Understanding how Madgraph handles the ttgamma syntax

Asked by Tom Cornelis on 2018-08-17

Dear Madgraph authors,

We are currently working on a ttgamma analysis and when reviewing again our process cards, we want to make sure we understand the syntax correctly:

import model sm-ckm_no_b_mass
define p = p b b~
define j = j b b~
define part = ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ ta- ta+
generate p p > t t~ a, t > b part part, t~ > b~ part part @1
add process p p > t t~, t > b part part a, t~ > b~ part part @2
add process p p > t t~, t > b part part , t~ > b~ part part a @3

As you can see, the goal is to have a 2->7 ttgamma process, where the photon could be emitted by the top, ISR or the top decay products. Now, if we would look at the produced diagrams, you would see that some diagrams appear in both @1 and @2, and similar for @1 and @3. Indeed, also in @2 and @3, the photon could still be emitted from the top quarks.

Therefore we want to make sure there is no risk of double counting here. We do not know how exactly Madgraph handles these situations, though I would assume double counting is avoided. Can you confirm this?

Thanks,
Tom

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Last query:
2018-08-17
Last reply:
2018-08-17

Hi,

The syntax does not prevent double counting.
So apriori you do have double counting with that syntax.

Now you do not have double counting in the Narrow-Width Approximation which means
1) you have force the top to be exactly onshell (in that case, you should use madspin in the onshell mode for the decay).
2) You should have a hard cut on your photon otherwise the NWA will not hold. (for photon of the order of a couple of times the width, the interference needs to be taken into account)

You might be able to find a sweet spot with some value of bw-cut, width of the top and of the cut on the photon which prevents the double counting and where NWA is a good enough approximation.
But I have never investigate if such sweet spot exists.

Cheers,

Olivier

PS: Did you try to implement one diagram restriction for the @1 such that,
you do not have at all the top FSR in that case. If combined with a large enough value of bw-cut, you should get a quite good way to do such computation (need to be checked obviously)

> On 17 Aug 2018, at 12:12, Tom Cornelis <email address hidden> wrote:
>
> New question #672405 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/672405
>
> Dear Madgraph authors,
>
> We are currently working on a ttgamma analysis and when reviewing again our process cards, we want to make sure we understand the syntax correctly:
>
> import model sm-ckm_no_b_mass
> define p = p b b~
> define j = j b b~
> define part = ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ ta- ta+
> generate p p > t t~ a, t > b part part, t~ > b~ part part @1
> add process p p > t t~, t > b part part a, t~ > b~ part part @2
> add process p p > t t~, t > b part part , t~ > b~ part part a @3
>
> As you can see, the goal is to have a 2->7 ttgamma process, where the photon could be emitted by the top, ISR or the top decay products. Now, if we would look at the produced diagrams, you would see that some diagrams appear in both @1 and @2, and similar for @1 and @3. Indeed, also in @2 and @3, the photon could still be emitted from the top quarks.
>
> Therefore we want to make sure there is no risk of double counting here. We do not know how exactly Madgraph handles these situations, though I would assume double counting is avoided. Can you confirm this?
>
> Thanks,
> Tom
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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