Invariant mass distribution (top) and $\langle \cos 2 (\varphi - \Psi) \rangle$ as a function of $m_{\ell\ell}$ (bottom) of opposite-sign dimuons (left) with $2 < p_{\rm T}< 4$ GeV/$c$ and $2.5 < y < 4$ and dielectrons (right) with $2 < p_{\rm T} < 6$ GeV/$c$ and $|y| < 0.9$, in semi-central (20--40\%) Pb-Pb collisions.