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Hadron

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Inparticle physics,ahadron(/ˈhædrɒn/ⓘ;fromAncient Greekἁδρός(hadrós)'stout, thick') is acomposite subatomic particlemade of two or morequarksheld togetherby thestrong interaction.They are analogous tomolecules,which are held together by theelectric force.Most of themassof ordinarymattercomes from two hadrons: theprotonand theneutron,while most of the mass of the protons and neutrons is in turn due to thebinding energyof their constituent quarks, due to the strong force.

A hadron is acomposite subatomic particle.Every hadron must fall into one of the two fundamental classes of particle,bosonsandfermions.

Hadrons are categorized into two broad families:baryons,made of an odd number ofquarks(usually three) andmesons,made of an even number of quarks (usually two: one quark and oneantiquark).[1]Protons and neutrons (which make the majority of the mass of anatom) are examples of baryons;pionsare an example of a meson."Exotic" hadrons,containing more than three valence quarks, have been discovered in recent years. Atetraquarkstate (anexotic meson), named theZ(4430)−,was discovered in 2007 by theBelle Collaboration[2]and confirmed as a resonance in 2014 by theLHCbcollaboration.[3]Twopentaquarkstates (exotic baryons), namedP+
c(4380)andP+
c(4450),were discovered in 2015 by theLHCbcollaboration.[4]There are several more exotic hadron candidates and other colour-singlet quark combinations that may also exist.

Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within anatomic nucleus) are believed to beunstableand eventually decay into other particles. The only known possible exception is free protons, whichappear to be stable,or at least, take immense amounts of time to decay (order of 1034+years). By way of comparison, free neutrons are thelongest-lived unstable particle,and decay with ahalf-lifeof about 611 seconds, and have a mean lifetime of 879 seconds,[a][5]seefree neutron decay.

Hadron physics is studied by colliding hadrons, e.g. protons, with each other orthe nuclei of dense, heavy elements,such aslead(Pb) orgold(Au), and detecting the debris in the producedparticle showers.A similar process occurs in the natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions ofcosmic rayswith rarefied gas particles in the outer atmosphere.[6]

Terminology and etymology

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The term "hadron" is anew Greekword introduced byL. B. Okunin aplenary talkat the 1962International Conference on High Energy PhysicsatCERN.[7]He opened his talk with the definition of a new category term:

Notwithstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles"is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called" non-leptonic".This definition is not exact because" non-leptonic "may also signify photonic. In this report I shall call strongly interacting particles" hadrons ", and the corresponding decays" hadronic "(the Greekἁδρόςsignifies "large", "massive", in contrast toλεπτόςwhich means "small", "light" ). I hope that this terminology will prove to beconvenient. —L. B. Okun(1962)[7]

Properties

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All types of hadrons have zero total color charge (three examples shown).

According to thequark model,[8]the properties of hadrons are primarily determined by their so-calledvalence quarks.For example, aprotonis composed of twoup quarks(each withelectric charge++2⁄3,for a total of +4⁄3together) and onedown quark(with electric charge−+1⁄3). Adding these together yields the proton charge of +1. Although quarks also carrycolor charge,hadrons must have zero total color charge because of a phenomenon calledcolor confinement.That is, hadrons must be "colorless" or "white". The simplest ways for this to occur are with a quark of one color and anantiquarkof the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type ofmeson,and those with the second arrangement are a type ofbaryon.

Massless virtual gluons compose the overwhelming majority of particles inside hadrons, as well as the major constituents of its mass (with the exception of the heavycharmandbottom quarks;thetop quarkvanishes before it has time to bind into a hadron). The strength of thestrong-forcegluonswhich bind the quarks together has sufficient energy (E) to have resonances composed of massive (m) quarks (E≥mc2). One outcome is that short-lived pairs ofvirtualquarks and antiquarks are continually forming and vanishing again inside a hadron. Because the virtual quarks are not stable wave packets (quanta), but an irregular and transient phenomenon, it is not meaningful to ask which quark is real and which virtual; only the small excess is apparent from the outside in the form of a hadron. Therefore, when a hadron or anti-hadron is stated to consist of (typically) two or three quarks, this technically refers to the constant excess of quarks versus antiquarks.

Like allsubatomic particles,hadrons are assignedquantum numberscorresponding to therepresentationsof thePoincaré group:JPC(m), whereJis thespinquantum number,Pthe intrinsic parity (orP-parity),Cthe charge conjugation (orC-parity), andmis the particle'smass.Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due tomass–energy equivalence,most of the mass comes from the large amount of energy associated with thestrong interaction.Hadrons may also carryflavor quantum numberssuch asisospin(G-parity), andstrangeness.All quarks carry an additive, conserved quantum number called abaryon number(B), which is++1⁄3for quarks and−+1⁄3for antiquarks. This means that baryons (composite particles made of three, five or a larger odd number of quarks) haveB= 1 whereas mesons haveB= 0.

Hadrons haveexcited statesknown asresonances.Eachground statehadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about 10−24seconds) via the strong nuclear force.

