Velocity $ \beta $ measured with the TOF detector as a function of the rigidity $ p/z $. For this figure a selection band of -1.5 to 3$\sigma$ around the mean of the TPC specific energy-loss distribution is required Negatively (positively) charged particles are shown on the left (right) side, with positive tracks in blue and negative tracks in green. The dashed vertical line represents the cut on the rigidity $p/z = 2$ GeV/$c$ (applied only for positively charged particles). The insert shows the $m^2/z^2$ distributions obtained from the data points shown in the main figure. For details, see text.