MadGraph5_aMC@NLO

MadSpin unable to generate amplitudes from process that's already done

Asked by Radha Mastandrea

I am trying to use MadSpin to decay a loop process involving the higgs. I first run the following in MadGraph 3.5.1:

```
import model loop_sm
generate g g > H QCD=2 QCD=2 [QCD]
output test1
launch
```

And that all runs fine.

I then launch MadSpin and run
```
import ../test1/Events/run_01/unweighted_events.lhe.gz
decay h > b b~
launch
```

But this fails with the error ` raise NoDiagramException("No amplitudes generated from process %s. Please enter a valid process" % \
madgraph.core.diagram_generation.NoDiagramException: No amplitudes generated from process Process: g g > h QCD=2 @1. Please enter a valid process`. I am a little confused because it seems like MadSpin is trying to redecay the original process, but I thought the point of MadSpin was to avoid this and just focus on the lower-level decays?

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Radha Mastandrea (rrm50) said : 		#1

Ok, I did some digging and I have to do set spinmode None for higgs decays. I think that resolved the issue.

I did have another question -- I am trying to get the higgs to decay to 2 photons, which requires a different model. So in MadSpin, I am running

```
import ../test1/Events/run_0/unweighted_events.lhe.gz
import model loop_qcd_qed_sm-with_b_mass /global/homes/r/rmastand/MG5_aMC_v3_5_1/models/loop_qcd_qed_sm/restrict_with_b_mass.dat --bypass_check
set spinmode None
decay h > a a [noborn=QED]
launch
```

This seems to run without errors, and the resulting lhe file looks ok. However, I am worried about possible inconsistencies that might come from using 2 different models for one decay chain. Is there any reason to expect that things would not work?

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#2

In spinmode=none, they are not interplay between the production matrix-element and decay matrix-element.
So they are no real potential problem.

The only real problem is obviously the consistency between the two model benchmark.
In general, expect the particles that you decay in madspin that need to have the same mass in both model, the rest is not relevant.
Obviously, this statement might be too generic and for some process it might be important to have other mass and width to be identical.

Cheers,

Olivier

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