Baryon

Baryons ( from Ancient Greek βαρύς Barys, hard ',' overweight ', analogous to the "light" leptons and the " moderate " mesons ) are particles that consist of three quarks (or antibaryons of three antiquarks ). About this they are subject to the strong interaction, ie they are among the hadrons, together with the mesons, which are each composed of a quark and an antiquark. In addition, baryons are subject to the weak interaction of gravity and, if they are loaded, also the electromagnetic force.

Baryons are fermions, that is, they have half-integer spin and are described by the Fermi -Dirac statistics, making them obey the Pauli exclusion principle 's.

The class of baryons include the proton and the neutron (collectively: nucleons ) as well as a number of other, heavier particles which hyperons so-called.

The proton (or antiproton ) is the only baryon, which is stable as a free particle, since it is the lightest baryon with baryon number 1 (or -1), and this is an absolute conserved quantity according to the standard model of elementary particle physics. The neutron decays on the other hand, if it is not bound in the atomic nucleus with other protons and neutrons.

Baryon multiplets

General

In 1964 it succeeded Murray Gell-Mann and Yuval Ne'eman, the known baryons due to group-theoretic considerations in certain schemes ( the Eightfold path, Eng. Eightfold way ) to order. In this the x-axis is given by the third component of the isotopic spin and the y - axis by the strangeness; diagonal, one may define the axes electric charge and hypercharge. The experimentally confirmed Gell-Mann - Nishijima formula can be read at the position of the axes.

An additional, then still unknown baryon has been postulated with the quark content sss from the model. The later discovery of the omega baryon in the predicted mass and with the predicted properties was one of the early successes of the quark model.

The baryons are composed of three fundamental particles, called the Gell-Mann quarks. All quarks are fermions with spin 1/2. The original quark hypothesis came from three different quarks, the up, the down and the strange quark (u, d and s quarks ). The up and down quark are combined into a Isospindublett because of their similar masses. Of them, the strange- quark differs mainly by its greater mass and a property that strangeness is called. The charge of the u -quark is the 2/3-fache the elementary charge, the charge of d -and s- quark the (-1 / 3) times the elementary charge.

The idea now is that any way to put together three quarks, corresponding to a baryon, where the properties of quarks determine the properties of the baryon. First, let the spins of the three quarks couple to a total spin of 1/2 or 3/2. In the first case, there is limited by the Pauli principle eight possibilities ( Baryonoktett ), in the second case there are ten ( Baryondekuplett ). Since it is assumed that the quarks in the ground state have no relative total orbital angular momentum, the parity of all baryons is positive.

The baryon octet (JP = 1/ 2 )

U-and d - quarks, we can join together UUD to the combinations and udd ( the combinations are uuu and ddd by the Pauli principle, prohibited). Indeed, there are in nature two non -strange Spin-1/2-Baryonen, the proton and the neutron. UUD The combination has an overall charge of 1, so we assign them to the proton, corresponding to the neutron is udd the particles. The isotopic spin of the three quarks each couple to ± 1 /2, therefore, form a proton and neutron Isospindublett (clearly: since both combinations ud contain the isospin is inherited directly from the supernumerary third quark ).

For the baryons the strands combinations uus, uds and dds are available. The isospin of the two non - strands quarks couple this to a triplet, the sigma- baryon, and a singlet, the lambda baryon.

The baryon decuplet (JP = 3/ 2 )

Similarly, can also explain the decuplet, which also symmetric quark combinations are allowed, eg the Δ with uuu.

The Pauli principle requires this, however, the introduction of an additional degree of freedom, the so-called color. It postulates namely that the wave functions of fermions must be antisymmetric. This means in the case of the baryon, that the wave function is replaced by a minus sign, when one exchanges the quantum numbers of two of the three participating particles.

The wave function of a baryon has interests in real space, in the spin space and in isospin space:

• The spatial wave function is symmetric for the Δ , since the three up quarks are indistinguishable;
• The spins 1/2 of the three participating quarks couple to a total spin 3/2, the spin wave function is therefore also symmetric;
• This applies analogously to the isospin wave function.

The previous composite wave function of the Δ would be symmetrical.

In order to satisfy the Pauli principle, therefore a further quantum number for quarks must be postulated, the color: it can the states " red", " green" and " blue " assume. We further postulate that there is always close together the quarks in the color space to an antisymmetric wave function, ie clear that the resulting particle always "knows" is, for example, in the baryon through the merger of a "red", a "green" and one "blue" quarks.

