How spinmode none works in MadSpin

They are no spin-correlation between production and decay but they are spin correlation within the production
and within the decay. So you might want to check how Pythia is handling the three body decay to see if you do have correlation between them or not. This might be the difference.

Cheers,

Olivier

> On 18 Mar 2024, at 23:05, D <email address hidden> wrote:
>
> New question #709600 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/709600
>
> I'm observing some unexpected behavior in what seems to be decays handled by MadSpin.
>
> I'm importing an RPV SUSY model (see https://feynrules.irmp.ucl.ac.be/wiki/RPVMSSM), and configuring it such that only one decay chain is allowed (which I've verified). Here are the processes:
>
> generate p p > su1 > b x1+
> add process p p > su1~ > b~ x1-
>
> Here, su1 is set to be 100% the right-handed stop. All other SUSY particle masses are set to 1000 TeV, and only the UDD RPV coupling that allows this process is non-zero.
>
> I then use the following MadSpin lines to decay the chargino (x1+):
>
> set spinmode none
> decay x1+ > b~ d~ s~
> decay x1- > b d s
>
> I'm using a special patch prepared by Olivier that allows me to use bridge mode to decay both final states since they are set to have the same 100% BR. See here for more details: https://answers.launchpad.net/mg5amcnlo/+question/708760.
>
> As far as I understand, setting MadSpin spinmode to none tells it to ignore spin correlations. However, I have been comparing a sample produced with this configuration to one made in Pythia, and I'm seeing some subtle but significant differences that seem to be related to the chargino decay and the pT and angles of the three daughter particles. These differences also seem a bit more pronounced when the mass difference between the stop and chargino is smaller (e.g. mStop = 1.5 TeV, mChargino = 1.4 TeV).
>
> In Pythia, I’m giving it the SLHA table defining these exact same decays (BR = 1) Then, I set the mass of all other sparticles to 1000 TeV and turn on the appropriate UDD coupling. I'm also using the same PDF sets as for MadGraph.
>
> I understand that this is not a forum for debugging Pythia, and it's possible that something strange is going on behind the hood of one of these generators. However, as far as I understand, if spin correlations are not being taken into account and everything is decoupled, then these two generators should be statistically identical, so I'm curious if there is some other reason I'm missing as to why they would be different. Given that chargino decay handled by MadSpin seems to treat the decay products different, I'm wondering if there's something about the MadSpin spinmode that I don't understand.
>
> Investigations into the exact differences between the Pythia and MadGraph samples are ongoing, but I'm happy to provide as many details as possible into what we do know so far as well as any further clarifications I can make about my configuration.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I see. I was not aware that this is what spinmode none does. Is there some documentation about this? I couldn't find any official MadSpin documentation and can only find statements that it means "no spin correlation", but to me, it wasn't clear that it only means no spin correlation between production and decay.

Best,
Devin

In itself the spinmode=none is too naive/limited to be on a paper on its own.
It will likely be properly documented when madspin2 paper will be out.

The paper where spinmode=none was introduced (and made for) is this one:
https://arxiv.org/pdf/1507.00020.pdf
1507.00020
PDF Document · 1,9 MB

Where you can find the following statement (at the end of appendix A)

Finally, a new MadSpin option allows to simulate arbitrary decays (including three-
body and loop-induced decays) but without any spin correlation (between production and
decay) and without BreitWigner smearing. The detailed description of this new functional-
ity is deferred to a future work, and here we limit ourselves to mentioning that this feature
can be used by adding the line ‘set spinmode none’ at the beginning of the MadSpin
configuration file.

The wiki documentation about madspin is here:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/MadSpin
and has the following:
set spinmode [none|onshell|madspin][default:madspin]
madspin default mode with full-spin correlation and off-shell effect
none(new in 2.3) No spin-correlation between production and decay. No off-shell effects. However this mode allows to handle three-body decay and loop induced processes. Due to the lack of spin-correlation it is mainly intended for scalar particle (and in particular the Higgs).
onshell (new in 2.5.3) Keep the full spin-correlation but keep the particle exactly on-shell. Supports three-body decay but not loop-induced processes (in either production or decay). Intended for particle with small width without any two body decay. This mode requires f2py program to be available on your machine.

Cheers,

Olivier

> On 19 Mar 2024, at 14:55, D <email address hidden> wrote:
>
> Question #709600 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/709600
>
> D posted a new comment:
> Hi Olivier,
>
> I see. I was not aware that this is what spinmode none does. Is there
> some documentation about this? I couldn't find any official MadSpin
> documentation and can only find statements that it means "no spin
> correlation", but to me, it wasn't clear that it only means no spin
> correlation between production and decay.
>
> Best,
> Devin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Thanks, this is helpful. I could not immediately find this twiki from the MadGraph page or other tutorials. I see now that it's under "Tools and plugins" but did see this on a quick glance.