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In particle physics, a hadron is a composite subatomic particle made of two or more bound state by the strong nuclear force. Pronounced , the name is derived . They are analogous to , which are held together by the electromagnetism. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.
Hadrons are categorized into two broad families: , made of an odd number of (usually three), and , made of an even number of quarks (usually two: one quark and one antiparticle). Protons and neutrons (which make the majority of the mass of an atom) are examples of baryons; are an example of a meson. A tetraquark state (an exotic meson), named the Z(4430), was discovered in 2007 by the Belle experiment and confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states (), named and , were discovered in 2015 by the LHCb collaboration. There are several other Exotic hadrons candidates and other colour-singlet quark combinations that may also exist.
Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an atomic nucleus) are believed to be particle decay and eventually decay into other particles. The only known possible exception is free protons, which Proton decay, or at least, take immense amounts of time to decay (order of 1034+ years). By way of comparison, free neutrons are the longest-lived unstable particle, and decay with a half-life of about 611 seconds, and have a mean lifetime of 879 seconds (see Free neutron decay).
Hadron physics is studied by colliding hadrons, e.g. protons, with each other or the nuclei of dense, heavy elements, such as lead (Pb) or gold (Au), and detecting the debris in the produced . 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 of with rarefied gas particles in the outer atmosphere.
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 heavy charm quark and ; the top quark vanishes before it has time to bind into a hadron). The strength of the strong-force which bind the quarks together has sufficient energy () to have resonances composed of massive () quarks (). One outcome is that short-lived pairs of virtual particle 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 all subatomic particles, hadrons are assigned corresponding to the representations of the Poincaré group: (), where is the spin quantum number, the intrinsic parity (or P-parity), the charge conjugation (or C-parity), and is the particle's mass. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction. Hadrons may also carry flavor quantum numbers such as isospin (G-parity), and strangeness. All quarks carry an additive, conserved quantum number called a baryon number (), which is e for quarks and e for antiquarks. This means that baryons (composite particles made of three, five or a larger odd number of quarks) have = 1 whereas mesons have = 0.
Hadrons have known as resonances. Each ground state hadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about ) via the strong nuclear force.
In other phases of matter the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics (QCD) predicts that quarks and will no longer be confined within hadrons, "because the strength of the strong interaction diminishes with energy". This property, which is known as asymptotic freedom, has been experimentally confirmed in the energy range between 1 GeV (gigaelectronvolt) and 1 TeV (teraelectronvolt). All free particle hadrons proton decay are unstable.
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, and , both discovered in 2015 by the LHCb collaboration.
Because mesons have an even number of quarks, they are also all , with integer spin, i.e., 0 ħ, +1 ħ, or −1 ħ. They have baryon number Examples of mesons commonly produced in particle physics experiments include and . Pions also play a role in holding atomic nuclei together via the residual strong force.
Terminology and etymology
The term "hadron" is a new Greek word introduced by Lev Okun in a plenary talk at the 1962 International Conference on High Energy Physics at CERN. He opened his talk with the definition of a new category term:
Properties
According to the quark model, the properties of hadrons are primarily determined by their so-called . For example, a proton is composed of two (each with electric charge e, for a total of + e together) and one down quark (with electric charge e). Adding these together yields the proton charge of +1 e. Although quarks also carry color charge, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". The simplest ways for this to occur are with a quark of one color and an antiparticle of the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type of meson, and those with the second arrangement are a type of baryon.
Baryons
are hadrons containing an odd number of valence quarks (at least 3). Most well-known baryons such as the proton and neutron have three valence quarks, but 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 all , i.e., they have half-integer spin. As quarks possess baryon number B = , baryons have baryon number B = 1. Pentaquarks also have B = 1, since the extra quark's and antiquark's baryon numbers cancel.
Mesons
are hadrons containing an even number of valence quarks (at least two). Most well known mesons are composed of a quark–antiquark pair, but possible (four quarks) and (six quarks, comprising either a dibaryon or three quark–antiquark pairs) may have been discovered and are being investigated to confirm their nature. Several other hypothetical types of exotic meson may exist which do not fall within the quark model of classification. These include and (mesons bound by excited ).
See also
Footnotes
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