As hadrons ( from Ancient Greek ἁδρός hadrós, thick ',' strong ' ) refers to particles that are subject to the strong interaction. The best known hadrons are nucleons ( neutrons and protons), which are part of the atomic nuclei.
The term hadron was founded in 1962 by Lev Okun, introduced in response to the discovery of ever new particles, which were subject to the strong interaction. Two years later, Murray Gell-Mann postulated the existence of quarks, from which the hadrons are built. This meant that the hadrons are no longer assigned to the elementary particles.
Depending on the spin hadrons are divided into 2 types:
- Mesons 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 of three antiquarks ). Examples of baryons are the proton and neutron.
In addition to the above-mentioned Examples, there are numerous other hadrons.
Hadrons are often simplified as assumed spherical and have a radius of about 10-15 m.
All hadrons are unstable, except proton, which does not decay have been detected. The decays of hadrons can take place via the strong, weak or electromagnetic interactions. For example, decomposes the neutral pi meson ( pion ) via the electromagnetic interaction into two photons.
The transitions between quarks of different flavor quantum numbers (up, down, strange, and very much more serious charm, bottom, top) are caused by the weak interaction, which thus allows for transitions between different hadrons. Since it is generated by the exchange of heavy W - bosons, these decays are relatively slow. Neutron decay, for example, one electron and antineutrino into protons ( beta decay ), and their total number is in nuclei only stable since the neutrons by the binding energy of a lower energy than would an additional proton. There are also unstable nuclei, taking into account their life depends crucially on the nature of the decay from ( beta decay via weak interaction, alpha decay via the tunnel effect, etc. ).
The strong interaction is described on " fundamental level " by the quantum chromodynamics as the exchange of gluons, or - as in nuclear physics mainly customary - on the " phenomenological level " through the exchange of mesons, especially the light pions. Quark flavors are not altered by the strong interaction, but it can be exchanged, for example, via quark mesons between baryons.
In high-energy physics is observed in scattering experiments not only quarks but also gluons. Therefore One imagines the construction of a hadron as before, except that the " basic building blocks " of a hadron, the so-called valence quarks that define its quantum numbers, nor gluons and a cloud of virtual quark- antiquark pairs are present. Virtual means be that, according to quantum field theory from the vacuum constantly produces such pairs of particles and antiparticles and the same again destroyed. General stirred at hadrons of light quarks (up, down ) the mass for the most part not of the masses of the valence quarks ago. Rather, this mass is generated dynamically by the strong interaction.
Many hadrons are extremely short-lived excited states, the resonances that are observed in inelastic scattering experiments. Theoretically, it can arbitrarily high mass hadrons give ( if one leaves aside the mass range in which the gravity is important). The heavier a hadron, the short-lived, it is in general.
It will also discuss the existence of exotic hadrons such as the hitherto purely hypothetical pentaquark, which were made of four quarks and one antiquark and accordingly would be baryons, or the also hypothetical tetra- quarks which consist of two quarks and two antiquarks and accordingly would mesons. More exotic hadrons would be called hybrids (states besides quarks gluonische suggestions included) or purely consisting of gluons glueballs.
In addition to the hadrons there are new states of matter such as the quark -gluon plasma, for which there is evidence in collision experiments with heavy ions may.