Due to the short bunch length of the positron ( ) and proton beam bunches ( ) positron-proton interactions occur within a window of less than around the time the bunch centers cross in the center of ZEUS. The detector timing is set for all sub-detectors such that for particles originating from the nominal interaction point a time of is reconstructed.
Any event which occurs outside of ZEUS can therefore be recognized if a sub-detector reports an event time outside a window around . This is illustrated in figure 5.4 for a beam-gas collision event.