Quark degrees of freedom in a hybrid approach to heavy ion collisions

We present recent results obtained with the UrQMD hybrid model for heavy ion collisions. In this approach an intermediate hydrodynamic stage is used to model the hot and dense phase of heavy ion collisions. An equation of state that incorporates a chiral as well as a deconfinement phase transition in accordance with lattice results is implemented and its effects on different bulk observables is studied. While hadron multiplicities and bulk flow observables seem independent on the underlying degrees of freedom we observe a dependence of di-lepton and photon production. Furthermore we show how the model can be used to separate the system, created in a heavy ion collision, into an equilibrated core and a dilute corona. In such a scenario we use the hydrodynamic approach only for the dense core of the system while the dilute outer region is described by the transport model. Within our model, such a separation is able to explain observables like the position and with of the horn in the K+/π+ ratio as a transition from systems out of equilibrium to systems that are in chemical and thermal equilibrium.