Common to all ZEUS physics data analyses is the need for the rejection of events which stem from non positron-proton collisions. At a nominal HERA luminosity of the event rate due to deep inelastic positron-proton scattering is only about while the rate of background events is in the order of .
If the trigger as described in section 5.3 were the only trigger used to identify charged current events a large number of background events present at HERA would be picked up.
To illustrate the problem the distribution of a random selection of events which have passed the first level trigger excluding the cut for unpaired proton and open bunches is shown in figure 5.3. Triggers in the open bunch are mostly due to cosmic muons traversing the detector, while the triggers in the unpaired proton bunch stem from proton beam-gas interactions. The histograms indicate that for both beam-gas and cosmic muon events many events with occur. From those histograms it is possible to give an estimate of the number of events which would pass the cut :
The data shown in figure 5.3 correspond to about of the running period. They are events that have been randomly selected at the SLT with a pre-scale of . This results in about proton beam gas events and about cosmic muon events in the data for this period.
The trigger system rejects most events which are inconsistent with positron-proton collisions. The various cuts are briefly described here.