In otherphasesofmatterthe hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory ofquantum chromodynamics(QCD) predicts that quarks andgluonswill no longer be confined within hadrons, "because thestrengthof the strong interactiondiminishes with energy".This property, which is known asasymptotic freedom,has been experimentally confirmed in the energy range between 1GeV(gigaelectronvolt) and 1TeV(teraelectronvolt).[9]Allfreehadronsexcept (possibly) the proton and antiprotonareunstable.

Baryons

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Main articles:BaryonandExotic baryon

Baryonsare hadrons containing an odd number of valence quarks (at least 3).[1]Most well-known baryons such as theprotonandneutronhave three valence quarks, butpentaquarkswith five quarks—three quarks of different colors, and also one extra quark-antiquark pair—have also been proven to exist. Because baryons have an odd number of quarks, they are also allfermions,i.e.,they have half-integerspin.As quarks possessbaryon numberB=1⁄3,baryons have baryon numberB= 1. PentaquarksalsohaveB= 1, since the extra quark's and antiquark's baryon numbers cancel.

Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example, just as a proton is made of two up quarks and one down quark, its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.

As of August 2015, there are two known pentaquarks,P+
c(4380)andP+
c(4450),both discovered in 2015 by theLHCbcollaboration.[4]

Mesons

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Main articles:MesonandExotic meson

Mesonsare hadrons containing an even number of valence quarks (at least two).[1]Most well known mesons are composed of a quark-antiquark pair, but possibletetraquarks(four quarks) andhexaquarks(six quarks, comprising either a dibaryon or three quark-antiquark pairs) may have been discovered and are being investigated to confirm their nature.[10]Several other hypothetical types ofexotic mesonmay exist which do not fall within the quark model of classification. These includeglueballsandhybrid mesons(mesons bound by excitedgluons).

Because mesons have an even number of quarks, they are also allbosons,with integerspin,i.e.,0, +1, or −1. They have baryon numberB=⁠1/3⁠−⁠1/3⁠= 0.Examples of mesons commonly produced in particle physics experiments includepionsandkaons.Pions also play a role in holdingatomic nucleitogether via theresidual strong force.

See also

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Footnotes

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  1. ^ The proton and neutrons' respective antiparticles are expected to follow the same pattern, but they are difficult to capture and study, because they immediately annihilate on contact with ordinary matter.

References

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  1. ^abc Gell-Mann, M. (1964). "A schematic model of baryons and mesons".Physics Letters.8(3): 214–215.Bibcode:1964PhL.....8..214G.doi:10.1016/S0031-9163(64)92001-3.
  2. ^ Choi, S.-K.; et al. (Belle Collaboration) (2008). "Observation of a resonance-like structure in the
    π±
    Ψ′ mass distribution in exclusive B→K
    π±
    Ψ′ decays ".Physical Review Letters.100(14): 142001.arXiv:0708.1790.Bibcode:2008PhRvL.100n2001C.doi:10.1103/PhysRevLett.100.142001.PMID18518023.S2CID119138620.
  3. ^Aaij, R.; et al. (LHCb collaboration) (2014). "Observation of the Resonant Character of the Z(4430)−State ".Physical Review Letters.112(22): 222002.arXiv:1404.1903.Bibcode:2014PhRvL.112v2002A.doi:10.1103/PhysRevLett.112.222002.PMID24949760.S2CID904429.
  4. ^ab Aaij, R.; et al. (LHCb collaboration) (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
    b    → J/ψK−p decays ".Physical Review Letters.115(7): 072001.arXiv:1507.03414.Bibcode:2015PhRvL.115g2001A.doi:10.1103/PhysRevLett.115.072001.PMID26317714.S2CID119204136.
  5. ^Zyla, P. A. (2020)."n MEAN LIFE".PDG Live: 2020 Review of Particle Physics.Particle Data Group.Retrieved3 February2022.
  6. ^Martin, B. R. (2017).Particle physics(Fourth ed.). Chichester, West Sussex, UK.ISBN9781118911907.{{cite book}}:CS1 maint: location missing publisher (link)
  7. ^ab Okun, L. B.(1962). "The theory of weak interaction".Proceedings of 1962 International Conference on High-Energy Physics at CERN.International Conference on High-Energy Physics (plenary talk). CERN, Geneva, CH. p. 845.Bibcode:1962hep..conf..845O.
  8. ^ Amsler, C.; et al. (Particle Data Group) (2008)."Quark Model"(PDF).Physics Letters B.Review of Particle Physics.667(1): 1–6.Bibcode:2008PhLB..667....1A.doi:10.1016/j.physletb.2008.07.018.hdl:1854/LU-685594.
  9. ^ Bethke, S. (2007). "Experimental tests of asymptotic freedom".Progress in Particle and Nuclear Physics.58(2): 351–386.arXiv:hep-ex/0606035.Bibcode:2007PrPNP..58..351B.doi:10.1016/j.ppnp.2006.06.001.S2CID14915298.
  10. ^ Mann, Adam (2013-06-17)."Mysterious subatomic particle may represent exotic new form of matter".Science.Wired.Retrieved2021-08-27.— News story aboutZ(3900) particle discovery.
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  • The dictionary definition ofhadronat Wiktionary
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