Left panel: Invariant-mass distribution of \sig\ candidates in 0.0 $ p_{\rm T} $ 4.0~$\rm GeV/\it{c}$ (black markers) with MC template fits: signal (blue markers), background (green markers), and total (red markers). The used missing-mass reconstruction method leads to a non-Gaussian shape of the distribution. The ratio of the data and the MC total distribution is shown in the lower panel.\\Right panel: Comparison of the \pt\ spectrum of \sig\ using the reconstruction method introduced in this paper with the corresponding spectrum measured in Ref.~. The ratio of the spectra is shown in the lower panel. The spectra are in good agreement, indicating that the reconstruction method and the purity determination work well over the full \pt\ range.