A pentaquark revival

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 identified among the decay products of so-called bottom lambda baryons produced in proton–proton collisions. Granted, we’ve heard this story before. In 2003, four independent groups claimed to have observed a less massive pentaquark, but the finding was eventually dismissed as a statistical fluke. The new discovery seems to rest on much firmer ground. The LHCb collaboration analyzed some 26 000 promising decay events—more than 10% of which are thought to have yielded pentaquarks—and report a statistical significance of more than 9σ. Although the particles’ existence seems nearly certain, their structure remains a mystery: It’s unclear whether the quarks form one tight-knit cluster of five (depicted here), two loosely bound clusters of two and three, or other, more complicated arrangements. (The LHCb collaboration, Phys. Rev. Lett. 115, 072001, 2015, doi:10.1103/PhysRevLett.115.072001 ; image by Daniel Dominguez/CERN.)