Mass splitting

Since the various lines of the multiplets differ by the number of strange quarks ( the strangeness takes each downward toward ), the mass difference between the strange and non- strange quarks provides a measure of the mass splitting of the individual Isospinmultipletts.

Further, a basic split between the masses exists in octet and decuplet, which is due to the (color- magnetic ) spin-spin interaction. Thus, for example, the quark combination ( uus ), depending on the spin different masses ( with spin 1/2 has m = 1189.37 MeV/c2 and with spin 3/2 has m = 1382.8 MeV/c2 ); in the adjacent figure of Dekupletts this distinction is not shown.

The small mass splitting within the Isospinmultipletts (eg proton-neutron splitting about 1.3 MeV/c2 ) can be explained in part on the different charge of the quarks involved.

More baryons

It is now known that there are in addition to the previously mentioned light quarks has three other heavier quarks ( charm, bottom and top). With them other baryons can be produced. By example, the strange- quark replaced by a charm quark at the lambda particles ( uds ), we obtain that with a larger to about 1200 MeV / c ² ground.

Besides the described ground states of baryons, there are still a huge number of excited states, the so-called baryon resonances.

Nomenclature

• Baryons are dependent on the number of light quarks (d, u ) and the isospin with the letters N ( nucleon ), Δ (delta), Λ (lambda ), Σ (Sigma ), Ξ (Xi) and Ω (Omega ) denotes. a baryon made ​​of three u and / or d- quarks is called nucleon (N ) if there has isospin 1/2, and Δ, if there has isospin 3/2. For the two charge states of the nucleon in the ground state the names proton (p) and neutron apply (s).
• A baryon with two u and / or d- quarks and spin 1 /2 is a Λ ( isospin 0) or Σ ( isospin 1). When the third Quark is a C, B or T, this is indicated as an index.
• A baryon with a u or d quark is a Ξ. Quarks heavier than s are again expressed as an index. (Example: a baryon the composition usc is a Ξc, a baryon the composition ucc is a Ξcc. )
• A Baryon without u - and d-quark is a Ω. Quarks heavier than s are again expressed as an index.

Baryon

Experimentally, it is observed that the number of baryons retained minus the number of antibaryons. Therefore, it assigns the baryons, the baryon number B = 1 and the antibaryons B = -1, corresponding to the quarks B = 1 / 3 and anti-quarks B = -1 / 3 The baryon number is an additive quantum number, ie, the quantum numbers of the individual constituents add for systems of several particles on the quantum number of the total system. In contrast to other preserved quantum numbers is known for the baryon number is no associated symmetry.

In theories that go beyond the Standard Model of particle physics, the baryon number is generally not well received. Processes that violate the Baryonenzahlerhaltung, must in such theories but be extremely rare, so as not to come into conflict with experimental results, especially for the average lifetime of the proton of more than 2.1 x 1029 years.

State of research

The model outlined above for the Baryonzusammensetzung is incomplete according to the current state of research. Today it is assumed that the mass, spin and other properties of the baryons can not be read directly from the properties of quarks involved; so, for example, makes the spin of the three quarks in the proton is only about a quarter of its total spin off (" spin puzzle ", " spin crisis ").

Since the 1970s, there is the quantum chromodynamics ( QCD ) is a quantum field theory for the strong interaction, ie the interaction between the quarks. This theory is, however, difficult to handle and not perturbatively treatable especially in lower energy ranges. Instead, here meshed possible discrete lattice using ( lattice gauge theory). An example is the calculation of each other Baryonenmassen relative.

The biggest unanswered question is still, as is clear from the basics of the QCD so far only postulierbare color confinement (English: confinement) can be derived. This is the fact described above that observed in the natural particle always "knows" are what in particular has the observability of free quarks result.

For a theoretical treatment, is therefore dependent on effective theories and quark models. A commonly observed characteristic of such quark models is the prediction of far more Baryonzuständen than previously observed. The search for such missing resonances (English: missing resonances ) is one of the main fields of experimental research on baryons. In addition, research is at the electroweak properties (such as form factors ) and the decays of baryons instead.

Baryonic matter in cosmology

As baryonic matter is referred to in cosmology and astrophysics, matter made ​​up of atoms, to distinguish them from dark matter, dark energy and electromagnetic radiation. In the visible universe, there are more baryons than antibaryons, this asymmetry is called baryon asymmetry.