3 body decay factor of two -- solved

Hi Thibaud,

The syntax:
> X83 > X53 w+ ,(X53 > w+ t).
assumes the the X53 is on shell (more exactly between -15 to +15 times the width).
So I don't understand how you can even have a non zero contribution in this case.

The way to generate this in this case is:

X83 > X53 > w+ w+ t

Cheers,

Olivier

On Dec 10, 2012, at 1:36 PM, Thibaud Vantalon <email address hidden> wrote:

> New question #216395 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/216395
>
> Hello,
>
> I have a question about a behavior of Madgraph.
>
> I implemented a model using FeynRules. It is the standard model (the one implemented in the FeynRules website) with two extra states called X83 and X53. When I compute the decay width of the X83, I have the following behaviors:
>
> The decay chain goes has followed X83 > X53 w+ ,(X53 > w+ t). So the final states is t w+ w+ and there is one off shell propagator X53. (I have the same behavior for the decay X53 > w+ t,(t>w+ b) if I set the decay width of the top to 0.
>
> If the decay width of X53 is set to 0 (not really physical I confess) the cross section is exactly two times bigger than a X53 with a width of 0.001 gev. (The mass of the two states is 400 gev)
>
> I checked the matrix element for one point in the phase space using the output standalone command and there are almost the same in the 2 cases(the difference is only due to the 0.001 width.
>
> Having done the computation using Mathematica, the good result seems to be the one with 0 width (if I have not forgotten a factor of two.
>
> I am quite confuse by this behavior. Where do you think it come from?
>
> (if you want I can send you the files)
>
> Thank you for your time
>
> Thibaud
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Hi,

Thanks for you quick answer. But I used generate X83 > w+ w+ t. The other way of writing was just here in order to make the decay chain more understandable.

Cheers,

Thibaud

Hello Thibaud,

Please do send the UFO model files and the cards, my email is jalwall_at_ntu.edu.tw.

Thanks,
Johan

This was actually a bug, the fix for which has been released. See Bug #1089199.

Yes I know, actually the bug report has been made by Johan Alwall because of this post.

Thanks for you great works.