Suggestion to use the IVOA HEIG nomenclature, which allows both particle name or PDGID. This solves the issue of "background", where there is not one single particle type. From the _IVOA HEIG Extension Draft_: > **messenger** > - `cosmic-ray`: the messenger is a cosmic-ray; > - `muon`: the messenger is a muon; > - `neutrino`: the messenger is a neutrino (any type); > - `photon`: the messenger is a photon; > - `proton`: the messenger is a proton or anti-proton; > - `pdgid`*$\pm$`nn`*: the messenger is a particle whose PDG ID[^1] is > *$\pm$nn*. > > The value "pdgid*$\pm$nn*" (where the sign is mandatory) allows a > messenger particle not listed in the table to be specified by reporting > the particle's Particle Data Group Identifier as *$\pm$nn*. For example, > a muon, $\mu^{-}$, may be specified as "pdgid+13". We note that > permissible *messenger* values will vary from facility to facility and > from instrument to instrument. > > [^1]: Particle Data Group Identifier, see > <https://www.phy.bnl.gov/twister/bee/particles/>