Official review: pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf
Section 15.3 discusses mesons; section 15.5.2, printed p. 19, explains that adding the charm quark gives flavour , which is strongly broken by the large charm quark mass but remains useful for classification. This supports distinguishing an approximate flavour symmetry from the existence of a four-flavour quark model.
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