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

We study the effects of bulk viscosity on pT spectra and elliptic flow in heavy ion collisions. For this purpose we compute the dissipative correction df 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 RHIC. We find significant corrections due to bulk viscosity in hadron pT spectra and the differential elliptic flow parameter v2(pT). These corrections are dominated by viscous corrections to the distribution function. We find that the relation between df and the bulk viscosity is different in the quark gluon plasma and hadronic phases. Reliable bounds on the bulk viscosity require accurate calculations of df in a hadronic resonance gas. Based on v2 spectra at RHIC we conservatively estimate zeta/s <= 0.05 near freeze-out. We also find that effects of the bulk viscosity on the pT integrated v2 are small.