Invariant mass distribution (a) and $\langle \cos 2 (\phi - \Psi) \rangle$ as a function of $m_{\mu\mu}$ (b) of OS dimuons with $2 \leq p_{\rm T}< 4$ GeV/$c$ and $2.5 < y < 4$ in semi-central (20\%--40\%) Pb-Pb collisions.