View source for Physics:Pentaquark

{{Short description|Human-made subatomic particle}}
{{multiple image|image1=Pentaquark-generic.svg|image2=Meson-Baryon-molecule-generic.svg|header=Two models of a generic pentaquark|caption1=A five-quark "bag"|caption2=A "meson-baryon molecule"|footer=A ''q'' indicates a [[Physics:Quark|quark]] and a {{overline|''q''}} an [[Physics:Antiquark|antiquark]]. Gluons (wavy lines) mediate [[Physics:Strong interaction|strong interaction]]s 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 human-made [[Physics:Subatomic particle|subatomic particle]], consisting of four [[Physics:Quark|quark]]s and one [[Physics:Antiquark|antiquark]] [[Physics:Bound state|bound together]]; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them.

As quarks have a [[Physics:Baryon number|baryon number]] of {{sfrac|+|1|3}}, and antiquarks of {{sfrac|−|1|3}}, the pentaquark would have a total baryon number of 1, and thus would be a [[Physics:Baryon|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 [[Physics:Exotic baryon|exotic baryon]]. The name pentaquark was coined by Claude Gignoux ''et al.'' (1987)<ref>
{{cite journal
 |last1=Gignoux |first1=C.
 |last2=Silvestre-Brac |first2=B.
 |last3=Richard |first3=J.M.
 |date=1987-07-16
 |title=Possibility of stable multiquark baryons
 |journal=Physics Letters&nbsp;B
 |volume=193 |issue=2 |pages=323–326
 |doi=10.1016/0370-2693(87)91244-5 |bibcode=1987PhLB..193..323G
}}
</ref> and Harry J. Lipkin in 1987;<ref name="Lipkin1987">
{{cite journal
 |year=1987
 |title=New possibilities for exotic hadrons — anticharmed strange baryons
 |journal=[[Physics:Physics Letters B|Physics Letters B]]
 |volume=195 |issue=3 |pages=484–488
 |bibcode= 1987PhLB..195..484L
 |doi=10.1016/0370-2693(87)90055-4
}}
</ref> however, the possibility of five-quark particles was identified as early as 1964 when [[Biography:Murray Gell-Mann|Murray Gell-Mann]] first postulated the [[Physics:Quark model|existence of quarks]].<ref name="LHCb-public">
{{cite press release
 |title=Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ{{su|p=0|b=b}}→J/ψpK{{sup|−}} decays
 |date=14 July 2015
 |publisher=[[Organization:CERN|CERN]]
 |department=LHCb
 |url=http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Penta
 |access-date=2015-07-14
}}
</ref> 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.<ref name="NewScientist2003">
{{cite magazine
 |first=H. |last=Muir
 |date=2 July 2003
 |title=Pentaquark discovery confounds sceptics
 |magazine=New Scientist
 |url=https://www.newscientist.com/article/dn3903-pentaquark-discovery-confounds-sceptics.html
 |access-date=2010-01-08
}}
</ref>

