View source for Exotic meson

{{short description|Meson particles which do not fit into the quark model}}
{{Refimprove|date=May 2011}}
[[Image:Exotic mesons.svg|thumb|right|300px|Identities and classification of possible [[tetraquark]] mesons, where {{mvar|I}} denotes [[isospin]].
{{legend inline|#60f7a2|{{math|1=''I'' = 0}} states}}; 
{{legend inline|#8a95ed|{{math|1=''I'' = 1/2}} states;}} 
{{legend inline|#ed8a8a|{{math|1=''I'' = 1}} states.}}
 The vertical axis is the mass.]]

In [[particle physics]], '''exotic mesons''' are [[meson]]s that have [[quantum number]]s not possible in the [[quark model]]; some proposals for non-standard quark model mesons could be:
;glueballs or gluonium: [[Glueball]]s have no valence [[quark]]s at all.
;tetraquarks: [[Tetraquark]]s have two valence quark–antiquark pairs.
;hybrid mesons: Hybrid mesons contain a valence quark–antiquark pair and one or more [[gluon]]s.

All exotic mesons are classed as mesons because they are [[hadron]]s and carry zero [[baryon number]]. Of these, glueballs must be flavor singlets – that is, must have zero [[isospin]], [[strangeness]], [[charm quark|charm]], [[bottom quark|bottomness]], and [[top quark|topness]]. Like all particle states, exotic mesons are specified by the quantum numbers which label representations of the [[Poincaré group|Poincaré symmetry]], q.e., by the [[mass]] (enclosed in parentheses), and by {{math|''J''{{sup|PC}}}}, where {{mvar|J}} is the [[angular momentum]], {{mvar|P}} is the [[intrinsic parity]], and {{mvar|C}} is the [[charge conjugation]] parity; One also often specifies the [[isospin]] {{mvar|I}} of the meson. Typically, every [[quark model]] meson comes in [[SU(3)]] [[flavor (particle physics)|flavor]] nonet: an octet and an associated flavor singlet. A glueball shows up as an extra (''supernumerary'') particle outside the nonet. 

In spite of such seemingly simple counting, the assignment of any given state as a glueball, tetraquark, or hybrid remains tentative even today, hence the preference for the more generic term ''exotic meson''. Even when there is agreement that one of several states is one of these non-quark model mesons, the degree of mixing, and the precise assignment is fraught with uncertainties. There is also the considerable experimental labor of assigning quantum numbers to each state and crosschecking them in other experiments. As a result, all assignments outside the quark model are tentative. The remainder of this article outlines the situation as it stood at the end of 2004.

==Lattice predictions==
[[Lattice QCD]] predictions for glueballs are now fairly settled, at least when [[virtual particle|virtual]] quarks are neglected. The two lowest states are
::0<sup>++</sup> with mass of {{val|1.611|0.163|ul=GeV/c2}} and
::2<sup>++</sup> with mass of {{val|2.232|0.310|u=GeV/c2}}
The 0<sup>−+</sup> and exotic glueballs such as 0<sup>−−</sup> are all expected to lie above {{val|2|u=GeV/c2}}. Glueballs are necessarily [[isoscalar]] (both for [[strong isospin]], and [[triviality (mathematics)|trivially]], [[weak isospin]]), with {{nobr|{{math|''I'' {{=}} ''T'' {{=}} 0}} .}}

The ground state ''hybrid mesons'' 0<sup>−+</sup>, 1<sup>−+</sup>, 1<sup>−−</sup>, and 2<sup>−+</sup> all lie a little below {{val|2|u=GeV/c2}}. The hybrid with exotic quantum numbers 1<sup>−+</sup> is at {{val|1.9|0.2|u=GeV/c2}}. The best lattice computations to date are made in the [[quenched approximation]], which neglects [[virtual particle|virtual]] quarks loops. As a result, these computations miss mixing with meson states.

==0<sup>++</sup> states==
The data show five isoscalar resonances: {{math|f}}{{sub|0}}(500), {{math|f}}{{sub|0}}(980), {{math|f}}{{sub|0}}(1370), {{math|f}}{{sub|0}}(1500), and {{math|f}}{{sub|0}}(1710). Of these the {{math|f}}{{sub|0}}(500) is usually identified with the {{mvar|σ}} of [[chiral model]]s. The decays and production of {{math|f}}{{sub|0}}(1710) give strong evidence that it is also a meson.

===Glueball candidate===
The {{math|f}}{{sub|0}}(1370) and {{math|f}}{{sub|0}}(1500) cannot both be a quark model meson, because one is [[wikt:supernumerary|supernumerary]]. The production of the higher mass state in two [[photon]] reactions such as {{nowrap|{{math|2γ → 2π}}}} or {{nowrap|{{math|2γ → 2K}}}} reactions is highly suppressed. The decays also give some evidence that one of these could be a glueball.

