Hadron

Hadrons (from the ancient Greek ἁδρός hadrós 'thick', 'strong') are subatomic particles held together by the strong interaction. The best known hadrons are the nucleons (neutrons and protons), which are part of the atomic nuclei.

The term hadrons was introduced in 1962 by Lew Okun in response to the discovery of more and more new particles that were subject to the strong interaction. Two years later, Murray Gell-Mann postulated the existence of quarks, of which the hadrons are composed. This led to the hadrons no longer being considered elementary particles.

Depending on their spin, 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 of three antiquarks. Examples of baryons are the proton and the neutron.

Hadrons are often assumed to be spherical (spherical) in a simplified way and have a radius of approximately 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 thus also allows transitions between different hadrons. Since it consists in the exchange of heavy W bosons, these decays are relatively slow. Neutrons, for example, decay into protons by giving up an electron and antineutrinos (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 Coulomb repulsion.

The strong interaction is described on the "fundamental level" by quantum chromodynamics as an exchange of gluons, or - as is mostly the case in nuclear physics - on the "phenomenological level" by the exchange of mesons, especially the light pions. Quark flavours are not changed by the strong interaction, but quarks can be exchanged between baryons via mesons, for example.

In high-energy physics, not only quarks but also gluons are observed in scattering experiments. Therefore, one imagines the structure of a hadron in such a way that apart from 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 created from the vacuum and immediately annihilated again. In general, for hadrons of light quarks (up, down), the mass does not derive for the most part from the masses of the valence quarks. Rather, this mass is dynamically generated by the strong interaction.

Many hadrons are extremely short-lived excited states, the resonances observed in inelastic scattering experiments. Theoretically, there can be hadrons of arbitrarily high mass (leaving aside the mass range where gravity becomes important). The heavier a hadron is, the shorter-lived it is in general.

The existence of exotic hadrons like tetraquarks, consisting of two quarks and two antiquarks, and pentaquarks, consisting of four quarks and one antiquark, is also discussed. Other exotic hadrons would be so-called hybrids (states containing gluonic excitations in addition to quarks) or glueballs consisting purely of gluons.

In addition to hadrons, there may be new states of matter such as the quark-gluon plasma. This is supported by evidence from collision experiments with heavy ions.