The source project of this merge request has been removed.
Adding pNGB DM model from 2107.12389 provided by Uli Haisch.
The model has been tested and works in 21.6,latest,AthGeneration
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In the txt file I add the twiki that will eventually hold recommendation for parameter settings to be used in this model. At the moment we don't have very concrete suggestions except of a set of initial recommendations provided by Uli. We want to add this model in the ATLAS repo to allow people to develop jO and study it further for Run 3.