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.<ref name="Ohio">
{{cite web
 |first=K. |last=Hicks
 |date=23 July 2003
 |title=Physicists find evidence for an exotic baryon
 |url=http://www.phy.ohiou.edu/~hicks/thplus/thplus-v0.html
 |publisher=[[Organization:Ohio University|Ohio University]]
 |access-date=2010-01-08
 |archive-url=https://web.archive.org/web/20160908045044/http://www.phy.ohiou.edu/~hicks/thplus/thplus-v0.html
 |archive-date=8 September 2016
 |url-status=dead
}}
</ref> 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.<ref name=PDG2008>See p.&nbsp;1124 in
{{cite journal
 |first1=C. |last1=Amsler
 |collaboration=[[Physics:Particle Data Group|Particle Data Group]]
 |display-authors=0
 |year=2008
 |title=Review of particle physics
 |url=http://pdg.lbl.gov/2008/download/rpp-2008-plB667.pdf
 |journal=[[Physics:Physics Letters B|Physics Letters B]]
 |volume=667 |issue=1–5 |pages=1–6
 |bibcode=2008PhLB..667....1A
 |doi=10.1016/j.physletb.2008.07.018
|hdl=1854/LU-685594
 |s2cid=227119789
 |hdl-access=free
 }}
</ref> On 13 July 2015, the LHCb collaboration at [[Organization:CERN|CERN]] reported results consistent with pentaquark states in the decay of bottom Lambda baryons ({{nowrap|Λ{{su|p=0|b=b}}}}).<ref name="LHCb2015">
{{cite journal
 |first1=R.  |last1=Aaij
 |collaboration=LHCb collaboration
 |display-authors=0
 |year=2015
 |title=Observation of J/ψp resonances consistent with pentaquark states in Λ{{su|p=0|b=b}}→J/ψK<sup>−</sup>p decays
 |journal=[[Physics:Physical Review Letters|Physical Review Letters]]
 |volume=115 |issue=7 |pages=072001
 |doi=10.1103/PhysRevLett.115.072001
|pmid=26317714
 |arxiv = 1507.03414 |bibcode = 2015PhRvL.115g2001A |s2cid=119204136
}}
</ref>
On 26 March 2019, the LHCb collaboration announced the discovery of a new pentaquark that had not been previously observed.<ref name="LHCb-2019">{{cite web |title=LHCb experiment discovers a new pentaquark |url=https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark |publisher=[[Organization:CERN|CERN]] |date=26 March 2019 |access-date=26 April 2019}}</ref> On 5 July 2022, the LHCb collaboration announced the discovery of the {{nowrap|P{{su|p=Λ|b=ψs}}(4338)<sup>0</sup>}}{{efn|The bracketed number is the calculated mass of the particle, measured in MeV.}} pentaquark.<ref>
{{cite web
 |url=https://lhcb-outreach.web.cern.ch/2022/07/05/observation-of-a-strange-pentaquark-a-doubly-charged-tetraquark-and-its-neutral-partner/
 |title=Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner. 
 |date=July 5, 2022
 |access-date=July 5, 2022
}}</ref>

Outside of particle research laboratories, pentaquarks might be produced naturally in the processes that result in the formation of [[Astronomy:Neutron star|neutron star]]s.<ref name="Guardian">
{{cite news
 |first=I.  |last=Sample
 |date=14 July 2015
 |title=Large Hadron Collider scientists discover new particles: pentaquarks
 |newspaper={{wipe|The Guardian}}
|url=https://www.theguardian.com/science/2015/jul/14/large-hadron-collider-scientists-discover-new-particles-pentaquarks
 |access-date=2015-07-14
}}
</ref>

==Background==
{{main|Physics:Quark}}

A [[Physics:Quark|quark]] is a type of [[Physics:Elementary particle|elementary particle]] that has [[Physics:Mass|mass]], [[Physics:Electric charge|electric charge]], and colour charge, as well as an additional property called [[Physics:Flavour (particle physics)|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 [[Physics:Antiparticle|antiquark]] are known as [[Physics:Meson|meson]]s, while those made of three quarks are known as [[Physics:Baryon|baryon]]s. These 'regular' hadrons are well documented and characterized; however, there is nothing in theory to prevent quarks from forming [[Physics:Exotic hadron|'exotic' hadrons]] such as [[Physics:Tetraquark|tetraquark]]s with two quarks and two antiquarks, or pentaquarks with four quarks and one antiquark.<ref name="NewScientist2003"/>

==Structure==
[[File:Pentaquark.svg|thumb|right|A diagram of the {{nowrap|P{{su|p=+|b=c}}}} type pentaquark possibly discovered in July 2015, showing the flavours of each quark and one possible colour configuration.|alt=five circles arranged clockwise: blue circle marked "c", yellow (antiblue) circle marked "c" with an overscore, green circle marked "u", blue circle marked "d", and red circle marked "u".]]

