If this is the case, then the so called "background" are not backgrounds at all. They could be good electron triggers but only show up without correct ADC information. Below are 3 figures which may support this theory, from run 4972.
Figure 1. PS vs. SH for gr 4 (here I chose gr4 instead of gr2 to gain more statistics), with a loose cut of fbTDC gr4 EN trigger of >-500. Note: =-500 means no signal.
Figure 2. same as Figure 1, but with a tight cut of fbTDC gr4 EN trigger between 300 and 1100. This roughly correspond to the gap seen in fbTDC of the previous entry, and could potentially be the ADC gate.
Figure 3. Cerenkov ADC sum for gr4 EN trigger > -500 (black) vs. between 300 and 1100 (red).
As can be seen, using a tight fbTDC cut the "background" in both PS vs. SH and Cherenkov is gone.
Suggestion for future analysis: If you want to have meaningful ADC
information, should use a ~400ns fbTDC cut. Since channel 280 is where
the "background" is starting to be seen, probablly (280,1080) is a good
cut to use.
A copy of this log entry has been emailed to: rom,pkpan,dwang,deng,yqiang,kkumar,rsubedi