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ve to sidebar hide top 1 background 2 structure 3 history toggle history subsection 3 1 mid 2000s 3 2 2015 lhcb results 3 3 studies of pentaquarks in other experiments 3 4 2019 lhcb results 3 5 2022 lhcb results 4 applications 5 see also 6 footnotes 7 references 8 further reading 9 external links toggle the table of contents pentaquark 33 languages afrikaans العربية беларуская català čeština deutsch ελληνικά español فارسی suomi français עברית हिन्दी bahasa indonesia italiano 日本語 한국어 македонски bahasa melayu nederlands norsk bokmål polski português română русский slovenščina தமிழ் ไทย tagalog türkçe українська tiếng việt 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia subatomic particle with five quarks two models of a generic pentaquark a five quark bag a meson baryon molecule a q indicates a quark and a q an antiquark gluons wavy lines mediate strong interactions between quarks red green and blue colour charges must each be present while the remaining quark and antiquark must share a colour and its anticolour in this example blue and antiblue shown as yellow a pentaquark is a subatomic particle consisting of four quarks and one antiquark bound together evidence for the existence of pentaquarks has been found in decays of the bottom lambda baryon 1 as quarks have a baryon number of 1 3 and antiquarks of 1 3 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 a k a triquarks it is classified as an exotic baryon the name pentaquark was coined by claude gignoux et al 1987 2 and harry j lipkin in 1987 3 however the possibility of five quark particles was identified as early as 1964 when murray gell mann first postulated the existence of quarks 1 although predicted for decades pentaquarks proved surprisingly difficult to discover and some physicists were beginning to suspect that an unknown law of nature prevented their production 4 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 5 however other researchers were not able to replicate the leps results and the other pentaquark discoveries were not accepted because of poor data and statistical analysis 6 on 13 july 2015 the lhcb collaboration at cern reported results consistent with pentaquark states in the decay of bottom lambda baryons λ 0 b 7 on 26 march 2019 the lhcb collaboration announced the discovery of a new pentaquark that had not been previously observed 8 on 5 july 2022 the lhcb collaboration announced the discovery of the p λ ψs 4338 0 a pentaquark 9 outside of particle research laboratories pentaquarks might be produced naturally in the processes that result in the formation of neutron stars 10 background edit main article quark a quark is a type of elementary particle that has mass electric charge and colour charge as well as an additional property called flavour which describes what type of quark it is up down strange charm top or bottom due to an effect known as colour confinement quarks are never seen on their own instead they form composite particles known as hadrons so that their colour charges cancel out hadrons made of one quark and one antiquark are known as mesons while those made of three quarks are known as baryons these regular hadrons are well documented and characterized however there is nothing in theory to prevent quarks from forming exotic hadrons such as tetraquarks with two quarks and two antiquarks or pentaquarks with four quarks and one antiquark 4 structure edit a diagram of the p c type pentaquark possibly discovered in july 2015 showing the flavours of each quark and one possible colour configuration a wide variety of pentaquarks are possible with different quark combinations producing different particles to identify which quarks compose a given pentaquark physicists use the notation qqqq q where q and q respectively refer to any of the six flavours of quarks and antiquarks the symbols u d s c b and t stand for the up down strange charm bottom and top quarks respectively with the symbols of u d s c b t corresponding to the respective antiquarks for instance a pentaquark made of two up quarks one down quark one charm quark and one charm antiquark would be denoted uudc c 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 b 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 11 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 pion exchange the same force that binds atomic nuclei in a meson baryon molecule 1 12 13 history edit mid 2000s edit the requirement to include an antiquark means that many classes of pentaquark are hard to identify experimentally if the flavour of the antiquark matches the flavour of any other quark in the quintuplet it will cancel out and the particle will resemble its three quark hadron cousin for this reason early pentaquark searches looked for particles where the antiquark did not cancel 11 in the mid 2000s several experiments claimed to reveal pentaquark states in particular a resonance with a mass of 1540 mev c 2 4 6 σ was reported by leps in 2003 the θ 14 this coincided with a pentaquark state with a mass of 1530 mev c 2 predicted in 1997 15 the proposed state was composed of two up quarks two down quarks and one strange antiquark uudd s following this announcement nine other independent experiments reported seeing narrow peaks from n k and p k 0 with masses between 1522 mev c 2 and 1555 mev c 2 all above 4 σ 14 while concerns existed about the validity of these states the particle data group gave the θ a 3 star rating out of 4 in the 2004 review of particle physics 14 two other pentaquark states were reported albeit with low statistical significance the φ ddss u with a mass of 1860 mev c 2 and the θ 0 c uudd c with a mass of 3099 mev c 2 both were later found to be statistical effects rather than true resonances 14 ten experiments then looked for the θ but came out empty handed 14 two in particular one at belle and the other at clas had nearly the same conditions as other experiments which claimed to have detected the θ diana and saphir respectively 14 the 2006 review of particle physics concluded 