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 5.15: A track in the central tracking detector can be approximated by a helix in three dimensions or a circle in the $X-Y$ plane as shown in this figure. Shown is also one of the parameters of the helix, $\rho _H$, the distance of closest approach to the beam axis.

Figure 5.16: Closest approach of the track helix to the beam axis for all ``good tracks'' in charged current Monte Carlo events (a) and real data (b). For charged current Monte Carlo events most tracks come very close to the beam axis while for real data many tracks stay far away from the beam axis.

In figure 5.16 $\rho _H$ is shown for good tracks in charged current Monte Carlo and real data. Most of the tracks in the Monte Carlo event sample come closer than to the beam axis. For real data many tracks stay far away from the beam axis.

In figure 5.17 the ratio of good tracks that are closer than to the beam axis over the total number of good tracks is shown for charged current Monte Carlo data and real data.

Figure: Ratio of number of good tracks that come closer than to the beam axis over the total number of good tracks for Monte Carlo (a) and real data events (b). Events with a ratio bigger than $0.4$ are kept.

Figure 5.18: ${P_y}$ vs. ${P_x}$ for all events after applying the beam wall collision cut described in the text. The enhancement of the number of events visible in figure 5.13 has disappeared.

Events with a ratio bigger than $0.4$ are kept. Figure 5.18 shows the distribution ${P_y}$ versus ${P_x}$ for all events which pass the cut described above. The enhancement visible in figure 5.13 has disappeared.

Through this cut events are rejected and events remain.