High width BSM particle not in LHE file

Asked by Ferdinand Schenck on 2018-07-04

Hi,

So let me start that this is not just a case of this faq: https://answers.launchpad.net/mg5amcnlo/+faq/2173

We are using a model with Vector Like Quarks, and for our example case (VLQ at 1400GeV, kappa=0.4), our decay has a width of around 144GeV. (MG gets this when using auto, also confirmed by our analytical calculations)

In the case where I specify my process by stating the only the final states and restrictions on coupling orders, my VLQ does not appear in the LHE file, regardless of what I set me bwcutoff to be.

I did some tests, with widths of 1, 10, 100 and 130 GeV, the VLQ is written into the LHE file (given a large enough bwcutoff, but as the width is so large, 15 is just fine).

But when testing with 140GeV, the particle is no longer written to the LHE file, it is not necessarily required, but would be useful for some sanity checks we are doing.

To preempt some questions:
-using MG 2.6.2
-We are also considering interference (separately)

Basically the two commands I am trying to compare are:
1) generate p p > j VLQ bb / p t t~ tp tp~ bp bp~ x x~ z h a, (VLQ > WW bb, WW > ferm ferm)
and
2) generate p p > j ferm ferm bb bb / p t t~ tp tp~ bp bp~ x x~ z h a YWB=2

This should give us the same processes, and looking at the feynman diagrams this seems to be the case.
Cross sections are very similar: 0.070pb vs 0.069pb.

When reconstructing the VLQ mass (using the W and b quark) there is a slight difference in the shape of the resonance, but probably not enough to lose any sleep about. See here: https://imgur.com/nuccSlt

1) is the Full chain and 2) is the fermion final case.

In the case where we specify the full decay chain (command 1) the VLQ does appear in the LHE file.

Our reasoning behind this is as we specify our interference using only the final states and coupling orders we want to be consistent with that, although it seems the difference is small enough that we probably need not worry.

So, is there a reason the particles do not appear in the LHE file, and is the discrepancy in the shape something to worry about?

Cheers,

Ferdi

Question information

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Last query:
2018-07-04
Last reply:
2018-07-04

Hi,

> So, is there a reason the particles do not appear in the LHE file,

Yes Pythia author ask us to put a threshold for the ratio width/mass above which the particle is never written inside the lhe file. This makes fully sense since including this particle within the file forbids that mass to be reshuffled by the parton-shower algorithm.
You want to forbid such reshuffling only for particle *really* onshell and not otherwise.

> and is the discrepancy in the shape something to worry about?

I'm not working this week. And this would require too much time to pronounce myself.
(I would need to reproduce your plot to be sure that I know EXACTLY what you are plotting. Which
seems too time consuming for this week)
I will try to take a look next week.

Cheers,

Olivier

> On 4 Jul 2018, at 15:27, Ferdinand Schenck <email address hidden> wrote:
>
> New question #670657 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/670657
>
> Hi,
>
> So let me start that this is not just a case of this faq: https://answers.launchpad.net/mg5amcnlo/+faq/2173
>
> We are using a model with Vector Like Quarks, and for our example case (VLQ at 1400GeV, kappa=0.4), our decay has a width of around 144GeV. (MG gets this when using auto, also confirmed by our analytical calculations)
>
> In the case where I specify my process by stating the only the final states and restrictions on coupling orders, my VLQ does not appear in the LHE file, regardless of what I set me bwcutoff to be.
>
> I did some tests, with widths of 1, 10, 100 and 130 GeV, the VLQ is written into the LHE file (given a large enough bwcutoff, but as the width is so large, 15 is just fine).
>
> But when testing with 140GeV, the particle is no longer written to the LHE file, it is not necessarily required, but would be useful for some sanity checks we are doing.
>
> To preempt some questions:
> -using MG 2.6.2
> -We are also considering interference (separately)
>
> Basically the two commands I am trying to compare are:
> 1) generate p p > j VLQ bb / p t t~ tp tp~ bp bp~ x x~ z h a, (VLQ > WW bb, WW > ferm ferm)
> and
> 2) generate p p > j ferm ferm bb bb / p t t~ tp tp~ bp bp~ x x~ z h a YWB=2
>
> This should give us the same processes, and looking at the feynman diagrams this seems to be the case.
> Cross sections are very similar: 0.070pb vs 0.069pb.
>
> When reconstructing the VLQ mass (using the W and b quark) there is a slight difference in the shape of the resonance, but probably not enough to lose any sleep about. See here: https://imgur.com/nuccSlt
>
> 1) is the Full chain and 2) is the fermion final case.
>
> In the case where we specify the full decay chain (command 1) the VLQ does appear in the LHE file.
>
> Our reasoning behind this is as we specify our interference using only the final states and coupling orders we want to be consistent with that, although it seems the difference is small enough that we probably need not worry.
>
> So, is there a reason the particles do not appear in the LHE file, and is the discrepancy in the shape something to worry about?
>
> Cheers,
>
> Ferdi
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Ferdinand Schenck (fschenck) said : #2

Hi Oliver,

Thanks for the answer. That explains the LHE file.

I was hoping there was something obvious that would explain the discrepancy in width, but I guess that's not the case.

Thanks,

Ferdi

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