Constraining models of initial state with v2 and v3 data from LHC and RHIC

We present a combined analysis of elliptic and triangular flow data from LHC and RHIC using viscous relativistic hydrodynamics. Elliptic flow v2 in hydrodynamics is proportional to the participant eccentricity ε2 and triangular flow is proportional to the participant triangularity ε3, which means vn=κnεn, where κn is the linear response coefficient in harmonic n. Experimental data for v2 and v3 combined with hydrodynamic calculations of κn thus provide us with the rms values of initial anisotropies ε2 and ε3. By varying free parameters in the hydro calculation (in particular the shear viscosity), we obtain an allowed band in the (rms ε2, rms ε3) plane. Comparison with Monte Carlo models of the initial state allows us to exclude several of these models. We illustrate that the effect of changing the granularity of the initial state is similar to changing the medium properties, making these effects difficult to disentangle.