No valid decay for particle 6 with mad spin

Hi,

The issue is that the decay that you asked is unphysical.
Since you do not specify the branching ratio for your "top", madspin request madwidth to do the computation.
MadWidth detects that the decay is below the QCD scale and discard the decay channel since such computation is meaningless.

If you ask the decay directly, Madgraph does not test if the decay does makes sense or not and therefore return a (non physical) result. I would think to check if they are a way to easily detects such type of decay and forbide them as well within madgraph.

Cheers,

Olivier

Thank you for your answer, but I don't understand your suggestion.
Do you suggest forbidding my decay within madgrpah, as in not looking at this decay at all?
If I misunderstood what you meant, I have a different question, Is there a way to change the BR directly?
I am changing the decay width of the "top", but does it affect Madspin aswell?

David

Hi,

Yes, I would not use/trust any result for that syntax since you are going to be dominated by non perturbative physics.

For MG5aMC, the only input used is the total width (all BR information will be ignored).

MadSpin is only going to use the provided total width if you also have the Branching Ratio information.
Otherwise, it will compute the total width (and all the BR) and use that one.
So by providing the width/BR for all particle in madspin, you can bypass the issue that MadSpin detects that the decay is non perturbative.

Olivier

Thank you again.

Lastly, I couldn't find a way to change the BR/width specifically in madspin.
Is it done through the param_card or is it based on the model I use?

I have changed the width of the "top" in the param_card and the error still persists, so I figured it is not the former.

David

Yes,

It is done via the param_card according to the community standard for that file (see SLHA1: Susy LesHouches Accord)
You have to have the BR information included for all particles that have a line decay in the madspin_card to avoid that madspin triggers the automatic computation of the width.

But can you try to generate a physical process as a test, running such unphysical process leads you to unphysical regime where all code are expected to either crash (best case scenario) or to return non-sense. Trying to avoid those crash is just a waste of time.
I do not really understand what you are hoping to achieve here.

Cheers,

Olivier

Thank you very much, this does help me run the simulation.

For the reason for me doing this, I am using this process as a probe of the polarization and spin correlation of both of the "top" particles. From the mass of these "tops", I am trying to look at the decaying bottoms, and as far as I understand it, this process fits my needs.

I used this process for actual tops as well, and using madspin for the decay resulted in better output. This is why I am trying to make it work through madspin.

Best,
David