Comment 2 for bug 1732756

Dear Ilaria,

I only reproduce your error when setting “dynamical_scale_choice” to -1 (which is the default).
But using the default dynamical scale choice is not really adapted to computation of interference.
This is what creates the real problem. I would suggest to either use a fix scale or a pure kinematical function for the scaled (i.e. having dynamical_scale_choice set to a positive number)

Cheers,

Olivier

> On Nov 16, 2017, at 19:17, Ilaria Brivio <email address hidden> wrote:
>
> Public bug reported:
>
> Hi,
>
> I am having problems generating some processes with 4-fermion operators.
> I am using the SMEFTsim UFO attached and I get errors when computing the interference term (the p contains b b~)
>
> p p > t t~ b b~ NP^2==1
>
> when four-quark operators with color singlet contractions are included.
> For instance this happens setting cud13x3x3x3 = 1 while leaving the
> other NP parameters to zero. (there's a restriction file in the UFO
> attached)
>
> The event generation does not crash, but an error appears, referred to
> the channel with gg initial state. I could not find any other flag
> output in the folder, apart from a file named "error", which says just
>
> Error: failed to reduce to color indices: 1 0
>
> I think that the UFO is ok, because the color contractions in the vertex
> (V_184) look ok, and also the same operators work with other processes.
> What can this be then?
>
> Thanks a lot in advance,
> Ilaria
>
> ** Affects: mg5amcnlo
> Importance: Undecided
> Status: New
>
> ** Attachment added: "ufo model with restriction"
> https://bugs.launchpad.net/bugs/1732756/+attachment/5010125/+files/SMEFTsim_tb1.tar.gz
>
> --
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1732756
>
> Title:
> Error with 4-fermion interactions: failed to reduce to color indices
>
> Status in MadGraph5_aMC@NLO:
> New
>
> Bug description:
> Hi,
>
> I am having problems generating some processes with 4-fermion operators.
> I am using the SMEFTsim UFO attached and I get errors when computing the interference term (the p contains b b~)
>
> p p > t t~ b b~ NP^2==1
>
> when four-quark operators with color singlet contractions are
> included. For instance this happens setting cud13x3x3x3 = 1 while
> leaving the other NP parameters to zero. (there's a restriction file
> in the UFO attached)
>
> The event generation does not crash, but an error appears, referred to
> the channel with gg initial state. I could not find any other flag
> output in the folder, apart from a file named "error", which says just
>
> Error: failed to reduce to color indices: 1 0
>
> I think that the UFO is ok, because the color contractions in the
> vertex (V_184) look ok, and also the same operators work with other
> processes. What can this be then?
>
> Thanks a lot in advance,
> Ilaria
>
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