MadGraph5_aMC@NLO

eMELA interface with LO calculations

Asked by Samuel Homiller

Hi,

We are attempting to implement lepton PDFs for calculations of high-energy (multi-TeV) e+e- or mu+mu- collisions in MadGraph5, using files in an LHAPDF-like format, and the existing interfaces in MG5. With a couple of minor tweaks, we managed to get them working with the LHAPDF6 interface, but (as we should have expected) ran into issues with the phase space integration when trying to use the muon PDFs that peak at x -> 1.

To our understanding, the known way of dealing with this is to instead use the interface with eMELA, where the limit as x -> 1 is replaced with the known analytic result.

Unfortunately, when trying to use the eMELA interface for LO processes, the MadGraph interface seems to reject setting 'pdlabel' to 'emela' (it accepts 'emela' as an option in the run card when doing one-loop calculations by appending [QED], but then we run into other issues, and can't generate events, and we are ultimately interested in LO calculations with BSM models anyway). Is there any way to use this interface with the current version of MG5, or perhaps with minor changes?

Thanks for any help you can provide,

Sam

Question information

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Status:
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MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Marco will comment why emela is not supported at LO, it can likely be done.
In the meantime you can do
[LOonly=QED] or [LOonly=QCD]
and then you should be able to use emela at LO (but from the NLO code and not from the LO code)

Cheers,

Olivier

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marco zaro (marco-zaro) said : 		#2

Hi,
for the moment, the eMELA interface works only for NLO-type code.
The reason is mostly that eMELA is the only provider for NNL-accurate PDFs, which are used in that scope.
Porting eMELA to LO (madevent) is possible, we can certainly work on it.
The workaround suggested by Olivier is the way to go if you want to use eMELA.
Note that there are some standalone PDFs for ISR (only e^+ in e^+, no photon) inside Source/PDF/lep_densities, for example one can set pdlabel= isronlyll
Best,

Marco

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Samuel Homiller (shomiller) said : 		#3

Hi Marco, Olivier,

Thanks, this indeed does the trick for us --- much appreciated! Your reason makes a lot of sense. It might be occasionally useful to have it ported to madevent in the future (this makes it a little bit easier to use with BSM models that aren't NLO ready, it seems), but this is probably a relatively niche use case.

Thanks again for the help!

Best, Sam

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Samuel Homiller (shomiller) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

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Samuel Homiller (shomiller) said : 		#5

Hi all,

Thanks again for the help. We had one other question: is there any way to call a different PDF set for each beam in eMELA with the LHAPDF interface? (i.e., set lhaid1 and lhaid2 to different values?)

I assume this would require a lot of the metadata in the PDFs to be the same, so that e.g., alpha_S(Q^2) is set in a consistent manner. We are hoping to do this to implement PDFs from leptons with different helicities, which we are simply calling as different set numbers as opposed to somehow duplicating the particles in a PDF.

Thanks in advance,

Sam

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Olivier Mattelaer (olivier-mattelaer) said : 		#6

As far as I know, no

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marco zaro (marco-zaro) said : 		#7

I confirm, there is no such possibility at the moment

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