As hadrons (from ancient Greek ἁδρός hadrós , thick ',' strong ') refers to subatomic particles , which by the strong interaction are held together. The most famous hadrons are the nucleons ( neutrons and protons ), which are part of the atomic nucleus .
|Mesons||Hadrons with integer spin ( bosons )|
|Quarkonia||J / ψ , Υ ...||heavy quark and its antiquark|
|other q q||π , K , η , ρ , D ...||general quark and antiquark|
|exotic||Tetraquarks , Glueballs ...||partly hypothetical|
|Baryons||Half-integer spin hadrons ( fermions )|
|Nucleons||p , n , N resonances||Baryons from u and d quarks with isospin 1 ⁄ 2|
|Δ-baryons||Δ ++ (1232) ...||Baryons from u and d quarks with isospin 3 ⁄ 2|
|Hyperons||Λ , Σ , Ξ , Ω||Baryons with at least one s-quark|
|other||Λ c , Σ c , Ξ b ...||Baryons with heavier quarks|
|exotic||Pentaquarks ...||consisting of more than three quarks|
The term hadrons was introduced in 1962 by Lew Okun in response to the discovery of ever new particles that were subject to the strong interaction. Two years later, Murray Gell-Mann postulated the existence of quarks that make up hadrons. As a result, the hadrons are no longer seen as elementary particles.
Depending on the spin , the hadrons are divided into two types:
- Mesons , they have integer spin and are therefore bosons . They consist of a quark and an antiquark, the antiparticle of a quark. Examples of mesons are pi meson and K meson .
- Baryons , they have half-integer spin and are therefore fermions . They consist of three quarks; Antibaryons from three antiquarks . Examples of baryons are protons and neutrons.
Hadrons are often assumed to be spherical (spherical) and have a radius of approx. 10 −15 m.
All free hadrons are unstable, except for the proton, for which no decays have yet been detected. The decays of the hadrons can take place via the strong, the weak or the electromagnetic interaction . For example, the neutral pi meson ( pion ) decays through the electromagnetic interaction, usually into two photons.
The transitions between quarks of different flavor quantum numbers (up, down, strange, as well as the much heavier charm, bottom, top) are caused by the weak interaction , which also enables transitions between different hadrons. Since it consists in the exchange of heavy W bosons , these decays are relatively slow. Neutrons decay z. B. emitting an electron and antineutrino in protons ( beta decay ). In an atomic nucleus, however, the neutron can be stable because the conversion into a proton would reduce the binding energy due to the Coulomb repulsion.
The strong interaction is described on the "fundamental level" by quantum chromodynamics as an exchange of gluons , or - as is mostly common in nuclear physics - on the "phenomenological level" by the exchange of mesons , especially the light pions . Quark flavors are not changed by the strong interaction, but z. B. Quarks are exchanged between baryons via mesons.
In high-energy physics , scattering experiments not only observe quarks, but also gluons . The structure of a hadron is therefore imagined in such a way that, in addition to the “basic building blocks” of a hadron, the so-called valence quarks , which determine its quantum numbers , there are also gluons and a cloud of virtual quark-antiquark pairs. Virtual means that, according to quantum field theory, such pairs of particles and antiparticles are constantly generated from the vacuum and immediately destroyed again. In general, the mass of hadrons from light quarks (up, down) does not come largely from the masses of the valence quarks. Rather, this mass is generated dynamically by the strong interaction.
Many hadrons are extremely short-lived excitation states, the resonances that are observed in inelastic scattering experiments. Theoretically there can be hadrons of arbitrarily high mass (if one leaves aside the mass range in which gravitation becomes important). In general, the heavier a hadron, the more short-lived it is.
Also discussed is the existence of exotic hadrons such as tetraquarks , which consist of two quarks and two antiquarks, and pentaquarks , which consist of four quarks and one antiquark. Other exotic hadrons would be so-called hybrids (states that contain gluonic excitations in addition to quarks) or glueballs consisting purely of gluons .
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