how do you automatically switch off decay channel if no phase space?

Asked by nblinov on 2021-03-04

Hello,

im trying to use madspin to decay a particle; there are many possible channels, depending on the mass of the particle. Is there any way to tell madspin to not use those channels that are kinematically forbidden automatically (instead of manually commenting out those channels in the madspin card)? If i include all channels, madspin will crash even if one of the channels has no phase space, while others are fine. A brute force solution would be to just write a new madspin card for every decaying particle mass, but I was hoping there was something more elegant. Thanks!

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2021-03-04
Last query:
2021-03-04
Last reply:
2021-03-04

I have never face any crash related to no phase-space.
for example the default process for w decay is
decay w+ > all all
which include decay to top quark.

Are you using one of the special mode of madspin? In that case it can make sense indeed.

Cheers,

Olivier

> On 4 Mar 2021, at 23:00, nblinov <email address hidden> wrote:
>
> New question #695867 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695867
>
> Hello,
>
> im trying to use madspin to decay a particle; there are many possible channels, depending on the mass of the particle. Is there any way to tell madspin to not use those channels that are kinematically forbidden automatically (instead of manually commenting out those channels in the madspin card)? If i include all channels, madspin will crash even if one of the channels has no phase space, while others are fine. A brute force solution would be to just write a new madspin card for every decaying particle mass, but I was hoping there was something more elegant. Thanks!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

nblinov (nblinov) said : #2

Yes, I forgot to mention that i use "spinmode none"; some of the decays are 3 body decays.

my suggestion would be to used multiparticles if possible.
If you do have a single line with both valid and invalid processes this should be fine.

Otherwise you can hack the line where the crash come from.
this should be given by the file me5_debug such that madspin ignore that issue and just set that contribution to 0.

Cheers,

Olivier

> On 4 Mar 2021, at 23:15, nblinov <email address hidden> wrote:
>
> Question #695867 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/695867
>
> nblinov posted a new comment:
> Yes, I forgot to mention that i use "spinmode none"; some of the decays
> are 3 body decays.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

nblinov (nblinov) said : #4

great, thanks!

nblinov (nblinov) said : #5

Thanks Olivier Mattelaer, that solved my question.