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The researchers concluded that that was the mass of a fleeting "charmonium" pentaquark containing two up quarks, one down quark, one charm quark, and one anticharm quark.

The particle contains two ‘up’ quarks, a ‘down’ quark, and a ‘charm’ quark-antiquark pair, making it a ‘charmonium’ pentaquark. 

This has a mass that places it in a dense forest of charmonium states – particles that are made up of a charm quark and a charm antiquark.

Crucially though, the Z is electrically charged whereas all charmonium states must be neutral, clearly marking it out as something unusual.