Adding Pythia8 UserHook to enable decay to SUEP

Simone Pagan Griso requested to merge spagan/athena:21.6-sp-seup into 21.6

This Pythia UserHook implements a decay of a Higgs-like (or any, as far as the code is concerned) particle to a Soft-Unclustered-Energy-Pattern.

The theory behind it is described in arXiv:1612.00850.

The code is largely based on the available public code provided by one of the author and available at

Right now, to avoid dependencies and adding too many additional files, all the code lives in a single file. I'm happy if the preference is to port two of the main files containing the auxiliary class needed as separate files in the atlas repo to make the UserHook file more easily readable, if this is the preference.

Also I'm happy to move all of this in a separate package if you think it's best.

Tagging for info (as they helped me through this) @mshapiro and @zmarshal.

Edited by Tomasz Bold

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