Using MadSpin to get branching ratios for W'

Hi,

1) If you are just interested in the branching ratio, you do not need MadSpin, just set the W' mass to "Auto" and then we will use this method: https://arxiv.org/pdf/1402.1178.pdf <https://arxiv.org/pdf/1402.1178.pdf>
to do the computation (assuming that your particle does not have loop induced decay and that NWA is valid).
MadSpin is actually using that tool (assuming no three body decay) for computing the BR

2) You likely mess up with the decay in the madspin_card:
I see that MadSpin request the following matrix-element
> INFO: generate e- e+ > t t~, (t~ > b~ w- , w- > all all QCD=99), (t~ > b wp- , wp- > all all QCD=99), (t > b w+ , w+ > all all QCD=99), (t > b~ wp+ , wp+ > all all QCD=99) --no_warning=duplicate;

and madgraph replies that such matrix element does not exits.
I believe that the following decay is not existing in your model:
> (t~ > b wp- , wp- > all all QCD=99)
should not it be
> (t~ > b~ wp- , wp- > all all QCD=99)
(same for the other branch)

Cheers,

Olivier

> On 17 Jun 2020, at 09:35, Olivier Dupon <email address hidden> wrote:
>
> New question #691364 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/691364
>
> Dear experts
>
> I am currently studying Lepton Flavor Violation for the FCC consindering e- e+ --> t t~.
> For this I'm trying to use MadGraph, together with a model that implements W', to generate some les houches files which I then can put into pythia8 and delphes so that I can generate events with W' in.
>
> Initially I got a problem where it didn't generate W' particles because the les houches file didn't have its branching ratios as such I tried to use MadSpin so that they would be included in the les houches files, but I constantly receive the following error:
>
> ************************************************************
> * *
> * W E L C O M E to M A D S P I N *
> * *
> ************************************************************
> INFO: Extracting the banner ...
> INFO: process: e- e+ > t t~
> INFO: options:
> INFO: detected model: WEff_UFO. Loading...
> Set group_subprocesses to Auto
> Note that you need to regenerate all processes
> set golem to /cvmfs/sft.cern.ch/lcg/views/LCG_92/x86_64-slc6-gcc62-dbg/lib
> set fastjet to fastjet-config
> set lhapdf to lhapdf-config
> set max_weight_ps_point 400 # number of PS to estimate the maximum for each event
> decay t > w+ b, w+ > all all
> decay t > wp+ b~, wp+ > all all
> decay t~ > w- b~, w- > all all
> decay t~ > wp- b, wp- > all all
> decay w+ > all all
> decay wp+ > all all
> decay w- > all all
> decay wp- > all all
> decay z > all all
> launch
> INFO: Will use seed 866976815
> INFO: We need to recalculate the branching fractions for t~,w-,w+,wp-,t,wp+,z
> INFO: using the FeynRules formula present in the model (arXiv:1402.1178)
> WARNING: The LO estimate for the width of particle 34
> WARNING: differs from the one in the banner by 69 percent
> INFO:
> INFO: decay channels for t~ : ( width = 1.466877 GeV )
> INFO: BR d1 d2
> INFO: 1.000000e+00 b~ w-
> INFO:
> INFO:
> INFO: decay channels for w- : ( width = 2.002524 GeV )
> INFO: BR d1 d2
> INFO: 3.164088e-01 d u~
> INFO: 3.162879e-01 s c~
> INFO: 1.111345e-01 e- ve~
> INFO: 1.111342e-01 mu- vm~
> INFO: 1.110519e-01 ta- vt~
> INFO: 1.699462e-02 s u~
> INFO: 1.698820e-02 d c~
> INFO:
> INFO:
> INFO: decay channels for w+ : ( width = 2.002524 GeV )
> INFO: BR d1 d2
> INFO: 3.164088e-01 d~ u
> INFO: 3.162879e-01 s~ c
> INFO: 1.111345e-01 e+ ve
> INFO: 1.111342e-01 mu+ vm
> INFO: 1.110519e-01 ta+ vt
> INFO: 1.699462e-02 s~ u
> INFO: 1.698820e-02 d~ c
> INFO:
> INFO:
> INFO: decay channels for wp- : ( width = 33.08451 GeV )
> INFO: BR d1 d2
> INFO: 2.528061e-01 d u~
> INFO: 2.415825e-01 b t~
> INFO: 2.399192e-01 s c~
> INFO: 8.426872e-02 e- ve~
> INFO: 8.426871e-02 mu- vm~
> INFO: 8.426832e-02 ta- vt~
> INFO: 1.288640e-02 s u~
> INFO:
> INFO:
> INFO: decay channels for t : ( width = 1.466877 GeV )
> INFO: BR d1 d2
> INFO: 1.000000e+00 b w+
> INFO:
> INFO:
> INFO: decay channels for wp+ : ( width = 33.08451 GeV )
> INFO: BR d1 d2
> INFO: 2.528061e-01 d~ u
> INFO: 2.415825e-01 b~ t
> INFO: 2.399192e-01 s~ c
> INFO: 8.426872e-02 e+ ve
> INFO: 8.426871e-02 mu+ vm
> INFO: 8.426832e-02 ta+ vt
> INFO: 1.288640e-02 s~ u
> INFO:
> INFO:
> INFO: decay channels for z : ( width = 2.411596 GeV )
> INFO: BR d1 d2
> INFO: 1.520727e-01 d~ d
> INFO: 1.520719e-01 s~ s
> INFO: 1.504304e-01 b~ b
> INFO: 1.178265e-01 u~ u
> INFO: 1.177064e-01 c~ c
> INFO: 6.878490e-02 vt~ vt
> INFO: 6.878490e-02 vm~ vm
> INFO: 6.878490e-02 ve~ ve
> INFO: 3.453867e-02 e+ e-
> INFO: 3.453839e-02 mu+ mu-
> INFO: 3.446033e-02 ta+ ta-
> INFO:
> INFO: generating the production square matrix element
> INFO: generate e- e+ > t t~;
> INFO: Done 8.41
> INFO: generating the full matrix element squared (with decay)
> INFO: generate e- e+ > t t~, (t~ > b~ w- , w- > all all QCD=99), (t~ > b wp- , wp- > all all QCD=99), (t > b w+ , w+ > all all QCD=99), (t > b~ wp+ , wp+ > all all QCD=99) --no_warning=duplicate;
> Command "generate_events run_01" interrupted with error:
> InvalidCmd : The number of of incoming/outcoming fermions are different
> quit
> INFO: storing files of previous run
> gzipping output file: unweighted_events.lhe
> INFO: Done
> INFO:
>
> I guess this is due to the nature of my experiment, but I don't know how I'll be able to generate my events then. I need these events badly so any advice is greatly appreciated.
>
> Kind regards
> Olivier Dupon
>
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