Hunting for impossible particles made of four quarks

A team used QCD sum rules to predict masses of hidden-heavy tetraquarks with the exotic quantum number 0−−, which ordinary two-quark mesons cannot possess. They found four candidates in the hidden-bottom sector around 10.8–11.1 GeV and four hidden-charm partners near 4.3–4.6 GeV. These particles would be smoking-gun evidence for exotic four-quark structures and should be within reach of Belle II and LHCb experiments.