Probing elliptic flow of QCD matter by lepton pairs

The strong collectivity of the medium formed in high energy heavy-ion collisions is quantified through the elliptic flow, v2, which measures the azimuthal correlation of produced particle with respect to the reaction plane. The initial spatial anisotropy in heavyion collisions gets converted to anisotropy in momentum space due to interactions among the constituents of the matter. Therefore, v2 may be treated as a measure of the momentum anisotropy. v2 measurements at RHIC and LHC have been used to establish the collectivity at the partonic phase, constrain the equation of state and extract the transport properties of the QCD matter.