# Behavior of on-shell decay chains

Asked by Gabriele Ferretti on 2018-04-25

Hi,
I was working on a long decay chain in a BSM model and I noticed a strange behavior.

The point is that if I have something like a > b c with Ma < Mb+Mc I get zero result, of course, because of phase space.
However, if I let, say, b decay further: a > b c, b > d e with Mc + Md + Me < Ma < Mb + Mc the process goes through even though the b cannot be on shell.

I can reproduce it in the sm model UFO file that comes with the MG distribution, so let me use this example.
Try
generate g g > t t~ , t > w+ b , t~ > w- b~
vs.
generate g g > t t~ , ( t > w+ b, w+ > e+ vl) , ( t~ > w- b~, w- > e- vl~ )
with an unphysical bottom mass of say 120 GeV.

The first process fails, as it should, but the second goes through. I am sure you are aware of it, but it makes me uneasy and I cannot find it discussed in previous bugs/questions.

Gabriele

## Question information

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Status:
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Solved by:
Olivier Mattelaer
Solved:
2018-04-25
Last query:
2018-04-25
2018-04-25
 Olivier Mattelaer (olivier-mattelaer) said on 2018-04-25: #1

Hi,

This syntax does not forces the particles to be exactly onshell.
the invariant mass should be between Mass-N*WIDTH and Mass + N*WIDTH
The value of N can be chosen in the run_card.dat and is by default set to 15.

Now this syntax should technically not be used in that situation, but I do not want to strictly enforce it.

Cheers,

Olivier

> On 25 Apr 2018, at 15:02, Gabriele Ferretti <email address hidden> wrote:
>
> New question #668245 on MadGraph5_aMC@NLO:
>
> Hi,
> I was working on a long decay chain in a BSM model and I noticed a strange behavior.
>
> The point is that if I have something like a > b c with Ma < Mb+Mc I get zero result, of course, because of phase space.
> However, if I let, say, b decay further: a > b c, b > d e with Mc + Md + Me < Ma < Mb + Mc the process goes through even though the b cannot be on shell.
>
> I can reproduce it in the sm model UFO file that comes with the MG distribution, so let me use this example.
> Try
> generate g g > t t~ , t > w+ b , t~ > w- b~
> vs.
> generate g g > t t~ , ( t > w+ b, w+ > e+ vl) , ( t~ > w- b~, w- > e- vl~ )
> with an unphysical bottom mass of say 120 GeV.
>
> The first process fails, as it should, but the second goes through. I am sure you are aware of it, but it makes me uneasy and I cannot find it discussed in previous bugs/questions.
>
> Gabriele
>
> --

 Gabriele Ferretti (ferretti-o) said on 2018-04-25: #2

Hi Oliver,
Thank you for your prompt replay!
Indeed, shrinking bwcutoff * WW gives zero result.
I was aware of this point but I though my original chain was already obeying this.
I think what happens in the original case (that I will spare you) is that, being the chain longer, the "fuzziness" of the on-shell conditions propagate, if you what I mean.
Thanks again!
Gabriele

 Gabriele Ferretti (ferretti-o) said on 2018-04-25: #3

Thanks Olivier Mattelaer, that solved my question.