Do I need to provide decay information for PYTHIA8 when using MG5

Asked by Li on 2018-01-09

Hi,

    I have set decay for new particles to "auto" in param_card.dat. When using PYTHIA8 installed in MG5, will Pythia use the calculated decay width and decay branch ratio?

   And, can pythia8 deal with the massive resonance, like the top, higgs, new particles, decay in a right way? Shall I do the decay when generate event?

Best,
Li

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2018-01-10
Last query:
2018-01-10
Last reply:
2018-01-10

Hi Li,

I believe that asking PY8 author might be a better move here since they have a better understanding that us of their code.
In particular if they update such functionality recently.

My understanding is that PY8 will typically ignore BR information for SM particles but will take those into account for BSM particles.
For all two body decay, the decay is done in the rest frame of the particle following the spin information. Some N body decay might have the same property but I do not know the details here.
In any case, they loose the full spin correlation with the production and between decay. (as I said this is my understanding of their code).

> When using PYTHIA8 installed in MG5, will Pythia use the calculated decay width and decay branch ratio?

So I would say yes!

> And, can pythia8 deal with the massive resonance, like the top, higgs, new particles, decay in a right way?

You will have the BW peak correct, but not the full spin correlation

> Shall I do the decay when generate event?

Depend if your analysis is dependent of such spin correlation or not.

Cheers,

Olivier

> On 10 Jan 2018, at 00:57, Li <email address hidden> wrote:
>
> New question #662752 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/662752
>
> Hi,
>
> I have set decay for new particles to "auto" in param_card.dat. When using PYTHIA8 installed in MG5, will Pythia use the calculated decay width and decay branch ratio?
>
> And, can pythia8 deal with the massive resonance, like the top, higgs, new particles, decay in a right way? Shall I do the decay when generate event?
>
> Best,
> Li
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Li (huangli-itp) said : #2

Thanks Olivier Mattelaer, that solved my question.