about random coincidence rate

NEXT
Make New Entry, Make Followup Entry

User name X. Zheng

Log entry time 14:01:22 on December 02, 2009

Entry number 304164

This entry is a followup to: 304159

keyword=about random coincidence rate

The random rate observed on the left arm is about 2.5% of the normal
electron rate. In the simpliest case, this could come from two sources:

1) pion events contamination because there is a GC signal from another
(electron or background) event within 60ns ahead of it:

Rate = R(GC,all groups) * 60ns * R(pion) / (pion rejection of lead glass)
      = R(GC,all groups) * 60ns * R(electron) * (pi to e ratio) / (pion rejection of lead glass)
      = 400kHz * 60ns * R(electron) * (0.25) / (pion rej of lead glass)
      = 0.6% * R(electron) / (pion rej of lead glass)

where "pion rej of lead glass" is at least 10-30 for the left arm.

2) electron events with a GC signal from another (electron or background)
event within 60ns ahead of it:

Rate = R(GC,all groups) * 60ns * R(electron)
      = 400kHz * 60ns * R(electron)
      = 2.4% * R(electron)

All numbers here are for left arm. It can be seen that we are dominated
by the "electron-GC random coincidence", which is NOT a background during
production mode. The same situation is true if we delay the whole VETO
(not just the GC input to the VETO). Although we could hope to use FADC to
to find out the fraction of each type, we should think of a clever way
to measure the random coincidences.