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adding flags to allow polarised VM decay in Starlight+EvtGen

Mateusz Dyndal requested to merge mdyndal/athena:starlight-vm-nodecay into main

This MR aims to interface Straight with EvtGen for vector meson (VM) i.e. J/Psi, Psi(2S) etc. production in UPC (Starlight) and its decay (EvtGen). Two steering flags are introduced (both are false by default):

  • suppressVMdecay (Starlight) -> to undo VM decay (from the final-state leptons which Starlight can only produce) and put a VM with its original kinematics to the MC record
  • setVMtransversePol (EvtGen) -> it sets the correct polarisation of VM expected in photonuclear UPC (transverse) via evtPart->setVectorSpinDensity();

Some validation plots and numbers from the full interface chain are here: https://indico.cern.ch/event/1352659/contributions/5695694/subcontributions/453782/attachments/2763893/4813935/hadrons_2023data.pdf

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