Cuts based on Timing

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.

Figure 5.4: Schematic 2D view of a beam gas event. Shown here is the beam pipe (horizontal parallel lines) and a part of the FCAL and RCAL (hatched structures). The nominal vertex is at $(0,0)$. The particles originating from the beam-gas event that occurred behind the RCAL also hit FCAL. Since FCAL and RCAL are about apart the average time of FCAL and RCAL for such an event differs by about .

A rejection cut on the event time is orthogonal to the cuts used to accept events for a physics analysis and does not introduce any bias. The online trigger system uses the information from the Vetowall, the C5 and UCAL to reject events based on timing.