Exotic hadron
{{Short description|Subatomic particles consisting of quarks and gluons}}
File:Pentaquark-generic.svg: q is a quark and {{overline|q}} an antiquark; gluons (wavy lines) mediate strong interactions between quarks; red, green, and blue color charges must each be present, while the remaining quark and antiquark must share a color and its anticolor, in this example blue and antiblue (shown as yellow).]]
Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic.{{cite journal |first= F. E.| last= Close |year=1988 |title=Gluonic Hadrons |journal=Reports on Progress in Physics |volume=51 |number=6 |pages=833–882 |doi=10.1088/0034-4885/51/6/002 |bibcode=1988RPPh...51..833C| s2cid= 250819208 }} In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.{{cite journal |author= Belz, J., et al. (BNL-E888 Collaboration) |year=1996 |title=Search for the weak decay of an H dibaryon |journal=Physical Review Letters |volume=76 |issue= 18|pages=3277–3280 |arxiv=hep-ex/9603002 |bibcode=1996PhRvL..76.3277B |doi= 10.1103/PhysRevLett.76.3277 |pmid=10060926 |s2cid=15729745 |quote= The theory of quantum chromodynamics imposes no specific limitation on the number of quarks composing hadrons other than that they form color singlet states.}}
Consistent with ordinary hadrons, exotic hadrons are classified as being either fermions, like ordinary baryons, or bosons, like ordinary mesons. According to this classification scheme, pentaquarks, containing five valence quarks, are exotic baryons, while tetraquarks (four valence quarks) and hexaquarks (six quarks, consisting of either a dibaryon or three quark-antiquark pairs) would be considered exotic mesons. Tetraquark and pentaquark particles are believed to have been observed and are being investigated; hexaquarks have not yet been confirmed as observed.
Exotic hadrons can be searched for by looking for S-matrix poles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for such exotic hadrons had been seen by 2003 at the latest,See Tetraquark{{cite journal| url= http://pdg.lbl.gov/2007/reviews/nonqqbar_mxxx050.pdf |title= Note on non-q qbar mesons| journal= Journal of Physics G |volume= 33 |year= 2006| page= 1}} but they remain a topic of controversy in particle physics.
Jaffe and Low{{cite journal| first1= R. L. |last1= Jaffe| authorlink1= Robert Jaffe (physicist)| authorlink2= Francis E. Low | first2= F. E. |last2= Low| journal= Physical Review D| title= Connection between quark-model eigenstates and low-energy scattering| volume= 19| page= 2105 |year= 1979|issue= 7| doi= 10.1103/PhysRevD.19.2105|bibcode= 1979PhRvD..19.2105J}} suggested that the exotic hadrons manifest themselves as poles of the P matrix, and not of the S matrix. Experimental P-matrix poles are determined reliably in both the meson–meson channels and nucleon–nucleon channels.
History
When the quark model was first postulated by Murray Gell-Mann and others in the 1960s, it was to organize the states known then to be in existence in a meaningful way. As quantum chromodynamics (QCD) developed over the next decade, it became apparent that there was no reason why only three-quark and quark-antiquark combinations could exist. Indeed, Gell-Mann's original 1964 paper alludes to the possibility of exotic hadrons and classifies hadrons into baryons and mesons depending upon whether they have an odd (baryon) or even (meson) number of valence quarks.{{cite journal | first= M.| last= Gell-Mann |title=A Schematic Model of Baryons and Mesons |journal=Physics Letters |volume=8 |issue=3 |pages=214–215 |year=1964 |doi= 10.1016/S0031-9163(64)92001-3 |bibcode = 1964PhL.....8..214G }} In addition, it seemed that gluons, the mediator particles of the strong interaction, could also form bound states by themselves (glueballs) and with quarks (hybrid hadrons). Several decades have passed without conclusive evidence of an exotic hadron that could be associated with the S-matrix pole.
In April 2014, the LHCb collaboration confirmed the existence of the Z(4430)−, discovered by the Belle experiment, and demonstrated that it must have a minimal quark content of c{{overline|c}}d{{overline|u}}.{{cite journal|arxiv=1404.1903|title=Observation of the resonant character of the Z(4430)− state |date=7 April 2014|author=LHCb collaboration|doi=10.1103/PhysRevLett.112.222002|pmid=24949760|volume=112| issue= 22|pages=222002|journal=Physical Review Letters |bibcode=2014PhRvL.112v2002A|s2cid=904429}}
In July 2015, LHCb announced the discovery of two particles, named {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, which must have minimal quark content c{{overline|c}}uud, making them pentaquarks.{{cite journal |last= Aaij |first= R., et al. (LHCb collaboration) |year=2015 |title= Observation of J/ψp resonances consistent with pentaquark states in Λ{{su|p=0|b=b}}→J/ψK−p decays |journal=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 }}
Candidates
See also
Notes
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