The muon rejection algorithms employed on the trigger level reject most of the events in which a traversing muon is the only particle detected by the ZEUS detector. However, there are events in which the traversal of a cosmic or halo muon coincides with a beam-gas background or even genuine positron proton collision event. The probability for this to happen is not small: Given the bunch crossing rate of and assuming that the detector is hit by a beam related event at a rate of and by a cosmic at a rate of this leads to a rate of overlap events of .
The event sample which has passed all the cuts described so far does not contain that many overlap events. The reason is that the probability for these events to have high is small: Given the mean energy loss of a minimum ionizing particle in the UCAL a muon that traverses of material would only deposit of energy. A halo muon which traverses the entire length of the calorimeter of can deposit about and can then indeed cause . However, cosmics at very high momentum pose a problem because bremsstrahlung can cause large .
A sophisticated muon finder algorithm which is described in detail in chapter 6 is used to reject the cosmic and halo muon events remaining in the event sample.
Through this cut events are rejected and events remain. After this cut no events remain in the unpaired bunches. This indicates that the background due to non e-p collisions is small.