===Tetraquark candidate===
The {{math|f}}{{sub|0}}(980) has been identified by some authors as a tetraquark meson, along with the {{mvar|I}}&nbsp;=&nbsp;1 states {{math|a}}{{sub|0}}(980) and {{math|κ}}{{sub|0}}(800). Two long-lived (''narrow'' in the jargon of particle spectroscopy) states: the scalar (0{{sup|++}}) state {{PhysicsParticle|D|BR=s''J''|TR=*±}}(2317) and the vector (1{{sup|+}}) meson {{PhysicsParticle|D|BR=s''J''|TR=*±}}(2460), observed at [[CLEO (particle detector)|CLEO]] and [[BaBar experiment|BaBar]], have also been tentatively identified as tetraquark states. However, for these, other explanations are possible.

==2{{sup|++}} states==
Two isoscalar states are definitely identified: {{math|f}}{{sub|2}}(1270) and the {{math|f}}{{sub|2}}′(1525). No other states have been consistently identified by all experiments. Hence it is difficult to say more about these states.

==1{{sup|−+}} and other states==
The two [[isovector]] exotics {{math|π}}<sub>1</sub>(1400) and {{math|π}}<sub>1</sub>(1600) seem to be well established experimentally.<ref>{{Cite journal |doi=10.1103/PhysRevLett.104.241803 |title=Observation of a J{{sup|PC}}=1{{sup|−+}} exotic resonance in diffractive dissociation of 190&nbsp;GeV/''c''<sup>2</sup> π{{sup|−}} into π{{sup|−}}π{{sup|−}}π{{sup|+}} |journal=Physical Review Letters |volume=104 |issue=24 |year=2018 |arxiv=1802.05913 |last1=Alekseev |first1=M.G. |last2=Alexakhin |first2=V.Yu. |last3=Alexandrov |first3=Yu. |last4=Alexeev |first4=G.D. |last5=Amoroso |first5=A. |last6=Austregesilo |first6=A. |last7=Badełek |first7=B. |last8=Balestra |first8=F. |last9=Ball |first9=J. |last10=Barth |first10=J. |last11=Baum |first11=G. |last12=Bedfer |first12=Y. |last13=Bernhard |first13=J. |last14=Bertini |first14=R. |last15=Bettinelli |first15=M. |last16=Birsa |first16=R. |last17=Bisplinghoff |first17=J. |last18=Bordalo |first18=P. |last19=Bradamante |first19=F. |last20=Bravar |first20=A. |last21=Bressan |first21=A. |last22=Brona |first22=G. |last23=Burtin |first23=E. |last24=Bussa |first24=M.P. |last25=Chapiro |first25=A. |last26=Chiosso |first26=M. |last27=Chung |first27=S.U. |last28=Cicuttin |first28=A. |last29=Colantoni |first29=M. |last30=Crespo |first30=M.L. |page = 092003|pmid = 20867295|s2cid = 24961203|display-authors=6}}</ref><ref>{{cite journal  |doi=10.1103/PhysRevD.98.092003 |title=Light isovector resonances in π{{sup|−}}p → π{{sup|−}}π{{sup|−}}π{{sup|+}}p at 190&nbsp;GeV/''c''<sup>2</sup> |journal=Physical Review D |volume=98 |issue=9  |year=2018 |arxiv=0910.5842 |last1=Aghasyan |first1=M. |last2=Alexeev |first2=M.G. |last3=Alexeev |first3=G.D. |last4=Amoroso |first4=A. |last5=Andrieux |first5=V. |last6=Anfimov |first6=N.V. |last7=Anosov |first7=V. |last8=Antoshkin |first8=A. |last9=Augsten |first9=K. |last10=Augustyniak |first10=W. |last11=Austregesilo |first11=A. |last12=Azevedo |first12=C.D.R. |last13=Badełek |first13=B. |last14=Balestra |first14=F. |last15=Ball |first15=M. |last16=Barth |first16=J. |last17=Beck |first17=R. |last18=Bedfer |first18=Y. |last19=Bernhard |first19=J. |last20=Bicker |first20=K. |last21=Bielert |first21=E.R. |last22=Birsa |first22=R. |last23=Bodlak |first23=M. |last24=Bordalo |first24=P. |last25=Bradamante |first25=F. |last26=Bressan |first26=A. |last27=Büchele |first27=M. |last28=Burtsev |first28=V.E. |last29=Chang |first29=W.-C. |last30=Chatterjee |first30=C. |page=241803 |bibcode=2018PhRvD..98i2003A |s2cid=119247683 |display-authors=6}}</ref><ref>{{cite journal |title=Odd and even partial waves of ηπ{{sup|−}} and η′π{{sup|−}} in π{{sup|−}}p → η(′)π{{sup|−}}p at 191&nbsp;GeV/''c''<sup>2</sup> |journal=Physics Letters B |year=2015 |volume=740 |pages=303–311 |arxiv=1408.4286 |doi=10.1016/j.physletb.2014.11.058 |last1=Adolph |first1=C. |last2=Akhunzyanov |first2=R. |last3=Alexeev |first3=M.G. |last4=Alexeev |first4=G.D. |last5=Amoroso |first5=A. |last6=Andrieux |first6=V. |last7=Anosov |first7=V. |last8=Austregesilo |first8=A. |last9=Badełek |first9=B. |last10=Balestra |first10=F. |last11=Barth |first11=J. |last12=Baum |first12=G. |last13=Beck |first13=R. |last14=Bedfer |first14=Y. |last15=Berlin |first15=A. |last16=Bernhard |first16=J. |last17=Bicker |first17=K. |last18=Bielert |first18=E.R. |last19=Bieling |first19=J. |last20=Birsa |first20=R. |last21=Bisplinghoff |first21=J. |last22=Bodlak |first22=M. |last23=Boer |first23=M. |last24=Bordalo |first24=P. |last25=Bradamante |first25=F. |last26=Braun |first26=C. |last27=Bressan |first27=A. |last28=Büchele |first28=M. |last29=Burtin |first29=E. |last30=Capozza |first30=L. |display-authors=6}}</ref> A recent coupled-channel analysis has shown these states, which were initially considered separate, are consistent with a single pole. A second exotic state is disfavored.<ref>{{cite journal |title=Determination of the Pole Position of the Lightest Hybrid Meson Candidate |journal=Physical Review Letters |year=2019 |volume=122 |issue=4 |page=042002 |arxiv=1810.04171 |bibcode=2019PhRvL.122d2002R |doi=10.1103/PhysRevLett.122.042002 |last1=Rodas |first1=A. |last2=Pilloni |first2=A. |last3=Albaladejo |first3=M. |last4=Fernández-Ramírez |first4=C. |last5=Jackura |first5=A. |last6=Mathieu |first6=V. |last7=Mikhasenko |first7=M. |last8=Nys |first8=J. |last9=Pauk |first9=V. |last10=Ketzer |first10=B. |last11=Szczepaniak |first11=A.P. |pmid=30768338 |s2cid=73455324 |display-authors=6 |collaboration=Joint Physics Analysis Center}}</ref> The assignment of these states as hybrids is favored. Lattice QCD calculations show the lightest {{math|π}}{{sub|1}} with 1{{sup|−+}} quantum numbers has strong overlap with operators featuring gluonic construction.<ref>{{cite journal |title=Toward the excited isoscalar meson spectrum from lattice QCD |journal=Physical Review D |year=2013 |volume=88 |issue=9 |page=094505 |doi=10.1103/PhysRevD.88.094505 |bibcode=2013PhRvD..88i4505D |arxiv=1309.2608 |last1=Dudek |first1=Jozef J. |last2=Edwards |first2=Robert G. |last3=Guo |first3=Peng |last4=Thomas |first4=Christopher E.|s2cid=62879574 }}</ref>

