Elliptic flow from pQCD+saturation+hydro model

We have previously predicted multiplicities and transverse momentum spectra for the most central LHC Pb+Pb collisions at √ s N N = 5.5 TeV using pQCD + saturation + hydro (EKRT model) [1, 2]. We now extend these calculations for non-central collisions and predict low-p T elliptic flow. Our model is in good agreement with RHIC data for central collisions, and we show that our extension of the model is also in good agreement with minimum bias v 2 data from RHIC Au+Au collisions at √ s N N = 200 GeV. We obtain the primary partonic transverse energy production and the formation time in central AA collisions from the EKRT model [2]. With the assumption of immediate thermalization we can use these to estimate the initial state for hydrodynamic evolution. For centrality dependence we consider here two limits which correspond to models eWN and eBC in [3], where the profile and normalization are obtained from optical Glauber model, once the parameters in central collisions are fixed. In the eWN (eBC) model the energy density profile and normalization are proportional to the density and the number of wounded nucleons (binary collisions), respectively. These energy density profiles are used to initialize boost invariant hydro code with transverse expansion. We use the bag model equation of state with massless gluons and quarks (N f = 3), and hadronic phase with all hadronic states up to a mass 2 GeV included. Phase transition temperature is fixed to 165 MeV. Decoupling is calculated using standard Cooper-Frye formula, and all decays of unstable hadronic states are included. Freeze-out temperature is fixed from RHIC p T spectra for the most central collisions and is 150 MeV for binary collision profile [1] and 140 MeV for wounded nucleon profile. The same freeze-out temperatures are used at the LHC. Both initializations give a good description of the low-p T spectra for different centralities at RHIC. Left figure shows our calculations for p T dependence of minimum bias v 2 for positive pions. RHIC results are compared with STAR [4] and PHENIX [5] data. Our minimum bias centrality selection (0 − 80%) corresponds to the one used by STAR collaboration. Solid lines are calculations with the eBC model and dashed lines are from the eWN model. Thin lines are our results for RHIC Au+Au collisions at √ s N N = 200 GeV and thick lines show our predictions …