The vertex detector must also be able to find the vertex. As an aid in understanding this process, figs. 4-5 show the number of hits on the vertex detector vs. z for a p+p event and for a Au+Au event. Hits on the parallel strips (left side) and perpendicular strips (right side) and on the inner barrel (top) and outer barrel (bottom) are shown separately. For perpendicular strips the number of clusters of adjacent hits vs. z is shown --- not the raw number of hits. The real vertex positions are marked. By looking at these figures, it is clear that the distribution of hits on the parallel strips can be used to estimate the vertex position for Au+Au, but for p+p the small number of hits, coupled with the noise, makes it difficult to find the vertex from this information alone.
Figure 4: p+p - a sample event. The left side shows the number of hits vs. z (the beam direction) for parallel strips. The right side shows the number of hits vs. z for perpendicular strips. The top half shows hits on the inner barrel (R1) and the bottom half shows hits on the outer barrel (R2). The arrows mark the vertex position.
Figure 5: Au+Au - a sample event. The left side shows the number of hits vs. z
(the beam direction) for parallel strips. The right side shows the number of
hits vs. z for perpendicular strips. The top half shows hits on the inner
barrel (R1) and the bottom half shows hits on the outer barrel (R2). The arrows
mark the vertex position.