Like wishes, bad news, and backup singers, quarks—the elementary building blocks of hadronic matter—tend to come in threes. The most familiar hadrons, the proton and the neutron, each comprise a trio of up and down quarks. But quantum chromodynamics, the theory that describes the strong forces that bind quarks together, doesn’t rule out smaller and larger groupings. Indeed, quark pairs known as mesons frequently show up in the debris of high-energy particle collisions. And experiments at the KEK collider in Japan and at CERN’s Large Hadron Collider (LHC) have produced strong evidence of a tetraquark. (See Physics Today, September 2014, page 56.) Now CERN’s LHCb collaboration has discovered what appear to be two pentaquarks. Both are composed of two up quarks, a down quark, a charmed quark, and a charmed antiquark, but they have opposite parity. The particles, each nearly five times as massive as a proton, were...

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