How to keep the Z boson mass distribution as Breit-Wigner distribution after using Pythia to decay Z

Asked by Yue Xu on 2019-12-26

Dear MadGraph group,

We generated events with HC model with process:
generate p p > z x0, x0 > z z

and used Pythia to decay three Z bosons.

We found the Z boson mass distribution is a delta function not a Breit-Wigner.

We wonder if there is any option or method to keep the Z boson mass distribution as a Breit-Wigner distribution.

Cheers,
Yue

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Yue Xu
Solved:
2020-01-08
Last query:
2020-01-08
Last reply:
2020-01-08

This question was reopened

Hi,

In pythia, I do not know (you should ask Pythia authors)
Otherwise, you can use MadSpin and/or request the decay within madgraph
(additionally you will have the full spin correlation)

Cheers,

Olivier

> On 26 Dec 2019, at 06:28, Yue Xu <email address hidden> wrote:
>
> New question #687539 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/687539
>
> Dear MadGraph group,
>
> We generated events with HC model with process:
> generate p p > z x0, x0 > z z
>
> and used Pythia to decay three Z bosons.
>
> We found the Z boson mass distribution is a delta function not a Breit-Wigner.
>
> We wonder if there is any option or method to keep the Z boson mass distribution as a Breit-Wigner distribution.
>
> Cheers,
> Yue
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Yue Xu (yuexu) said : #2

Hi,

Thanks for the explanation !

Cheers,
Yue

Yue Xu (yuexu) said : #3

Hi Olivier,

We have asked Pythia expert Marjorie Shapiro.
He don't recommend this route for two reasons: 1) You will loose any spin correlations between the two bosons and that might have a significant effect on the acceptance and 2) I don't think you can conserve energy and momentum if you change the boson mass between the hard scattering in Madgraph and the decay in Pythia.

We would like to know the suggestions from MadGraph expert.

Could you help adding some comments or suggestions? Is MadSpin a better way to do it?

Cheers,
Yue

Hi,

MadSpin will have off-shell effects (up to a cut off) and full spin-correlation.
It obviously relies on the validity of the narrow width approximation (NWA) and the full cross-section will be given by the NWA formula.

I have check within the Standard Model and MadSpin can correctly support such type of production.

Another option is to do the following:
generate p p > x0 z , z > all all QCD=99, ( x0 > z z, z > all all QCD=99)

The differential cross-section is actually the same as the one produced by MadSpin (it has off-shell effects up to a threshold and full spin correlation). However the computation of the cross-section does not rely on the Narrow-width approximation, a full phase-space integral is done in that case (allowing to set up cuts on the final state which is not possible for MadSpin).
Note that you still rely on the NWA for the physics validity of the diagram filtering and for the preservation of the gauge invariance.

Now as said in another thread, I would rather generate such process via
generate p p > z z z , z > all all QCD=99
To avoid any issue with the validity of the NWA for the x0 particles.

Cheers,

Olivier

> On 8 Jan 2020, at 02:52, Yue Xu <email address hidden> wrote:
>
> Question #687539 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/687539
>
> Status: Solved => Open
>
> Yue Xu is still having a problem:
> Hi Olivier,
>
> We have asked Pythia expert Marjorie Shapiro.
> He don't recommend this route for two reasons: 1) You will loose any spin correlations between the two bosons and that might have a significant effect on the acceptance and 2) I don't think you can conserve energy and momentum if you change the boson mass between the hard scattering in Madgraph and the decay in Pythia.
>
> We would like to know the suggestions from MadGraph expert.
>
> Could you help adding some comments or suggestions? Is MadSpin a better
> way to do it?
>
> Cheers,
> Yue
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Yue Xu (yuexu) said : #5

Hi Olivier,

Thanks for the explanation and suggestion!
I asked some questions about syntax generate p p > z z z in another thread.

Cheers,
Yue

Just to add that indeed MadSpin seems to have some issue in this particular case if you start from
generate p p > z x0, x0 > z z
The issue is likely that you have z boson both from the production and the decay and therefore this can creates a lot of issues since you artificially broke the symmetry between the three Z.

So you can either use MadSpin from
generate p p > z z z
or if you want to/can force x0 to be onshell (thanks to the validity of the NWA) then you can also use:
generate p p > x0 z , z > all all QCD=99, ( x0 > z z, z > all all QCD=99)

Cheers,

Olivier