In the spring of 2013, the BESIII Collaboration reported the appearance of an electrically-charged particle -- called the "Zc(3900)" -- that was found to decay to a charged pion (consisting of either an up quark and an anti-down quark or a down quark and an anti-up quark) and a neutral J/y (consisting of a charm quark and an anti-charm quark).
Because of its decay to the J/y, the Zc(3900) particle must contain at least a charm quark and an anti-charm quark, a combination which has no electric charge. But since the Zc(3900) has one unit of electric charge, it must also contain additional quarks. Hence, the Zc(3900) must be (at least) a four-quark object.
Results of the research were published on Physical Review Letters on June 17, 2013.
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Observation of a Charged Charmoniumlike Structure at BESIII
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