Inparticle physics, ahadron is acomposite subatomic particle made of two or morequarksheld together by thestrong interaction. Pronounced/ˈhædrɒn/^ⓘ, the name is derived fromAncient Greekἁδρός (hadrós) 'stout, thick'. They are analogous tomolecules, which are held together by theelectric force. Most of themass of ordinarymatter comes from two hadrons: theproton and theneutron, while most of the mass of the protons and neutrons is in turn due to thebinding energy of their constituent quarks, due to the strong force.

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;pions are an example of a meson. Atetraquark state (anexotic meson), named theZ(4430)⁻, was discovered in 2007 by theBelle Collaboration^[2] and confirmed as a resonance in 2014 by theLHCb collaboration.^[3] Twopentaquark states (exotic baryons), namedP⁺
_c(4380) andP⁺
_c(4450), were discovered in 2015 by theLHCb collaboration.^[4] There are several other"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 beunstable and 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-life of 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 rays with rarefied gas particles in the outer atmosphere.^[6]

The term "hadron" is anew Greek word introduced byL. B. Okun in aplenary talk at the 1962International Conference on High Energy Physics atCERN.^[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 Greekἁδρός signifies "large", "massive", in contrast toλεπτός which means "small", "light"). I hope that this terminology will prove to beconvenient. —L. B. Okun (1962)^[7]

According to thequark model,^[8] the properties of hadrons are primarily determined by their so-calledvalence quarks. For example, aproton is composed of twoup quarks (each withelectric charge++2⁄3, for a total of +4⁄3 together) 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 anantiquark of 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 heavycharm andbottom quarks; thetop quark vanishes before it has time to bind into a hadron). The strength of thestrong-forcegluons which 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 ofvirtual quarks 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 numbers corresponding to therepresentations of thePoincaré group:J^PC(m), whereJ is thespin quantum number,P the intrinsic parity (orP-parity),C the charge conjugation (orC-parity), andm is 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 numbers such asisospin (G-parity), andstrangeness. All quarks carry an additive, conserved quantum number called abaryon number (B), which is++1⁄3 for quarks and−+1⁄3 for 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 states known asresonances. Eachground state hadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about 10⁻²⁴ seconds) via the strong nuclear force.

In otherphases ofmatter the 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 andgluons will no longer be confined within hadrons, "because thestrength of the strong interactiondiminishes with energy". This property, which is known asasymptotic freedom, has been experimentally confirmed in the energy range between 1 GeV (gigaelectronvolt) and 1 TeV (teraelectronvolt).^[9] Allfree hadronsexcept (possibly) the proton and antiproton areunstable.

Baryons are hadrons containing an odd number of valence quarks (at least 3).^[1] Most well-known baryons such as theproton andneutron have three valence quarks, butpentaquarks with 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. Pentaquarksalso haveB = 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 theLHCb collaboration.^[4]

Mesons are 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 meson may exist which do not fall within the quark model of classification. These includeglueballs andhybrid 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 includepions andkaons. Pions also play a role in holdingatomic nuclei together via theresidual strong force.

•   The dictionary definition ofhadron at Wiktionary