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{{overline|q}}'', where ''q'' and ''{{overline|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 [[Physics:Up quark|up]], [[Physics:Down quark|down]], [[Physics:Strange quark|strange]], [[Physics:Charm quark|charm]], [[Physics:Bottom quark|bottom]], and [[Physics:Top quark|top]] quarks respectively, with the symbols of {{overline|u}}, {{overline|d}}, {{overline|s}}, {{overline|c}}, {{overline|b}}, {{overline|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{{overline|c}}.

The quarks are bound together by the [[Physics:Strong force|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.{{efn|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.}} 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).<ref name="HadronPhysics">
{{cite book
 |first=J. |last=Pochodzalla
 |year=2005
 |chapter=Duets of strange quarks
 |title=Hadron Physics
 |page=268
 |publisher=IOS Press
 |isbn=978-1614990147
}}
</ref>

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 [[Physics:Nuclear force|pion exchange]] (the same force that binds [[Physics:Atomic nucleus|atomic nuclei]]) in a "meson-baryon molecule".<ref name="LHCb-public"/><ref name="NewScientist2015">
{{cite magazine
 |first=G. |last=Amit
 |date=14 July 2015
 |title=Pentaquark discovery at LHC shows long-sought new form of matter
 |magazine=New Scientist
 |url=https://www.newscientist.com/article/dn27892-pentaquark-discovery-at-lhc-shows-long-sought-new-form-of-matter/
 |access-date=2015-07-14
}}</ref><ref name="Heavy Pentaquarks">
{{cite journal
 |first1=T.D.  |last1=Cohen
 |first2=P.M.  |last2=Hohler
 |first3=R.F.  |last3=Lebed
 |year=2005
 |title=On the Existence of Heavy Pentaquarks: The large ''N''<sub>c</sub> and Heavy Quark Limits and Beyond
 |journal=[[Physics:Physical Review D|Physical Review D]]
 |volume=72 |issue=7 |pages=074010
 |arxiv=hep-ph/0508199  |bibcode=2005PhRvD..72g4010C
 |doi=10.1103/PhysRevD.72.074010  |s2cid=20988932
}}
</ref>

==History==

===Mid-2000s===
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.<ref name="HadronPhysics"/> In the mid-2000s, several experiments claimed to reveal pentaquark states. In particular, a [[Physics:Resonance (particle physics)|resonance]] with a [[Physics:Mass|mass]] of {{val|1540|ul=MeV/c2}} (4.6&nbsp;[[Standard deviation|σ]]) was reported by LEPS in 2003, the {{SubatomicParticle|Theta+}}.<ref name=PDG2006>
{{cite journal
 |first1=W.-M. |last1=Yao
 |collaboration=[[Physics:Particle Data Group|Particle Data Group]]
 |display-authors=0 
 |year=2006
 |title=Review of particle physics: {{SubatomicParticle|Theta+}}
 |journal=[[Physics:Journal of Physics G|Journal of Physics G]]
 |volume=33 |issue=1 |pages=1–1232
 |arxiv=astro-ph/0601168  |bibcode=2006JPhG...33....1Y
 |doi=10.1088/0954-3899/33/1/001
 |s2cid=262936640
 |url=http://pdg.lbl.gov/2006/reviews/theta_b152.pdf
}}</ref> This coincided with a pentaquark state with a mass of {{val|1530|u=MeV/c2}} predicted in 1997.<ref>
{{cite journal
 |first1=D.  |last1=Diakonov
 |first2=V.  |last2=Petrov
 |first3=M.  |last3=Polyakov
 |name-list-style=amp
 |year=1997
 |title=Exotic anti-decuplet of baryons: prediction from chiral solitons
 |journal=[[Physics:Zeitschrift für Physik A|Zeitschrift für Physik A]]
 |volume=359 |issue=3 |pages=305
 |arxiv=hep-ph/9703373
 |bibcode=1997ZPhyA.359..305D |doi=10.1007/s002180050406
 |citeseerx=10.1.1.44.7282 |s2cid=2322877
}}
</ref>
 
