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The best evidence yet for the existence of a new type of particle called a pentaquark has been unveiled by physicists working on the LHCb experiment on the Large Hadron Collider (LHC) at CERN. Containing five bound quarks, pentaquarks were first predicted to exist in the late 1970s, and evidence for their existence emerged from several labs in the 2000s, only to be contradicted by experiments done elsewhere. While this latest evidence from LHCb is very strong, the data do not reveal exactly how the five quarks are bound together – something that will be the subject of further studies at CERN.

Most known hadrons are either mesons, which contain a quark and an antiquark, or baryons, which comprise three quarks. A proton contains two "up" quarks and one "down" quark, while a positive kaon contains an up quark and a "strange" antiquark. But the theory of the strong force – quantum chromodynamics (QCD) – allows for other types of baryons, providing that the number of quarks minus the number of antiquarks is a multiple of three. In particular, it allows for particles containing four quarks and one antiquark.

In the early 2000s several independent groups of physicists reported the observation of pentaquarks, most with masses in the 1520–1560 MeV/c2 range. Since then, however, other research groups have failed to find further evidence for these pentaquarks, and by the start of this decade, the consensus in the particle-physics community was that the pentaquark had yet to be discovered...

LHCb claims discovery of two pentaquarks - physicsworld.com

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