The opposite-sign invariant mass spectrum of mass pairs, for the four transverse momentum intervals used in the data analysis, after mixed-event background subtraction. The kinematic domain is $2.5< y< 4$ and the centrality interval is 0--90\%. The result of one among the 60 $\chi^2$ minimization fit is reported, corresponding to a CB function for the signal, a variable-width Gaussian for the background, a $2< m_{\mu\mu}< 5$ GeV/$c^2$ fitting range, $2< m_{\mu\mu}< 8$ GeV/$c^2$ normalization range of the mixed-event background, and CB tails obtained from Monte Carlo. The contribution of the J/$\psi$ and $\psi(2S)$ resonances and of the background continuum as obtained from the fit are also reported.