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this is a picture of the MVD barrel showing only the silicon panels (postscript version)

Since the MVD barrel only has 2 layers of silicon strips, and in some places only 1 layer, true tracking to determine the vertex cannot be done. In the pseudo-tracking method, first hit clusters, composed of adjacent hit strips, are found, and then straight lines are projected from each outer-barrel cluster through each inner-barrel cluster (in the same phi-sector) to the z-axis. The z-axis intercepts are collected in a histogram, and the true vertex shows up as a clustering of intercepts, as shown in the cartoon below.

This histogram shows the distribution of z-intercepts for a Au-Au collision. The arrow indicates the true vertex position at 5.56 cm. The pseudo-tracking method found the vertex at 5.55 cm. (postscript version)

If the panels are not placed accurately, or their position is not precisely known, then the peak in this distribution will broaden and its maximum will come down. Eventually the algorithm that tries to locate the peak will fail more and more often, reducing the vertex finding efficiency.