UCAL Cosmic Muon Timing Cut

Cosmic muons that traverse the ZEUS detector can cause high to be measured by the calorimeter. Therefore these events are background to the charged current events. The rate of cosmic muons traversing the ZEUS detector is estimated to be about . This rate poses a significant problem in the ZEUS trigger system, so cosmic rejection is performed on both the second and third level trigger system.

A cosmic muon traversing BCAL can be recognized by calculating the average time in both the upper ( ) and lower halves ( ) of BCAL. For a cosmic muon the time difference, given the dimensions of BCAL (see figure 3.2) is significant: If the muon traverses BCAL from top to bottom in the center the average distance of the cells in the top to the bottom half is about which results in a time difference of about .

The cut requires that there be no energy in either FCAL or RCAL and more than is deposited in both the upper and lower half of BCAL. Events are rejected if . The cut is illustrated in figure 5.6.

Figure: 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.