Interaction Point Determination

The SLC collides bunches of electrons and positrons accelerated in the SLAC linac at a rate of 120 Hz. After colliding at energies of 45.6 GeV each, the bunches are extracted and dumped. For the 1993 run, the spatial extent of the bunches at the interaction point was typically m vertically, m horizontally, and m longitudinally[6]. The transverse position of the SLC collision region was stable, with variations of typically 5-10m over time periods measured in hours.

The spatial location of the interaction point is determined accurately in the transverse plane using tracks from hadronic decays [7]. A fit to a single point in the transverse plane is made using tracks from 30 successive events, in an iterative procedure. At each iteration, tracks that significantly reduce the quality of the fit are removed. Typically the fit uses about 300 tracks, and converges after a few iterations. The uncertainty in this determination is m for the 1993 data and m for the 1992 data. Non-Gaussian tails in the interaction point distribution may be represented conservatively by a second Gaussian with a standard deviation of 100 m for 0.25%(0.5%) of the 1993 (1992) events.