The proposed state was composed of two [[Physics:Up quark|up quark]]s, two [[Physics:Down quark|down quark]]s, and one strange antiquark (uudd{{overline|s}}). Following this announcement, nine other independent experiments reported seeing [[Physics:Resonance width|narrow peak]]s from {{SubatomicParticle|link=yes|neutron}}{{SubatomicParticle|link=yes|Kaon+}} and  {{SubatomicParticle|link=yes|proton}}{{SubatomicParticle|link=yes|Kaon0}}, with masses between {{val|1522|u=MeV/c2}} and {{val|1555|u=MeV/c2}}, all above 4&nbsp;σ.<ref name=PDG2006/> While concerns existed about the validity of these states, the [[Physics:Particle Data Group|Particle Data Group]] gave the {{SubatomicParticle|Theta+}} a 3-star rating (out of 4) in the 2004 ''Review of Particle Physics''.<ref name=PDG2006/> Two other pentaquark states were reported albeit with low statistical significance—the {{SubatomicParticle|Phi--}} (ddss{{overline|u}}), with a mass of {{val|1860|u=MeV/c2}} and the {{SubatomicParticle|Charmed Theta0}} (uudd{{overline|c}}), with a mass of {{val|3099|u=MeV/c2}}. Both were later found to be statistical effects rather than true resonances.<ref name=PDG2006/>

Ten experiments then looked for the {{SubatomicParticle|Theta+}}, but came out empty-handed.<ref name=PDG2006/> Two in particular (one at [[Physics:Belle experiment|BELLE]], and the other at [[Physics:CLAS detector|CLAS]]) had nearly the same conditions as other experiments which claimed to have detected the {{SubatomicParticle|Theta+}} (DIANA and SAPHIR respectively).<ref name=PDG2006/> The 2006 ''Review of Particle Physics'' concluded:<ref name=PDG2006/>

<blockquote>[T]here has not been a high-statistics confirmation of any of the original experiments that claimed to see the {{SubatomicParticle|Theta+}}; there have been two high-statistics repeats from [[Organization:Thomas Jefferson National Accelerator Facility|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 {{SubatomicParticle|Theta+}}; and all attempts to confirm the two other claimed pentaquark states have led to negative results. The conclusion that pentaquarks in general, and the {{SubatomicParticle|Theta+}}, in particular, do not exist, appears compelling.</blockquote>

The 2008 ''Review of Particle Physics'' went even further:<ref name=PDG2008/>

<blockquote>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.</blockquote>

Despite these [[Null result|null result]]s, LEPS results continued to show the existence of a narrow state  with a mass of {{val|1524|4|ul=MeV/c2}}, with a [[Statistical significance|statistical significance]] of 5.1&nbsp;σ.<ref>
{{cite journal
 |first1=T.  |last1=Nakano
 |display-authors=0
 |collaboration=LEPS Collaboration
 |year=2009
 |title=Evidence of the Θ<sup>+</sup> in the γd→K<sup>+</sup>K<sup>−</sup>pn reaction
 |journal=[[Physics:Physical Review C|Physical Review C]]
 |volume=79 |issue=2 |pages=025210
 |arxiv=0812.1035 |bibcode=2009PhRvC..79b5210N
 |doi=10.1103/PhysRevC.79.025210


