strange diagrams in madgraph

Asked by Aniello on 2018-05-03

Dear All,
I am trying to generate the following process:
pp -> Z' -> ZH, with Z to quarks and H to ZZ, with the Zs to leptons (charged and neutral).
I am using the proc card you can read in [*].

As you can see, I am generating this process by doing:
generate p p > vz, (vz > h z, z > q q, (h > z vl vl~ / vz, z > l+ l-))
add process p p > vz, (vz > h z, z > q q, (h > z l+ l- / vz, z > vl vl~))

The problem is that, when I look at the diagrams, I find also some diagrams with H-> tau tau, tau->Z tau

I would not expect to have these diagrams and in principle I would like to remove them.
Do you know how I can do that?

Best regards,
Aniello

[*]
set group_subprocesses Auto
set ignore_six_quark_processes False
set gauge unitary
set complex_mass_scheme False
import model Vector_Triplet_free_Width_UFO
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
define l+ = e+ mu+ ta+
define l- = e- mu- ta-
define p = g u c d s u~ c~ d~ s~
define q = u c d s b u~ c~ d~ s~ b~
generate p p > vz, (vz > h z, z > q q, (h > z vl vl~ / vz, z > l+ l-))
add process p p > vz, (vz > h z, z > q q, (h > z l+ l- / vz, z > vl vl~))
output Zprime_Zh_Zhadhinc_narrow_M1600 -nojpeg

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Aniello
Solved:
2018-05-04
Last query:
2018-05-04
Last reply:
2018-05-03

Hi,

> I would not expect to have these diagrams and in principle I would like to remove them.

Those diagrams exists even if they should be suppressed. I guess that this is fine to remove them since they should be gauge invariant on their own.
I would suggest to replace your syntax by

> generate p p > vz, (vz > h z, z > q q, (h > z > l+ l- vl vl~ / vz ta+ ta-))

Cheers,

Olivier

> On 3 May 2018, at 10:57, Aniello <email address hidden> wrote:
>
> New question #668541 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/668541
>
> Dear All,
> I am trying to generate the following process:
> pp -> Z' -> ZH, with Z to quarks and H to ZZ, with the Zs to leptons (charged and neutral).
> I am using the proc card you can read in [*].
>
> As you can see, I am generating this process by doing:
> generate p p > vz, (vz > h z, z > q q, (h > z vl vl~ / vz, z > l+ l-))
> add process p p > vz, (vz > h z, z > q q, (h > z l+ l- / vz, z > vl vl~))
>
> The problem is that, when I look at the diagrams, I find also some diagrams with H-> tau tau, tau->Z tau
>
> I would not expect to have these diagrams and in principle I would like to remove them.
> Do you know how I can do that?
>
> Best regards,
> Aniello
>
> [*]
> set group_subprocesses Auto
> set ignore_six_quark_processes False
> set gauge unitary
> set complex_mass_scheme False
> import model Vector_Triplet_free_Width_UFO
> define vl = ve vm vt
> define vl~ = ve~ vm~ vt~
> define l+ = e+ mu+ ta+
> define l- = e- mu- ta-
> define p = g u c d s u~ c~ d~ s~
> define q = u c d s b u~ c~ d~ s~ b~
> generate p p > vz, (vz > h z, z > q q, (h > z vl vl~ / vz, z > l+ l-))
> add process p p > vz, (vz > h z, z > q q, (h > z l+ l- / vz, z > vl vl~))
> output Zprime_Zh_Zhadhinc_narrow_M1600 -nojpeg
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Aniello (aniello.spiezia) said : #2

Dear Olivier,
thanks a lot. I am going to test your suggestion.
One question: with the syntax you are suggesting, I am removing also the Z -> tau tau diagrams, no? And these diagrams should be present, I guess. Do you know if this is true?
Best regards,
Aniello

Hi,

No you should have it. final state particles are not impacted by the / .
Internal particles are.

For example if you do
generate
g g > g g / g
then you have only the four point interaction feynman diagram.
This definition is off course meaningless since it breaks gauge invariance.

Cheers,

Olivier

> On 3 May 2018, at 14:32, Aniello <email address hidden> wrote:
>
> Question #668541 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/668541
>
> Status: Answered => Open
>
> Aniello is still having a problem:
> Dear Olivier,
> thanks a lot. I am going to test your suggestion.
> One question: with the syntax you are suggesting, I am removing also the Z -> tau tau diagrams, no? And these diagrams should be present, I guess. Do you know if this is true?
> Best regards,
> Aniello
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Aniello (aniello.spiezia) said : #4

Dear Olivier,
thanks again. I have tested it and the problem is solved.
I have an additional question, hoping it is not too naive.

I have produced the inclusive process, i.e.:
generate p p > vz, (vz > h z, z > q q)
and I get a cross section of 1076 picobarn.

I then try with the H->ZZ and the Z bosons into leptons, i.e.:
generate p p > vz, (vz > h z, z > q q, (h > z > l+ l- vl vl~ / vz ta+ ta-))
and I would expect a cross section given by the previous one multiplied by the BRs, i.e.
0.026 x (0.2 x 0.037 x 3) x 2 = 0.00115, where:
BR(H->ZZ) = 0.026
BR(Z->neutrinos) = 0.2
BR(Z->charged leptons) = 0.037*3
and the factor 2 is to take care of the two Z bosons.
So I expect a cross section of 1076*0.00115=1.242 picobarn

Instead I get 0.2188 picobarn.
So it seems to me that I am not considering some of the processes.
Furthermore, if I look at the diagrams, I see only the diagrams with Z to electrons or electronic neutrinos
(but I guess this is due to the fact that the diagrams are not repeated).

Do you have some suggestions on this issue?

Thanks again,
Aniello

Hi,

You should check this FAQ.
Also technically you can not use BR in this case since you have one Z which is offshell (so narrow width approximation does not hold)

Cheers,

Olivier

FAQ #2442: “why production and decay cross-section didn't agree.”.

Aniello (aniello.spiezia) said : #6

Dear Olivier,
thanks a lot.
Aniello