14 t here has not been a high statistics confirmation of any of the original experiments that claimed to see the θ 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 θ and all attempts to confirm the two other claimed pentaquark states have led to negative results the conclusion that pentaquarks in general and the θ in particular do not exist appears compelling the 2008 review of particle physics went even further 6 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 despite these null results leps results continued to show the existence of a narrow state with a mass of 1524 4 mev c 2 with a statistical significance of 5 1 σ 16 however this discovery was later revealed to be due to flawed methodology https www osti gov biblio 21513283 critical view claimed theta sup pentaquark 2015 lhcb results edit feynman diagram representing the decay of a lambda baryon λ 0 b into a kaon k and a pentaquark p c in july 2015 the lhcb collaboration at cern identified pentaquarks in the λ 0 b j ψk p channel which represents the decay of the bottom lambda baryon λ 0 b into a j ψ meson j ψ a kaon k and a proton p the results showed that sometimes instead of decaying via intermediate lambda states the λ 0 b decayed via intermediate pentaquark states the two states named p c 4380 and p c 4450 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 1 the two pentaquark states were both observed decaying strongly to j ψp hence must have a valence quark content of two up quarks a down quark a charm quark and an anti charm quark u u d c c making them charmonium pentaquarks 7 10 17 the search for pentaquarks was not an objective of the lhcb experiment which is primarily designed to investigate matter antimatter asymmetry 18 and the apparent discovery of pentaquarks was described as an accident and something we ve stumbled across by the physics coordinator for the experiment 12 studies of pentaquarks in other experiments edit a fit to the j ψp invariant mass spectrum for the λ 0 b j ψk p decay with each fit component shown individually the contribution of the pentaquarks are shown by hatched histograms the production of pentaquarks from electroweak decays of λ 0 b 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 it is expected that pentaquarks will be studied in electron proton collisions in hall b e12 12 001a 19 and hall c e2 16 007 20 experiments at jlab the major challenge in these studies is a heavy mass of the pentaquark which will be produced at the tail of photon proton spectrum in jlab kinematics for this reason the currently unknown branching fractions of pentaquark should be sufficiently large to allow pentaquark detection in jlab kinematics the proposed electron ion collider which has higher energies is much better suited for this problem an interesting channel to study pentaquarks in proton nuclear collisions was suggested by schmidt siddikov 2016 21 this process has a large cross section due to lack of electroweak intermediaries and gives access to pentaquark wave function in the fixed target experiments pentaquarks will be produced with small rapidities in laboratory frame and will be easily detected besides if there are neutral pentaquarks as suggested in several models based on flavour symmetry these might be also produced in this mechanism this process might be studied at future high luminosity experiments like after lhc 22 and nica 23 2019 lhcb results edit 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 8 designated p c 4312 p c 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 p c 4450 pentaquark actually were the average of two different resonances designated p c 4440 and p c 4457 understanding this will require further study 2022 lhcb results edit on 5 july 2022 the lhcb collaboration announced the discovery of a further new pentaquark 24 with a significance of 15 sigma designated p ψs λ 4338 0 its composition is described as udsc c representing the first confirmed pentaquark containing a strange quark 25 applications edit colour flux tubes produced by five static quark and antiquark charges computed in lattice qcd 26 confinement in quantum chromodynamics leads to the production of flux tubes connecting colour charges the flux tubes act as attractive qcd string like potentials 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 neutron stars 10 see also edit baryon triquark exotic matter heptaquark hexaquark list of particles quark model tetraquark footnotes edit the bracketed number is the calculated mass of the particle measured in mev the colour charges do not correspond to physical visible colours they are arbitrary labels used to help scientists describe and visualise the charges of quarks references edit 1 2 3 4 observation of particles composed of five quarks pentaquark charmonium states seen in λ 0 b j ψpk decays lhcb press release cern 14 july 2015 retrieved 2015 07 14 gignoux c silvestre brac b richard j m 1987 07 16 possibility of stable multiquark baryons physics letters b 193 2 323 326 bibcode 1987phlb 193 323g doi 10 1016 0370 2693 87 91244 5 lipkin h j 1987 new possibilities for exotic hadrons anticharmed strange baryons physics letters b 195 3 484 488 bibcode 1987phlb 195 484l doi 10 1016 0370 2693 87 90055 4 1 2 muir h 2 july 2003 pentaquark discovery confounds sceptics new scientist retrieved 2010 01 08 hicks k 23 july 2003 physicists find evidence for an exotic baryon ohio university archived from the original on 8 september 2016 retrieved 2010 01 08 1 2 see p 1124 in amsler c et al particle data group 2008 review of particle physics pdf physics letters b 667 1 5 1 6 bibcode 2008phlb 667 1a doi 10 1016 j physletb 2008 07 018 hdl 1854 lu 685594 s2cid 227119789 1 2 aaij r et al lhcb collaboration 2015 observation of j ψp resonances consistent with pentaquark states in λ 0 b j ψk p decays physical review letters 115 7 072001 arxiv 1507 03414 bibcode 2015phrvl 115g2001a doi 10 1103 physrevlett 115 072001 pmid 26317714 s2cid 119204136 1 2 lhcb experiment discovers a new pentaquark cern 26 march 2019 retrieved 26 april 2019 closier joel july 5 2022 observa...