Madwidth does not include all relevant decay modes

Asked by Rameswar Sahu

Hi Authors,
 I am having some issues generating the decays of gluino with Madwidth. I am using the MSSM+gravitino model file provided with Feynrules. I have generated the spectrum using SPheno. First, I made the gravitino and goldstino heavy in the param card and tried calculating the decay table for gluino. And I get the correct result. Below is the command I used and the result I got:
command : compute_widths go --path=param_card.dat --output=param_card2.dat --body_decay=3 --min_br=0.0001 --precision_channel=0.0001

# PDG Width
DECAY 1000021 2.168630e-06
# BR NDA ID1 ID2 ...
   4.568783e-01 3 -6 5 1000024 # 9.90799987729e-07
   4.566846e-01 3 -1000024 -5 6 # 9.90379924098e-07
   2.407050e-02 3 -6 6 1000022 # 5.2200008415e-08
   1.042179e-02 3 -5 5 1000022 # 2.26010064477e-08
   9.784979e-03 3 -1 1 1000022 # 2.121999900877e-08
   9.753162e-03 3 -3 3 1000022 # 2.115099970806e-08
   7.726537e-03 3 -6 6 1000025 # 1.675599993431e-08
   4.932607e-03 3 -2 2 1000022 # 1.069699951841e-08
   4.915546e-03 3 -4 4 1000022 # 1.0660000521979998e-08
   3.713035e-03 3 -2 2 1000025 # 8.052199092049999e-09
   3.699663e-03 3 -4 4 1000025 # 8.02320017169e-09
   1.835583e-03 3 -5 5 1000025 # 3.980700361289999e-09
   1.560985e-03 3 -1 1 1000025 # 3.3851989005499997e-09
   1.555498e-03 3 -3 3 1000025 # 3.3732996277399996e-09
   6.179016e-04 3 -1000024 -1 2 # 1.3399999468079999e-09
   6.176710e-04 3 -2 1 1000024 # 1.3394998607299999e-09
   6.158266e-04 3 -4 3 1000024 # 1.335500039558e-09
   6.158266e-04 3 -1000024 -3 4 # 1.335500039558e-09

Later, I used the same param_card but made the gravitino light (= 1eV). Technically, the gluino should decay to the gravitino (a two-body decay) and the other decay channels (3 body decays) I got earlier. I used the same command, but here is what I got as the decay table:
# PDG Width
DECAY 1000021 5.892100e-06
# BR NDA ID1 ID2 ...
   1.000000e+00 2 21 1000049 # 5.8921e-06

 I have calculated the above decay widths by hand, and they are correct. The only problem is that MW is not calculating three-body decay widths when the gravitino is light. And as we can see, the branching ratios are also significant.

P.S: I am using MG5_aMC_v3_4_0

Please look into the issue.


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Olivier Mattelaer (olivier-mattelaer) said :


Without the model/benchmark it is difficult to comment on the potential issue. (I tried with GldGr_COMPLEX_UFO but fail to get the exact behavior due to difference in benchmark).
So here I will only do generality. Allowing the computation of two and three body decay computation is obviously not a trivial task for a generic model. Because:
1) one need to be able to remove double counting (this is typically relatively simple to handle)
2) one need to remove radiative diagram. Which might be the issue here (but not clear to me)

If the diagram is not discarded due to those issues, then the issue is that it might be discarded due to some wrong estimator.
As you know, MadWidth use two successive estimator before doing the actual numerical computation.
Do you know which one of the two is preventing the computation?

Finally one final reason why some br is discarded can be related to the QCD scale in some cases the BR is so small than the particle will decay in hadronic mode rather than in quark/gluon consistuent. You are then in non perturbative regime and MadWidth discard those contributions as unreliable. (Like you would do for the bottom width computation)

Can you help with this problem?

Provide an answer of your own, or ask Rameswar Sahu for more information if necessary.

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