MadGraph5_aMC@NLO

Asymmetric beams with lhapdf

Asked by Claudio Severi

Hello everyone,
I was playing with asymmetric beams, when I noted something weird happening with the lhapdf interface to mg5.

I have these commands in my script:

set run_card ebeam1 10000.0
set run_card ebeam2 750.0
set run_card pdlabel1 lhapdf
set run_card pdlabel2 lhapdf
set run_card lpp1 1
set run_card lpp2 0

Given the asymmetry, the first time madgraph is run, the run card is updated, and goes from containing "pdlabel" to containing "pdlabel1" and "pdlabel2".

However, during event generation, when i get the usual message:
  > update the strong coupling value (alpha_s) to the value from the pdf selected: 0.130
the value of alphas that is updated is the one from the default pdlabel, nn23lo1, not the lhapdf one i chose for beam 1 !

I think sometimes madgraph still looks for "pdlabel", even when "pdlabel1" and "pdlabel2" are defined instead.

Can you please confirm?

Thanks a lot,
Claudio

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

You right, that update is coming "too early" and the run_card was not yet sanitized and therefore the entry was still allowed to be in a non consistent setup.

Here you will see the proper patch (including also your other bug report):
https://github.com/mg5amcnlo/mg5amcnlo/commit/d8a3d93d83506319137b97ff62a9e2ed8dda838c

The fact that the value of alphas is set to the wrong value is not really important actually (since nobody care in practise) but if you use the eft running module of madgraph (which can be important in your case)

Cheers,

Olivier

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