Momentum anisotropy at freeze out

The transition from a hydrodynamical modelling to a particle-based approach is a crucial element of the description of high-energy heavy-ion collisions. Assuming this ''freeze out'' happens instantaneously at each point of the expanding medium, we show that the local phase-space distribution of the emitted particles is asymmetric in momentum space. This suggests the use of anisotropic hydrodynamics for the last stages of the fluid evolution. We discuss how observables depend on the amount of momentum-space anisotropy at freeze out and how smaller or larger anisotropies allow for different values of the freeze-out temperature.