Less muons than expected from MadGraph sample

[Jose Ruiz]

Dear experts,

I'm currently seeing a strange effect at Delphes level, when simulating ttbar events at 13 TeV in proton-proton collisions for the leptonic final state considering only muons.

My process declaration at MadGraph level is:

generate p p > t t~, (t > w+ b, w+ > mu+ vm), (t~ > w- b~, w- > mu- vm~)

Which implies that all events have exactly two mouns at parton level. I have corroborated this with MadAnalysis. I have also checked that the eta and pt of the muons are in the acceptance of a CMS-like detector. I have found that only a small part (~10%) of muons are produced with |eta|>2.5

I then use MadGraph interface with pythia8 and Delphes to generate a detector level MC sample. The problem appears at Delphes, when I check the "Muon_Size" variable I see that a huge part (~50%)of events have only one muon.

For Delphes simulation I'm using the config card that comes with MadGraph for the CMS experiment.

Could you please help me figure out why am I seeing this behavior?

Thanks,
Jose

[Michele Selvaggi]

Hi,

muons in ttbar events are probably not very well isolated which might explain why you are losing so many.
Try to increase the PTRatioMax parameter in MuonIsolation in the card and you'll see how they increase.

Note that given you have two muons, the inefficiencies contribute twice to the N=1 ,multiplicity bin.
In other words 25% muon inefficiencies (acceptance, reconstruction, isolation) are enough to explain what you see.

Michele