The {{math|π}}(1800) 0{{sup|−+}}, {{math|ρ}}(1900) 1{{sup|−−}} and the {{math|η}}{{sup|2}}(1870) 2{{sup|−+}} are fairly well identified states, which have been tentatively identified as hybrids by some authors. If this identification is correct, then it is a remarkable agreement with lattice computations, which place several hybrids in this range of masses.

==See also==
*[[Quark model]], [[meson]]s, [[baryon]]s, [[quark]]s, and [[gluon]]s
*[[Exotic hadron]]s and [[exotic baryon]]s
*[[Quantum chromodynamics]], [[flavor (particle physics)|flavor]], and the [[QCD vacuum]]
*[[GlueX]], an experiment which will explore the spectrum of glueballs and exotic mesons

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

==Further reading==
*{{cite journal |first1=W.-M. |last1=Yao |display-authors=etal |collaboration=[[Particle Data Group]] |year=2006 |title=Review of Particle Physics: Non-q{{overline|q}} mesons |journal=[[Journal of Physics G]] |volume=33 |issue=1 |pages=1 |doi=10.1088/0954-3899/33/1/001 |arxiv=astro-ph/0601168 |bibcode=2006JPhG...33....1Y  |url=http://pdg.lbl.gov/2006/reviews/nonqqbar_mxxx050.pdf}}

{{particles}}

[[Category:Mesons]]
[[Category:Hypothetical composite particles]]