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In the pseudo-tracking algorithm, all hit clusters on the outer barrel
are connected with all hit clusters on the inner barrel, and projected onto
the z-axis.
This histogram shows the distribution of z-intercepts for one Au-Au
collision. The arrow indicates the true vertex position at 5.56 cm. The
pseudo-tracking method locates the highest bin, and found the vertex at
5.55 cm.
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The efficiency and resolution of the vertex finding depend on the event
multiplicity. In the top panel, for p+p events, the vertex is found in 70%
of the cases, with an rms of 939 um. Delta-z is the difference between the
true vertex and the found vertex z-position.
In the bottom panel, the results for Au+Au collisions are shown. Because of the much higher multiplicity, the efficiency is now 98%, with an rms of 177 um. |