Energy loss as the origin of an universal scaling law of the elliptic flow

It is shown that the excellent scaling of the elliptic flow found for all centralities, species and energies from RHIC to the LHC for $p_{T}$ less than the saturation momentum is a consequence of the energy lost by a parton interacting with the color field produced in a nucleus-nucleus collision. In particular, the deduced shape of the scaling curve describes correctly all the data. We discuss the possible extensions to higher $p_{T}$, proton-nucleus and proton-proton collisions as well as higher harmonics.