Fraction of reconstructed tracks coming from the primary interaction vertex. Two sets of cuts on the track distance of closest approach ($d_{0}$) to the primary vertex are shown: "loose'' with $|{\rm d}_{0,z}|< 3$ cm, ${\rm d}_{0,xy}< 3$ cm and "strict" with $|{\rm d}_{0,z}|< 2$ cm, ${\rm d}_{0,xy}< (0.0182+0.0350 GeV/c \> \> \pt^{-1}$) cm.