|s2cid=118475729
 }}</ref>

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===
[[File:Pentaquark-Feynman.svg|thumb|right|[[Physics:Feynman diagram|Feynman diagram]] representing the decay of a lambda baryon {{nowrap|Λ{{su|p=0|b=b}}}} into a kaon {{nowrap|K{{su|p=−}}}} and a pentaquark {{nowrap|P{{su|p=+|b=c}}}}.]]
In July 2015, the LHCb collaboration at [[Organization:CERN|CERN]] identified pentaquarks in the {{nowrap|Λ{{su|p=0|b=b}}→J/ψK{{su|p=−}}p}} channel, which represents the decay of the bottom lambda baryon {{nowrap|(Λ{{su|p=0|b=b}})}} into a J/ψ meson {{nowrap|(J/ψ)}}, a [[Physics:Kaon|kaon]] {{nowrap|(K{{su|p=−}})}} and a [[Physics:Proton|proton]] (p). The results showed that sometimes, instead of decaying via intermediate [[Physics:Lambda baryon|lambda]] states, the {{nowrap|Λ{{su|p=0|b=b}}}} decayed via intermediate pentaquark states. The two states, named {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, had individual [[Statistical significance|statistical significance]]s of 9&nbsp;σ and 12&nbsp;σ, respectively, and a combined significance of 15&nbsp;σ – enough to claim a formal discovery. The analysis ruled out the possibility that the effect was caused by conventional particles.<ref name="LHCb-public"/> The two pentaquark states were both observed decaying strongly to {{nowrap|J/ψp}}, hence must have a valence quark content of two [[Physics:Up quark|up quark]]s, a [[Physics:Down quark|down quark]], a [[Physics:Charm quark|charm quark]], and an anti-charm quark ({{Subatomic particle|up quark}}{{Subatomic particle|up quark}}{{Subatomic particle|down quark}}{{Subatomic particle|charm quark}}{{Subatomic particle|charm antiquark}}), making them charmonium-pentaquarks.<ref name="LHCb2015"/><ref name="Guardian"/><ref>
{{Cite news
 |first=P. |last=Rincon
 |date=14 July 2015
 |title=Large Hadron Collider discovers new pentaquark particle
 |website=BBC News
 |publisher=British Broadcasting Corporation
 |url=https://www.bbc.com/news/science-environment-33517492
 |access-date=2015-07-14
}}
</ref>

The search for pentaquarks was not an objective of the LHCb experiment (which is primarily designed to investigate [[Astronomy:Baryon asymmetry|matter-antimatter asymmetry]])<ref>
{{cite press release
 |title=Where has all the antimatter gone?
 |year=2008
 |publisher=[[Organization:CERN|CERN]]
 |department=LHCb
 |url=http://lhcb-public.web.cern.ch/lhcb-public/en/Physics/WhereIsAntimatter-en.html
 |access-date=2015-07-15
}}
</ref> and the apparent discovery of pentaquarks was described as an "accident" and "something we've stumbled across" by the Physics Coordinator for the experiment.<ref name="NewScientist2015"/>

===Studies of pentaquarks in other experiments===
[[File:J-psi p pentaquark mass spectrum.svg|thumb|right|A fit to the {{nowrap|J/ψp}} invariant mass spectrum for the {{nowrap|Λ{{su|p=0|b=b}}→J/ψK{{su|p=−}}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 {{nowrap|Λ{{su|p=0|b=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&nbsp;B E12-12-001A<ref>
{{cite web |title=Near threshold J/psi photoproduction and study of LHCb pentaquarks with CLAS12 |url=https://misportal.jlab.org/mis/physics/experiments/viewProposal.cfm?paperId=931 |website=[[Organization:Thomas Jefferson National Accelerator Facility|Jefferson National Laboratory]] |series=Physics experiment proposals |id=E12-12-001A}}
</ref> and Hall&nbsp;C E2-16-007<ref>
{{cite web |title=A search for the LHCb charmed "pentaquark" using photoproduction of J/Psi at threshold in hall&nbsp;C at Jefferson Lab |url=https://misportal.jlab.org/mis/physics/experiments/viewProposal.cfm?paperId=885 |website=[[Organization:Thomas Jefferson National Accelerator Facility|Jefferson National Laboratory]] |series=Physics experiment proposals |id=E2-16-007}}
</ref> experiments at [[Organization:Thomas Jefferson National Accelerator Facility|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).<ref>
{{cite journal
 |last1=Schmidt |first1=Iván
 |last2=Siddikov |first2=Marat
 |date=3 May 2016
 |title=Production of pentaquarks in pA-collisions
 |journal=Physical Review&nbsp;D
 |volume=93 |issue=9 |page=094005
 |doi=10.1103/PhysRevD.93.094005 |arxiv=1601.05621
 |bibcode=2016PhRvD..93i4005S |s2cid=119296044
}}
</ref> 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<ref>{{cite web |title=After@LHC |url=http://after.in2p3.fr/after/index.php/Main_Page}}</ref> and NICA.<ref>{{cite web |title=NICA |url=http://nica.jinr.ru/}}</ref>

