cannot generate events with QCD=0

Hi,

I'm in holliday so I did not had the time to check the following statement (but they are likely true)
This model is likely to contain operator like the chromo-magnetic operator which allows to have emission of gluon which are not propotional to alpha_s (and therefore you can have gluon emission even with QCD=0.

Such type of gluon emission is actually not fully supported in MG5aMC and is partly bugged in 2.6.5. So I would strongly suggest to use 2.6.6 where related bugs have been fixed and where you would have restriction in place preventing you to do stuff that are not supported (like MLM, using the LO default dynamical scale,...)

Now, I'm just slightly confused since in order to have such gluon, you would need two of such vertex (since you can not have a single gluon in inital/final state) which does not make sense in EFT theories. In general the UFO model should prevent you to have multiple insertion of such vertex. IF that's the case you should force yourself to restrict to one insertion (you can contact the model authors if needed)

Cheers,

Olivier

> On 3 Aug 2019, at 16:57, pietro govoni <email address hidden> wrote:
>
> New question #682582 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/682582
>
> Dear Madgraph experts,
>
> I am trying to generate VBF Higgs events with the SMEFTsim model, and I don't manage to let MG neglect QCD vertices.
> Within the release MG5_aMC_v2_6_5 I checked out the SMEFTsim package:
>
> cd MG5_aMC_v2_6_5/models
> wget http://feynrules.irmp.ucl.ac.be/raw-attachment/wiki/SMEFT/SMEFTsim_A_U35_MwScheme_UFO_v2.1.tar.gz
> tar xzf SMEFTsim_A_U35_MwScheme_UFO_v2.1.tar.gz
> cd SMEFTsim_A_U35_MwScheme_UFO_v2_1
> wget https://raw.githubusercontent.com/UniMiBAnalyses/D6EFTStudies/master/madgraph_model/restrict_SMlimit_massless.dat
>
> then I try to geneate a VBF Higgs production + decay:
>
> cd ../..
> ./bin/mg5_aMC
> import model SMEFTsim_A_U35_MwScheme_UFO_v2_1-SMlimit_massless
> define q = u c d s u~ c~ d~ s~
> generate p p > q q H, (H > W+ W-, W+ > l+ vl, W- > l- vl~) QCD=0 SMHLOOP=0
> output VBFHWW_SMlimit
> quit
>
> and despite the parameters, the production contains diagrams with SM loops and diagrams with virtual gluons exchanged.
> If I do non use the SMEFTsim model, i.e. I omit the "import ..." line, the generation works as I would expect and produces diagrams with EW vertices only.
>
> How can I recover this behaviour also in the case of EFT?
>
> Best regards,
>
> pietro
>
>
>
> --
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> contact for MadGraph5_aMC@NLO.

Dear Olivier,

thanks a lot for your prompt reaction.
I did update to the latest version of the code (thanks for the suggestion), and I eventually found the problem thanks to I. Brivio, who pointed out that the QCD=0 directive works when inserted before the comma in the process definition syntax.
Thanks also for the detailed discussion about the gluon operators. For the time being I am keeping that operator off, but I am aware now of the situation.

Best regards,

pietro