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To find the x-y position of the vertex, I draw a plane perpendicular to the z-axis, at the at the location of the found z-vertex. Then I play the same game as for the z-vertex pseudo-tracking: connect all inner-outer hits and project to the plane, where I look for the maximum.

Each of the 6 phi-sectors can make a projection along a line perpendicular to the plane of the silicon. Note that sectors 1 and 2 are fully populated with silicon panels, and sectors 3-6 only on the outer ends.

These are the projections made by each of the 6 phi-sectors along a line perpendicular to the silicon planes. The data are for a single AuAu collision. The x-axes are in cm, with the origin sitting at the crossing with the z-axis. The peak at 0 can be picked out easily in sectors 1 and 2, but it is less unambiguous for the partially populated sectors 3-6. (postscript) In the scatterplotplots below, only the information from the fully populated sectors is used: The highest bin in projections 1 and 2 is assumed to correspond to the xy vertex.

In the top two scatterplots, the calculated position is plotted on the x-axis and the true position on the y-axis, (x on the left, y on the right) for about 40 events. A good correlation can be seen. The bottom two plots are the diagonal projections, or (calculated-true). (postscript) These projections show that x and y of a single AuAu event vertex can be located to a few hundred microns.