"No FKS configuration with soft singularity" at NLO for VBF

Hello Matteo,

Sorry for the delay.

This is for fixed order NLO runs, right? (Not a MC@NLO one).

Which version of MG_aMC are you using? If it has not been one of the latest 3.4.x/3.5.x, could you try with one of those. I remember seeing a similar issue before and already fixed it. (Of course, you could have found another problem that requires some other fix).

Best,
Rikkert

Hi Rikkert,

many thanks for your reply!

There was indeed a typo in my question, it's not fixed order generation, but with Pythia8 shower and FxFx, sorry for that; the fixed order one works fine.

MG5 version is v3.4.2.

Please note that we are using a SM model we implemented, where we added a WCC coupling to explicit the Wqq vertex.
This is useful to select a subset of diagrams which are gauge-invariant: test_ME, test_MC and check_poles are successful, but MG5 crashes during the event generation, before the shower.

Might there be a problem in our model, even if it is all right in the fixed order case?

You can find the banner below,

Matteo

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Hello again,

A couple of points:

1. I'm not convinced that what you are doing is gauge invariant -- in particular from the EW point of view. (It might be, it's just that I do not see it at the moment). Are you sure it is? MG5_aMC's checks do not cover gauge invariance.

2. FxFx will not be correct for this process. This process includes jets at the lowest multiplicity Born process, which means that it cannot work, as explained for example here: http://amcatnlo.web.cern.ch/amcatnlo/FxFx_merging.htm

3. The part of the code that gives the error ("S-event contribution found, but no FKS configuration with soft singularity") is not used for fixed order calculations. Indeed, the requirements on consistency of the orders is greater at NLO+PS as compared to fixed order NLO. Hence, it might be that your model will work and is correct at NLO, but it cannot be used for NLO+PS.

4. I did a quick run for "Process p p > z j j $$ w+ w- / a z QCD=0 [QCD]" in the Standard Model (I believe to what you are trying to generate, except for an on-shell Z boson) and could not reproduce your error. (Again also for this setup, I'm not sure it's gauge invariant though).

best,
Rikkert

Thank you for the references and explanations!

What would be an alternative to FxFx for this process at NLO?

And also, do you have any idea about what might make the model unsuitable for NLO+PS, or how to check it? Is there anything general that usually causes the problem?

About gauge invariance, we think that if we only allow W bosons in the t-channel, jets would be of different generations and no crossing should be possible, so there shouldn't be any interference of the QCD with the EW diagrams... The interference without crossing should be null because it's proportional to the trace of the Gell-Mann matrices.

Cheers,

Matteo

- An alternative would be a normal NLO+PS generation using the MC@NLO method. FxFx is only needed when merging processes with multiple jet-multiplicities.

- Not sure. There might be some complications with the coupling orders, and/or expansion in the couplings, since the coupling orders are applied at the amplitude level, while the expansion in the couplings (which is more physical) is applied at the matrix element squared level.

- I think the consistent way of defining VBF production is to consider the two protons to belong to different copies of QCD that only talk through exchange of EW bosons. In other words the quarks and gluons from the left-incoming proton are distinct from the quarks and gluons from the right-incoming proton and they do not couple to each other apart through EW bosons. However, this would require quite a big overhaul of the model files, and maybe also some more hackery in the MG5aMC code since it expects only one copy of QCD.

Best,
Rikkert

Hi Rikkert,

When running the generation without FxFx, as you suggested, everything goes smooth, and events are generated and showered.

Best,

Matteo