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

Asked by Ogul Oencel on 2020-07-17

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?

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

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2020-07-24
2020-07-29
 Olivier Mattelaer (olivier-mattelaer) said on 2020-07-17: #1

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:
>
> 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?
>
>
> 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
>
>
> --

 Ogul Oencel (oguloncel) said on 2020-07-24: #2

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

 Olivier Mattelaer (olivier-mattelaer) said on 2020-07-29: #3

MadSpin decay are performed at tree-level precision.
All the details are included in the associated paper:
https://arxiv.org/abs/1212.3460

Cheers,

Olivier