Fivequark

Fivequark, commonly referred to as a pentaquark, is a hadron consisting of five valence quarks: four quarks and one antiquark. In the conventional quark model, baryons are three-quark states and mesons are quark–antiquark pairs; five-quark states are exotic hadrons that extend beyond these categories. A typical candidate has quark content such as uud c c̄, incorporating three light quarks plus a charm quark and an anti-charm quark, totaling five constituents.

Two broad structural pictures compete in interpreting fivequarks. One posits compact pentaquarks, where all five valence quarks are tightly bound in a single color-confined state, sometimes described by diquark–diquark–antiquark configurations. The other envisions hadronic molecules, in which a baryon and a meson form a bound state near open-flavor thresholds. Distinguishing between these pictures remains an active area of research.

Experimental history began with claims in the mid-2010s of pentaquark candidates observed by the LHCb collaboration in Lambda_b decays, notably Pc(4380) and Pc(4450). In 2019–2020, LHCb reported a refined set of states around masses near 4312, 4440, and 4457 MeV, interpreted as three pentaquark candidates. The exact nature—compact multiquark states versus molecular states—remains under study, with theoretical models and lattice QCD calculations contributing to the discussion. Earlier pentaquark claims in the early 2000s led to a cautionary history about experimental evidence for exotic hadrons.

Today, fivequarks are an established topic in hadron spectroscopy, with ongoing experimental and theoretical efforts at facilities such as LHCb and Belle II exploring their properties, production mechanisms, and internal structure.