MadGraph5_aMC@NLO

too much decay at MG level in madspin

Asked by Nima Hamed

Hi.
My idea is to generate u u~ > t t~ , (t > w+ b , w+ > l+ vl) , (t~ > w- b~ , w- > l- vl~).
Before that I define l- = l- ta- and l+ = l+ ta+.
so in this di-leptonic channel, I want to decay leptons, if and only if they are taus to electron or muons; something like this:
ta- ta+ > l- l+ vl- vl+ vt vt~.

I tried both in main process or in madspin but the following error appears:
decay ta- > l- vl~ vt
decay ta+ > l+ vl vt~

Exception : too much decay at MG level. this can not be done for the moment)

can you please let me know if there is a good way that my final states would be a pair of b-quarks + di-lepton channel (two leptons + needed neutrinos)?

I just want to be sure if tau created from W, they would decay to electron or muon.

thanks

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

They are plenty of issue with what you are trying to do.
1. Mixing madspin and the decay-chain syntax is something that is super complex since they are a huge amount of syntax to handle to have such generation working (not even speaking about efficiency here)
2. You are trying to do a three body decay which is not supported by MadSpin (at least in the full spin-correlation and off-shell effect that you are trying to use.

If you do not care about offshell effect and spin correlation, you can set madspin in a special mode
set spinmode none
In that case, you will bypass both problem (assuming that the tau width is not zero in your model).

Otherwise the best is likely to use the parton-shower to let the tau decay, you can use the pythia8 syntax to force a leptonic decay of the tau.

Cheers,

Olivier

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