Warn when charge conjugation in a combiner is a no-op
Particle combiner algorithms accept 'decay descriptors' which define the topology to reconstruct. The cc syntax expands a single descriptor string into two topologies which are then reconstructed separately:
"[D0 -> K- pi+]cc" → ["D0 -> K- pi+", "D~0 -> K+ pi-"]There are some decay descriptions which result in the same topologies when cc'd:
"[J/psi -> mu+ mu-]cc" → ["J/psi -> mu+ mu-", "J/psi -> mu+ mu-"]The result is that the combiner creates the same candidates, J/psi in this example, twice.
Combiner algorithms should warn or raise an error when using cc creates such duplication.
There is some ambiguity in cases where the topology is not identical but is not useful. For example:
"[D0 -> K+ K-]cc" → ["D0 -> K+ K-", "D~0 -> K+ K-"]The parent ID is different here, but the result will be a pair of D0 and D~0 candidates with identical kinematic and vertex properties. It is less clear whether the combiner should warn/error in this case. See Moore#64 for related discussion.