[Primexd] About eta decays in MC

[Eugene Chudakov]

The eta->gamma+gamma decay is simple - isotropic in the eta CM frame. The 3-body decays are isotropic, but different from the phase space. One may look into the analysis note by Jon Zarling on the eta cross section. There is a package decay_evtgen which does these decays. It reads in a hddm file and writes out a hddm file.
eta decays are turned off in bggen_upd. It sends etas to the output. The decays are left to GEANT(4). I am sure GEANT4 uses the right BRs, but not sure it does the 3-body decays right. GEANT3 did not. One may find it out, or may try to do the decays using decay_evtgen.

PS. PYTHIA uses a model for 3-body pseudoscalar decays.

Eugene

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