UCAL Beam-Gas Timing Cuts

Beam gas events which occur upstream of ZEUS and for which the UCAL is hit are rejected using the calorimeter timing. This is based on calculating an average event time in the UCAL regions and comparing the results to reference values.

In figure 5.5 the distribution is shown of the average time in RCAL, , as calculated by the UCAL SLT algorithm for a random selection of events which pass the first level trigger. Events are rejected if . Events are also rejected if .

Figure: The distribution of the average RCAL time for events at the second trigger level. Events in the hatched area ( ) are rejected.

More timing cuts are implemented on the third level trigger system. Here the average event time is calculated with higher precision using an energy-weighting algorithm and tighter cuts are used. At the TLT the systematic time shift of the ZEUS detector with respect to the bunch crossing time which is calculated by the C5 detector (see section 3.2.4) is taken into account.

The TLT also uses the error on the time average to reject events (see section 3.2.2 for ). Time averages are calculated by the TLT as error weighted averages and both the average time and the error on that time measurement are used:

One of the cuts is based on the global event time which is an average over the time reported by all calorimeter channels. An event is rejected if is outside a window around zero given by the maximum of and . This ensures that events with a poorly measured time average are still accepted. Table 5.2 list all timing cuts used at the TLT.