Figure 1

Left: track impact parameter resolution in the transverse plane asa function of $\pt$ in Pb-Pb collisions. For $\pt< 2 \gev/c$, pion identification by the TPC or TOF detectors is required; the results for data and simulation are shown The simulation includes the effect of the residual geometrical misalignment ofthe sensor modules of the Inner Tracking System (see text for details). Right: difference between the measured time-of-flight and that expected under the kaon hypothesis as a function of track momentum for the 20% most central Pb-Pb collisions.