Invariant mass distribution for opposite-sign muon pairs ($2.5< y< 4$, all $p_{\rm t}$),in the mass region $2< m_{\mu\mu}< 5$ GeV/$c^2$, with the result ofthe fit (see text for details). The fitted J/$\psi$ and $\psi(2S)$ contributions, as well as thebackground, are also shown.