===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.<ref name="LHCb-2019"/>

Designated P<sub>c</sub>(4312)<sup>+</sup> (P<sub>c</sub><sup>+</sup> identifies a charmonium-pentaquark while the number between parenthesis indicates a mass of about 4312&nbsp;MeV), the pentaquark decays to a proton and a J/ψ meson. The analyses revealed additionally that the earlier reported observations of the P<sub>c</sub>(4450)<sup>+</sup> pentaquark actually were the average of two different resonances, designated P<sub>c</sub>(4440)<sup>+</sup> and P<sub>c</sub>(4457)<sup>+</sup>. Understanding this will require further study.

===2022 LHCb results===
On 5 July 2022, the LHCb collaboration announced the discovery of a further new pentaquark,<ref>{{cite web|url=https://www.manchester.ac.uk/discover/news/large-hadron-collider-project-discovers-three-new-exotic-particles/|title=Large Hadron Collider project discovers three new exotic particles|publisher=University of Manchester|date=5 July 2022|access-date=1 September 2023}}</ref> with a significance of 15-sigma. Designated P<sub>ψs</sub><sup>Λ</sup>(4338)<sup>0</sup>, its composition is described as udsc{{overline|c}}, representing the first confirmed pentaquark containing a strange quark.<ref>{{cite journal|title = Observation of a ''J/ψ''Λ Resonance Consistent with a Strange Pentaquark Candidate in ''B<sup>‒</sup>'' → ''J/ψ''Λ{{overline|''p''}} Decays|author = Aaij, R. and 1072 others|journal = Physical Review Letters|volume = 131|issue = 3|pages = 031901–1‒031901–11|date=17 July 2023|doi = 10.1103/PhysRevLett.131.031901| pmid=37540878 | s2cid=252992880 |doi-access = free|arxiv = 2210.10346}}</ref>

==Applications==
[[File:PQ EB ape hyp geom5 B 3D.jpg|thumb|right|upright|Colour flux tubes produced by five static quark and antiquark charges, computed in [[lattice QCD]].<ref>
{{cite journal
 |author1=N. Cardoso |author2=M. Cardoso |author3=P. Bicudo  |name-list-style=amp |year=2013
 |title=Color fields of the static pentaquark system computed in SU(3) lattice QCD
 |journal=[[Physics:Physical Review D|Physical Review D]]
 |volume=87 |issue=3 |page=034504
 |arxiv=1209.1532
 |bibcode= 2013PhRvD..87c4504C
 |doi=10.1103/PhysRevD.87.034504
|s2cid=119268740 }}</ref> Confinement in quantum chromodynamics leads to the production of [[Astronomy:Flux tube|flux tube]]s connecting colour charges. The flux tubes act as attractive QCD string-like potentials.]]

The discovery of pentaquarks will allow physicists to study the [[Physics:Strong force|strong force]] in greater detail and aid understanding of [[Physics:Quantum chromodynamics|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 [[Astronomy:Neutron star|neutron star]]s.<ref name="Guardian"/>

==See also==
* [[Physics:Baryon|Baryon]] (triquark)
* [[Physics:Exotic matter|Exotic matter]]
* [[Physics:Heptaquark|Heptaquark]]
* [[Physics:Hexaquark|Hexaquark]]
* [[Physics:List of particles|List of particles]]
* [[Physics:Quark model|Quark model]]
* [[Physics:Tetraquark|Tetraquark]]
{{clear}}

