pentaquark

A pentaquark is a type of hadron that consists of four quarks and one antiquark (qqqq\bar{q}). This makes it an exotic hadron, in contrast to ordinary baryons (three quarks) and mesons (a quark and an antiquark). Pentaquarks can be envisioned as compact five-quark bound states or as hadronic molecules formed by a meson bound to a baryon. Their quark content can include up, down, strange, charm, or bottom quarks, leading to a range of possible charges and masses.

The idea of pentaquarks arose within the quark model in the 1960s as a natural extension of multiquark configurations. Early experimental claims, most notably the reported Theta+(1540) in 2003, generated significant interest, but subsequent experiments failed to confirm the state and it is now widely regarded as not established. For several years the existence of pentaquarks remained uncertain, with searches focusing on light-quark systems.

In 2015, the LHCb collaboration reported resonant structures in the decay Lambda_b0 → J/psi p K−, interpreted as two hidden-charm pentaquark states, Pc(4380)+ and Pc(4450)+. In 2019, LHCb further identified three narrow states, Pc(4312)+, Pc(4440)+, and Pc(4457)+, in the same decay channel. These observations are supported by detailed amplitude analyses, though their internal structure and quantum numbers are still under study. The prevailing interpretations include compact five-quark configurations and meson-baryon molecular bound states, with ongoing theoretical and experimental work to determine their nature.

Overall, the charm-sector pentaquarks discovered by LHCb constitute the first robust evidence for such exotic hadrons and continue to inform understanding of non-perturbative QCD.