added pointer to today’s:
LHCb collaboration, Observation of an exotic narrow doubly charmed tetraquark (arXiv:2109.01038)
LHCb collaboration, Study of the doubly charmed tetraquark (arXiv:2109.01056)
added pointer to what looks like the origin of the idea:
added references (here) on the recent experimental detection of the first presumably “true” tetraquark state
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