== Explanatory footnotes ==
{{notelist}}

== References ==
{{reflist|25em}}

==Further reading==
*{{cite web
 |first=David  |last=Whitehouse
 |date=1 July 2003
 |title=Behold the pentaquark 
 |website=BBC News
 |publisher=British Broadcasting Corporation
 |url=http://news.bbc.co.uk/1/low/sci/tech/3034754.stm
 |access-date=2010-01-08
}}
*{{cite journal
 |first1=Thomas E. |last1=Browder
 |first2=Igor R.   |last2=Klebanov
 |first3=Daniel R. |last3=Marlow
 |year=2004
 |title=Prospects for Pentaquark Production at Meson Factories
 |journal=[[Physics:Physics Letters B|Physics Letters B]]
 |volume=587 |issue=1–2 |pages=62
 |doi=10.1016/j.physletb.2004.03.003
 |arxiv=hep-ph/0401115 |bibcode=2004PhLB..587...62B |s2cid=15859315
}}
*{{cite arXiv
 |first=Akio |last=Sugamoto
 |year=2004
 |title=An attempt to study pentaquark baryons in string theory
 |eprint=hep-ph/0404019
}}
*{{cite journal
 |first=Kenneth |last=Hicks
 |year=2005
 |title=An experimental review of the {{SubatomicParticle|Theta+}} pentaquark
 |journal=[[Physics:Journal of Physics: Conference Series|Journal of Physics]]
 |volume=9 |issue=1 |pages=183–191
 |doi=10.1088/1742-6596/9/1/035 |arxiv=hep-ex/0412048
 |bibcode = 2005JPhCS...9..183H |s2cid=117703727
}}
* {{cite news
 |first=Mark |last=Peplow
 |date=18 April 2005
 |title=Doubt is cast on pentaquarks
 |journal=Nature
 |doi=10.1038/news050418-1
}}
* {{cite magazine
 |first=Maggie |last=McKie
 |date=20 April 2005
 |title=Pentaquark hunt draws blanks
 |magazine=New Scientist
 |url=https://www.newscientist.com/article.ns?id=dn7287
 |access-date=2010-01-08
}}
* {{cite web
 |publisher=[[Organization:Thomas Jefferson National Accelerator Facility|Thomas Jefferson National Accelerator Facility]]
 |date=21 April 2005
 |title=Is it or isn't it? Pentaquark debate heats up
 |website=SpaceDaily.Com
 |url=http://www.spacedaily.com/news/physics-05t.html
 |access-date=2010-01-08
 }}
*{{cite journal
 |first=Dmitri  |last=Diakonov
 |year=2005
 |title=Relativistic Mean Field Approximation to Baryons
 |journal=[[Physics:European Physical Journal A|European Physical Journal A]]
 |volume=24 |issue=1 |pages=3–8
 |bibcode=2005EPJAS..24a...3D |s2cid=120054432
 |doi=10.1140/epjad/s2005-05-001-3
}}
*{{cite journal
 |last=Schumacher  |first=R.A.
 |year=2006
 |title=The rise and fall of pentaquarks in experiments
 |journal=AIP Conference Proceedings
 |volume=842  |page=409
 |doi=10.1063/1.2220285  |arxiv=nucl-ex/0512042  |s2cid=16956276
 |citeseerx=10.1.1.285.7719  |bibcode=2006AIPC..842..409S
}}
* {{cite magazine
 |first=Kandice  |last=Carter
 |year=2006
 |title=The rise and fall of the pentaquark
 |magazine=Symmetry Magazine
 |volume=3 |issue=7 |pages=16
 |url=http://www.symmetrymagazine.org/article/september-2006/the-rise-and-fall-of-the-pentaquark
}}

==External links==
*{{cite web
 |title=Pentaquark
 |website=xstructure.inr.ac.ru
 |url=http://xstructure.inr.ac.ru/x-bin/theme3.py?level=2&index1=213533
}}

{{Particles}}

[[Category:Baryons]]
[[Category:Hadrons]]

{{Sourceattribution|Pentaquark}}