Zero three-body decay width for stop

Hi Daniele,

Thanks for using MG5 and to keep us updated with your trouble.

By running with the default param_card.dat, I successfully computed
non zero decay width for both channel.
So this is (at least in part) due to the spectrum of masses (or
couplings) that you input.

In particular, I suspect a problem with the cut on the invariant mass
that you include.
Let me explain this point. When you use a decay chain like:
> t2 > b1 w+, w+ > e+ ve
you force the w+ to be onshell (and NOT offshell). onshell meaning
that it should be close to it's invariant mass.

the following parameter in the run_card.dat defines this concept of
"close to it's invariant masss"
#**********************************
# BW cutoff (M+/-bwcutoff*Gamma)
#**********************************
   15 = bwcutoff ! (M+/-bwcutoff*Gamma)

so this might be responsible of your zero result.

I would strongly suggest to use instead

> generate t2 > b1 e+ ve
> add process t2 > b1 n1 b

Then you are not sensitive to this cut.

Cheers,

Olivier

On 27-mars-12, at 18:55, Daniele Bertolini wrote:

> New question #191860 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/191860
>
> Hello,
> I'm using MG5 with the MSSM in UFO format, which I got from
> FeynRules. I have replaced the param_card with one containing my
> spectrum/mixings/widths. The latter has been generated using SoftSusy
> +SDecay. The lowest states in my spectrum include the two stops
> (which are comparable admixtures of Left and Right states), one
> sbottom (mostly Left) and the lightest neutralino. I would like to
> study the decay t1 or t2 -> b1 w+. It cant proceed with both final
> states on-shell, so either b1 or w+ must be off-shell with a three
> body final state. I'm generating the processes with MadGraph and
> caluclating the decay width with MadEvent. My problems are:
> 1) while t2 > b1 w+, w+ > e+ ve has a non zero width i get zero for
> t2 > b1 w+, b1 > n1 b
> (the process b1 > n1 b alone has a non-vanishing decay width). Why
> does the diagram with off-shell b1 give zero width?
> 2) as I said, according to the mixing matrix in the param_card, t1
> and t2 are almost equal admixtures of Left and Right so the above
> decay with t2 replaced by t1 should exist as well. Again, the
> process is generated by MadGraph but the decay width that I get is
> zero, in this case both with a w+ or a b1 off-shell.
>
> Do you have any clue about how I could fix those problems? Your help
> would be greatly appreciated.
> Thanks a lot.
> Daniele
>
>
>
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