How can I identify which Br MG is using?

Hi Gabriel,

The short answer is that MG5aMC does not use any Branching Ratio.

Now some caveat: some code like MadSpin does use it, and it either use the branching ration written in the param_card or it is using the auto-width method to compute them (and therefore the information will be written in the param_card).

To come back to your question for context where BR could have been used, MG5aMC does the full phase-space integration (in some context not full due to onshell cut) of the propagator that depends only on the total width of the particle. This integral typically provide the 1/width part of the BR. The remaining part (the partial width) will be computed by integrating the phase-space of the decaying particle multiplied by the coupling (and lorentz structure and ...).
The advantage of this method is that
1) you are allowed to setup cuts on the final state particle
2) you do have off-shell effect (at least some or all depending of your process syntax)
3) you do have full spin correlation

Assuming that you did not apply any cut, and that you are not in any special case (like loop induce decay, NLO width,...). You can get the BR computation done by simply put the width to "auto" in the param_card, and the code will tell you what is the LO width and the associated BR. The BR being valid only if you use that total width in the above computation.

Cheers,

Olivier

Tell me how I can put the width to "auto" on param_card. I didn't find it.
Thanks in advance.

You can replace the line
DECAY 24 2.047600e+00 # WW
by
DECAY 24 Auto # WW

All details on what syntax does (and does not do, and assume,..) see the following paper: 1402.1178 [hep-ph]
It will also tell you all the different way you can run that command and for the other syntax, you do have the options to have more control on what the command does.

Cheers,

Olivier