Overlap in Zgamma and Zgammagamma samples

[Benjamin Kaplan <benjamin.kaplan@cern.ch>]

Hello,

I am trying to combine Zgamma and Zgammagamma samples and it is unclear if there is any overlap between these processes. I am filtering the samples to have at least 1 lepton and 2 photons.

In the Zgamma sample, I find a large number of events with 2 final state "true" photons photons. In all of these events, the truth record shows me that there are 2 initial state leptons and 1 initial state photon (status 3), but the decay chain is:

llgamma->llgammagamma

This looks like FSR, but is it? Furthermore, I assume there should be some photons in the sample from the underlying event. How can I identify those?

In the Zgammagamma sample all of the events look to be ISR. Specifically, I see llgammagamma produced with status 3. Is there truly no FSR in the Zgammagamma sample? If these is how do I identify it?

Finally, if I look at the relative rates of di-photon events in the two samples, I find that the di-photon cross section is higher in Zgamma than in Zgammagamma. How is this possible?

Thank you,

-Ben