisions

We study the effects of bulk viscosity on p_T spectra and elliptic flow in heavy-ion collisions. For this purpose, we compute the dissipative correction δf to the single-particle distribution functions in leading-log QCD, and in several simplified models. We consider, in particular, the relaxation time approximation and a kinetic model for the hadron resonance gas. We implement these distribution functions in a hydrodynamic simulation of Au + Au collisions at the Relativistic Heavy Ion Collider (RHIC). We find significant corrections due to bulk viscosity in hadron p_T spectra and the differential elliptic flow parameter v₂(p_T). We observe that bulk viscosity scales as the second power of conformality breaking ζ∼η(c_s²-1/3)², whereas δf scales as the first power. Corrections to the spectra are therefore dominated by viscous corrections to the distribution function, and reliable bounds on the bulk viscosity require accurate calculations of δf in the hadronic resonance phase. Based on viscous hydrodynamic simulations and a simple kinetic model of the resonance phase, which correctly extrapolates to the kinetic description of a dilute pion gas, we conclude that it is difficult to describe the v₂ spectra at RHIC unless ζ/s≲0.05 near freeze-out. We also find that effects of the bulk viscosity on the p_T integrated v₂ are small.