SMEFTatNLO event generation

Asked by Rahul Balasubramanian on 2020-08-11

Dear Madgraph Experts,

     We are generating samples to parameterise the contribution of EFT operators in SMEFTatNLO for gluon-gluon fusion Higgs production. A peculiar behaviour regarding the usage of restrict cards for particular values of the coefficients was observed and we would be glad if you could throw some light on the matter.

     Considering the generation command template below, where restrict cards are loaded for coefficients mentioned in the restrict are set to 1.0 and the rest are set to 0.

     import model SMEFTatNLO_U2_2_U3_3_cG_4F_LO_UFO-NLO_{restrict}
     generate p p > h [noborn=QCD] QED = 1 NP<=2 NP^2=={val}

  We see the following behaviour,
        When {restrict} is c3pl1 and c3pl2 the predictions are equivalent and opposite to cll1221 for val=2.
        When turning on two operators at a time (c3pl1_cll1221, c3pl2_cll1221), we see that if c3pl1 = cll1221 or c3pl2 = cll1221 in the restrict card we get a No Diagram Exception for val=2 and val=4.

   Furthermore, the prediction for NP^2==0 is the same as NP^2<=2 and NP^2<=4 which suggests that for c3pl1 = cll1221 (a) or c3pl2 = cll1221 (b), the contribution is the SM prediction. This behaviour also appears for the case c3pl1 = -c3pl2 (c). For any other parameter values that don't satisfy the previous equalities, we seem to avoid the no diagram exception.

   Is the No Diagram Exception for such a scenario a nominal behaviour ? Does the cancellation of the EFT contribution for NP^2==2 when any of (a) (b) (c) is true necessarily guarantee that the contribution at NP^2==4 is zero ? Thanks a lot for your insights.

Ana, Saskia, and Rahul

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Solved by:
Olivier Mattelaer
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I'm not familiar with that model so will not be able to comment on that.
However, restriction model behavior can indeed be disturbing when you do not read the associate manual.
The idea of the restriction model is that you remove all interaction with a zero coupling for the associated restriction_card.

The fact that you have after the restriction a "no diagram execption" indicates that MG5aMC removes the associate interaction since the coupling was evaluated to zero. If you look at the expression of the model, you should be able to confirm/infirm this.



PS: In top of that, a couple of parameter will be removed from the "new" param_card
- all those set to 0 in the restriction card
- all those set to 1 in the restriction card
- if two parameter are equal or opposite within a same block, then one of the two is removed.
This is not enough to guarantee that you can freely change the card without making a mistake (which I define that you set a parameter such that one of the vertices removed by the restriction are not zero anymore)

Dear Olivier,

     Thank you for your swift and clear explanation. Having looked into the couplings
expression of the model it does seem to be the case that the coupling goes to
zero for such cases.

Thanks a lot,

Thanks Olivier Mattelaer, that solved my question.