MadSpin does not decay both leptons in NLO tt̄ -> SS2L Channel in HeavyN Model

What about:

You should use a syntax like

> decay t > b w+, (w+ > n mu+ , n > mu+ j j)
> decay t~ > b~ w-, w- > j j

Cheers,

Olivier

> On 17 Jul 2020, at 14:20, Ogul Oencel <email address hidden> wrote:
>
> New question #691919 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/691919
>
> Dear MG Experts,
>
> I am a first time MG user who is trying to generate some NLO tt̄ events and decay them into SS2L channel using HeavyN [1] Model.
>
> The process I want to achieve is (See Feynman diagram on Fig.1 in [2])
>
> t -> b W+ (W+ -> N mu+) (N -> mu+ W*) (W* -> q q')
> tbar -> bW
>
> with mN < mW.
>
> Since I want NLO level events, I separate the work into two steps:
>
> 1) Generate tt̄ events with Proc_card, I do:
>
> =========================
> import model SM_HeavyN_NLO
> define p = g u c d s u~ c~ d~ s~
> define j = g u c d s u~ c~ d~ s~
> define n = n1 n2 n3
> generate p p > t t~ [QCD]
> =========================
>
> 2) In a second step I want to decay these top quarks using MadSpin:
>
> I decay top products subsequently and tbar as usual.
> APPROACH 1
> =========================
> decay t > b w+
> decay w+ > n mu+
> decay n > mu+ w-
> decay w- > j j
> decay t~ > b~ w-, w- > j j
> =========================
> When I check, I come across the problem that I am having only one muon, the muon that is supposed to be coming from n does not happen. Having read a bit more I found out that X > Y Z format enforces on-shell decay of particles. So I believe since mN<mW the decay of n is not working here?
>
> The tutorials suggest using multi-body decay that does not enforce on-shell, but then MS cannot calculate everything perfectly so one needs to use the option "set spinmode none" So I did the following as a test:
> APPROACH 2
> =========================
> set spinmode none
> decay t > b mu+ mu+ j j
> decay t~ > b~ w-, w- > j j
> =========================
>
> This now gives lot of SS2L events with reasonable plots. However I am not sure if decays here do take place through the decay chain I wish to happen? I want to somehow make sure that the decay chain follows from Majorana particle n. I then thought of this following approach:
> APPROACH 3
> =========================
> set spinmode none
> decay t > b w+
> decay w+ > mu+ n
> decay n > mu+ j j (Idea is that multi-body decay here should account for off-shellness of W*)
> decay t~ > b~ w-, w- > j j
> =========================
> However this again does not give 2 mu+. My question is whether APPROACH 2 is already the correct way and if not is there way to make an APPROACH 3 like syntax work?
>
>
> Thank you in advance!
> Cheers,
>
> Ogul
>
> Setup: OTF production on LXPLUS using MG5aMC@NLO 2.6.7
> [1] http://feynrules.irmp.ucl.ac.be/wiki/HeavyN
> [2] https://arxiv.org/pdf/1910.00749.pdf
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Oliver,

Thank you for your answer, that worked! I have now lots of 2L events.

However, I noticed that I am not seeing any virtual diagrams in the decay. Is that expected? I can see that couplings defined in Model file have at most QED=2 level couplings.

Cheers,

Ogul

Thanks Olivier Mattelaer, that solved my question.