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Trigger Rate

As the VSAT is positioned directly on the beampipe it is flooded by off-energy electrons that were bent off from the beamline. There are mainly two types of background electrons that hit the detector. If the interaction between a beam- and residual gas- particle results in a small energy loss, the deflection from the beam will also be small. This background thus comes from far away, has high energy and is focused in the vertical plane of the beam (Fig. 3.3).

Figure 3.3: The Y-position of the low and high energy background.
Figure 3.4: The energy of the low and high energy background.

If the energy loss with the beam-gas interference is large, the resulting background will be more deflected and have lower energy. This background comes from a region close to DELPHI as it can not travel very far before it hits the beampipe walls. The low energy background is spread over all of the four VSAT modules, whereas the high energy background is concentrated in the outer modules (Fig. 3.4).

The vacuum is better in the region close to the interaction point (Fig. 3.2), resulting in a much lower background rate from the low-energy background. The VSAT detector therefore has about a factor five lower background in the inner modules than in the outer (Fig. 3.5). It is also clear that the background at Z0 running is much lower, only reaching a total trigger rate of about 300-400 hz (compared to about 1200-1400 for high energy running). This amount of data would require a lot of storage space and dead time if it was recorded. The background trigger is therefore downscaled, and only a small fraction of the events are read out.

Figure 3.5: The online trigger rates for the inner and outer modules. These are downscaled individually to get a balanced rate below 1 Hz in the recorded data (blocked in terms of fills=one filling of the LEP ring).
Figure 3.6: The Bhabha trigger rate. The online total Bhabha trigger rate is the sum of the true and false Bhabhas. Some events are removed in the offline processing reducing the offline trigger rate somewhat.