Particle Containing 4 Quarks Is Confirmed for First Time: One side proposes that the particle is actually a union of two ordinary particles called mesons, which contain one quark and one antiquark. Zc(3900) particles could be made up of two mesons joined by a loose connection to form a molecule-like structure...
Other theorists have tentatively labelled the new particle a true tetraquark — four quarks stuck together tightly to form a compact ball... Such pairings do not occur in any known particle and would thus introduce new building blocks of matter...
Proponents of the tetraquark theory point out that a ‘molecule’ made of mesons should split easily into two halves, and that such a breakdown has not appeared in the data. “The signature of a molecule is not seen, which favours the tetraquark picture...”
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