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A pentaquark is a human-made subatomic particle, consisting of four and one antiquark bound state; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them.
As quarks have a baryon number of , and antiquarks of , the pentaquark would have a total baryon number of 1, and thus would be a baryon. Further, because it has five quarks instead of the usual three found in regular baryons ( "triquarks"), it is classified as an exotic baryon. The name pentaquark was coined by Claude Gignoux et al. (1987)
The first claim of pentaquark discovery was recorded at LEPS in Japan in 2003, and several experiments in the mid-2000s also reported discoveries of other pentaquark states.
Outside of particle research laboratories, pentaquarks might be produced naturally in the processes that result in the formation of .
The quarks are bound together by the strong force, which acts in such a way as to cancel the colour charges within the particle. In a meson, this means a quark is partnered with an antiquark with an opposite colour charge – blue and antiblue, for example – while in a baryon, the three quarks have between them all three colour charges – red, blue, and green. In a pentaquark, the colours also need to cancel out, and the only feasible combination is to have one quark with one colour (e.g. red), one quark with a second colour (e.g. green), two quarks with the third colour (e.g. blue), and one antiquark to counteract the surplus colour (e.g. antiblue).
The binding mechanism for pentaquarks is not yet clear. They may consist of five quarks tightly bound together, but it is also possible that they are more loosely bound and consist of a three-quark baryon and a two-quark meson interacting relatively weakly with each other via nuclear force (the same force that binds atomic nucleus) in a "meson-baryon molecule".
The proposed state was composed of two , two , and one strange antiquark (uudd). Following this announcement, nine other independent experiments reported seeing resonance width from and , with masses between and , all above 4 σ. While concerns existed about the validity of these states, the Particle Data Group gave the a 3-star rating (out of 4) in the 2004 Review of Particle Physics. Two other pentaquark states were reported albeit with low statistical significance—the (ddss), with a mass of and the (uudd), with a mass of . Both were later found to be statistical effects rather than true resonances.
Ten experiments then looked for the , but came out empty-handed. Two in particular (one at Belle experiment, and the other at CLAS detector) had nearly the same conditions as other experiments which claimed to have detected the (DIANA experiment and SAPHIR respectively). The 2006 Review of Particle Physics concluded:
The 2008 Review of Particle Physics went even further:
Despite these , LEPS results continued to show the existence of a narrow state with a mass of , with a statistical significance of 5.1 σ.
However this 'discovery' was later revealed to be due to flawed methodology (https://www.osti.gov/biblio/21513283-critical-view-claimed-theta-sup-pentaquark).
The search for pentaquarks was not an objective of the LHCb experiment (which is primarily designed to investigate baryon asymmetry)
It is expected that pentaquarks will be studied in electron-proton collisions in Hall B E12-12-001A
An interesting channel to study pentaquarks in proton-nuclear collisions was suggested by Schmidt & Siddikov (2016).
Designated Pc(4312)+ (Pc+ identifies a charmonium-pentaquark while the number between parenthesis indicates a mass of about 4312 MeV), the pentaquark decays to a proton and a J/ψ meson. The analyses revealed additionally that the earlier reported observations of the Pc(4450)+ pentaquark actually were the average of two different resonances, designated Pc(4440)+ and Pc(4457)+. Understanding this will require further study.
The discovery of pentaquarks will allow physicists to study the strong force in greater detail and aid understanding of quantum chromodynamics. In addition, current theories suggest that some very large stars produce pentaquarks as they collapse. The study of pentaquarks might help shed light on the physics of .
There has not been a high-statistics confirmation of any of the original experiments that claimed to see the ; there have been two high-statistics repeats from Jefferson Lab that have clearly shown the original positive claims in those two cases to be wrong; there have been a number of other high-statistics experiments, none of which have found any evidence for the ; and all attempts to confirm the two other claimed pentaquark states have led to negative results. The conclusion that pentaquarks in general, and the , in particular, do not exist, appears compelling.
There are two or three recent experiments that find weak evidence for signals near the nominal masses, but there is simply no point in tabulating them in view of the overwhelming evidence that the claimed pentaquarks do not exist... The whole story—the discoveries themselves, the tidal wave of papers by theorists and phenomenologists that followed, and the eventual "undiscovery"—is a curious episode in the history of science.
2015 LHCb results
In July 2015, the LHCb at CERN identified pentaquarks in the channel, which represents the decay of the bottom lambda baryon into a J/ψ meson , a kaon and a proton (p). The results showed that sometimes, instead of decaying via intermediate lambda baryon states, the decayed via intermediate pentaquark states. The two states, named and , had individual statistical significances of 9 σ and 12 σ, respectively, and a combined significance of 15 σ – enough to claim a formal discovery. The analysis ruled out the possibility that the effect was caused by conventional particles. The two pentaquark states were both observed decaying strongly to , hence must have a valence quark content of two , a down quark, a charm quark, and an anti-charm quark (), making them charmonium-pentaquarks.
Studies of pentaquarks in other experiments
The production of pentaquarks from electroweak decays of baryons has extremely small cross-section and yields very limited information about internal structure of pentaquarks. For this reason, there are several ongoing and proposed initiatives to study pentaquark production in other channels.
2019 LHCb results
On 26 March 2019, the LHCb collaboration announced the discovery of a new pentaquark, based on observations that passed the 5-sigma threshold, using a dataset that was many times larger than the 2015 dataset.
2022 LHCb results
On 5 July 2022, the LHCb collaboration announced the discovery of a further new pentaquark, with a significance of 15-sigma. Designated PψsΛ(4338)0, its composition is described as udsc, representing the first confirmed pentaquark containing a strange quark.
Applications
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Confinement in quantum chromodynamics leads to the production of connecting colour charges. The flux tubes act as attractive QCD string-like potentials.]]
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