Next, the X,Y of the vertex can be found by placing a plane at the found Z position, perpendicular to the Z-axis. Using only the fully populated bottom sectors, project tracks into this plane, and again locate the maximum.

These are the 3D vertex finding resolutions in X, Y and Z. The scale is in cm, so the typical resolution is better than 200 um sigma. In this particular run, the mean beam position was offset from the nominal z-axis by a full 2mm in X and 2mm in Y. With the new 3D cpability, the vertex-finding efficiency is now independent of the mean transverse beam offset.