For the C5 detector (see section 3.2.4) a hit requires coincidence of the two opposing scintillator plates within a short time window ( ). An event is vetoed if the hit occurs within a time window of centered around before the bunch crossing time. Any activity from an event occurring at the nominal interaction point would have a time later by and the C5 timing resolution is .
The C5 detector is also used to make fine adjustments of the ZEUS data acquisition system internal clock to the HERA clock. Since all sub-detectors have a known and fixed time offset to each other the only quantity to be determined is the exact time with respect to the ZEUS time at which the interactions occur inside ZEUS. This is done by measuring the average time at which the proton bunches pass the C5 detector, . This time is then corrected for the time of flight distance from the C5 detector to the nominal interaction point, resulting in a time offset. This offset is small, typically less than . It is measured before data taking starts and used as a constant time offset by the ZEUS higher level triggers.
The C5 detector also determines , the average time at which the positron bunches pass the C5 detector. This value, together with can be used to calculate the average location of the interaction point. This is used as an online check to ensure stable data taking conditions.