The opposite-sign invariant mass spectrum of mass pairs, for the four centrality intervals used in the data analysis, after mixed-event background subtraction. The kinematic domain is $2.5< y< 4$, $p_{\rm T}< 12$ GeV/$c$ ($0.3< p_{\rm T}< 12$ GeV/$c$ for the two most peripheral intervals). 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, $2< m_{\mu\mu}< 5$ GeV/$c^2$ fitting range, a $2< m_{\mu\mu}< 8$ 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.