MadGraph5_aMC@NLO

SMEFTsim linear contribution to cross section disappears for imaginary parts of couplings

Asked by barthoux maxime

Hello,

While generating top quark pairs production and decay, I tried to experiment with the ctg and ctw couplings.

I generated samples with all couplings to 0 except for one (ctgre, ctgim, ctwre or ctwim).

I also did the same while isolating SM/linear/quadratic contributions from each other.

The cross sections given by those samples show pretty intersting features, and my questions concern one of them : the imaginary couplings don't produce a linear contribution, only quadratic, as opposed to real couplings, who produce both.

My questions are the following : Is that supposed to happen ? And if yes, why ?

Have a nice day,
Maxime

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Which model are you speaking about? SMEFT model have only real coupling, so I'm in the dark here.
Anyway, it is likely that I'm not an expert in the model that you are using (even if it is one of the SMEFT model).
So it might be better to contact one expert of the model.

My (limited) knowledge of SMEFT is that some operator do have zero interference with the SM (which can be due to CP symmetry and or a missmatch of helicity configuration that contributes).

Cheers,

Olivier

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barthoux maxime (maximebarthoux) said : 		#2

The model I'm using is SMEFTsim_topU3l_alphaScheme.

I have discussed this with my professor previously and by trying some computations we found out that the interference term, corresponding to this linear part, would vanish when the coupling is imaginary if the Matrix element corresponding to the standard model was real. However we're not sure why that would be the case.

Thanks a lot for the answer, I will try to contact someone who might know more about this specific model.

Have a nice day !
Maxime

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