Debugging sensitivity to etal variable

Asked by Peter Steinberg on 2017-06-16

I am trying to calculate gamma+gamma -> mu+ mu- gamma, without requiring observation of the final state gamma (e.g. through the deviations from pure back to back emission of the muons). I am using elastic photons (lpp=2 for both beams), and I require the muons have pT>4 GeV and |eta|<2.4 (ATLAS acceptance).

What I am trying to understand is how to get a reliable calculation despite not measuring the photons. To avoid singularities, I require
1) photon energy is at least 1 MeV (ea = 0.001)
2) the opening angle of the photon relative to a muon is 0.01 (dral = 0.01)
3) *no* requirement on the photon eta (etaa = -1.0)
4) For the record, i assume each proton beam has 2510 GeV

(the cards are embedded in the output file - https://dl.dropboxusercontent.com/u/67108486/run_25_tag_1_banner.txt)

I found a few strange features, which I would like to understand better

1) when etal=2.5 or 2.6, the calculation runs successfully, however, when etal=2.7, the calculation takes much longer to run, generates a much larger cross section overall, and is unable to complete the requested number of events (a typical sign something is wrong). Presumably i'm running out of phase space, but i'm not sure how to test this (suggestions I received to set a minimum x for the elastic photon PDF can't be achieved since i don't see an option for this in MadGraph).

2) when I run the same script with 100k events, I get a much larger cross section than when i run 10k events, although the cross section for events with a large photon acoplanarity (== 1- delta-phi/pi) is the same. Thus it seems that small angle photon emission seems to change randomly despite having the same input cards file. For the record, both jobs finish with no obvious issues.

Thanks for any advice!

- Peter

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
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Last query:
2017-06-16
Last reply:
2017-06-16

Dear Peter,

I confirmed the problem with your cuts.
I think we need a (tiny) pta or etaa to avoid collinear emissions along the beam lines.

Kentarou

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