Standard Model

The Standard Model of elementary particle physics (SM ) is a physical theory which describes the known elementary particles and the interactions between them. The three described by the standard model interactions are the strong interaction, the weak interaction and the electromagnetic interaction. Only gravity is left out.

The Standard Model is a quantum field theory. Your fundamental objects are fields that can be changed only in discrete packets. In an appropriate representation, the discrete packets correspond to the observed particles. The standard model is designed so that the particles and fields described by him comply with the laws of special relativity. At the same time it contains the statements of quantum mechanics and quantum chromodynamics.

Many predictions of the standard model have been confirmed by experiments in particle physics. In particular, the existence of all the elementary particles of the model is demonstrated experimentally. At the same time far no elementary particles has been found that is not found in the standard model. Quantitative characteristics of the particles show a very good agreement between measurements and predictions of the Standard Model. A particularly clear example of this is the g factor of the electron.

Nevertheless, there are grounds for believing that the standard model is only one aspect of a more comprehensive theory. His statements lead, at high energies, as occurred during the Big Bang, to contradictions with the General Theory of Relativity. In addition to 18 parameters, whose values ​​do not emerge from the theory, are defined on the basis of experimental results. It is thus quite " flexible " so that it can adapt to a certain extent the observations actually made ​​. There are therefore numerous efforts to extend the standard model or replace.

Interactions

In the standard model the interaction of the matter fields is described by abstract (mathematical ) gauge symmetries, so that the standard model is a gauge theory. The gauge groups of the SM are, and. The respective charges of these symmetries are the (weak) hypercharge, the (weak) isospin and color charge. The three commonly enumerated as interactions of the SM interactions ( electromagnetic interaction, the weak interaction and the strong interaction ) arising from these gauge groups:

• The Higgs mechanism leads to electroweak symmetry breaking. This results in the groups and in the particle representation three effective exchange particles: the photon, the Z boson and the W boson. The massless photon is the exchange particle of the electromagnetic interaction, the Z and the W - boson is the massive exchange particles of the weak interaction.
• The local gauge group enforces the existence of the gluon fields, which mediate the color interaction between quarks and each other. The color interaction allows the exchange of bound quark- antiquark states ( pions ) between the building blocks of an atomic nucleus ( nucleons ). Depending on the nomenclature of the term is strong interaction used as follows: either it refers to the object created by pion exchange effective interaction between nucleons, or the color interaction itself is called directly as a strong interaction.

Elementary

Fermions: Matter particles

The fermions of the standard model and not elementary particles, which are composed of them are, by convention, the particles are referred to as "matter". Fermions that are subject to color change effect are called "quarks"; Fermions, which do not, are " leptons " ( light particles ). Both leptons and quarks are divided for practical purposes into three generations, each with a pair of particles. The particles of a pair differ in their behavior with respect to the gauge group, and thus in their electroweak interactions - especially noteworthy is their different electrical charge. Equivalents particles of different generations have almost identical properties, the most notable difference is the increasing with the mass generation.

Vector bosons: interaction particles

The bosonic elementary particles of the Standard Model differ in their spin, the vector bosons (photon, W, Z, gluon ) spin 1 and the Higgs boson have spin 0. The existence of the vector bosons is mathematically a necessary consequence of the gauge symmetries of the Standard Model. They mediate the interactions between particles, but can in principle also occur as separate particles (in particular, the photon, which a " quantum size " of electromagnetic waves is as elementary particles).

The gluons are gauge bosons and directly represent the degrees of freedom of the gauge group of the strong force. The W and Z bosons and the photon, however, not directly represent the degrees of freedom of the remaining gauge group, but are still occasionally referred to as gauge bosons. The vector bosons of the standard model are also called " messenger particles " or " exchange particles ".

Higgs boson

The Higgs boson is not a direct consequence of a gauge symmetry, does not therefore represent interaction in terms of the standard model and is therefore not considered as exchange particles. The Higgs boson is required, however, to break the electroweak SU (2) xU (1 ) symmetry and thus provide both the Z and the W - boson mass. On July 4, 2012, announced in a seminar at CERN, that a boson has been demonstrated by experiments at the Large Hadron Collider, which is identical for all previously studied properties of the Higgs boson.

Physics beyond the Standard Model

The Standard Model of particle physics can explain almost all observed previously in particle observations. However, it is incomplete because it does not describe the gravitational interaction. There are also some open questions that can not solve the Standard Model, such as the hierarchy problem and the unification of the three fundamental forces within the particle. Even the now confirmed nonzero rest mass of neutrinos leads beyond the theory of the standard model.

There are a variety of alternative models, based on which the established standard model is only added to other approaches in order to describe some problems better without changing its foundation as such. The most well known approaches for new models are attempts to unify the three occurring in the standard model interactions in a grand unified theory (GUT). Such models often include supersymmetry, a symmetry between bosons and fermions. These theories postulate to each particle of the standard model, with different partner particles from Originalteilchen spinning, none of which have been, however, still was able to be detected. Another approach for the extension of the standard model yields theories of quantum gravity. Such approaches include, for example, the string theories that also include GUT models, as well as the loop quantum gravity.

In summary, there remain open questions in the standard model:

• Has found the Higgs boson predicted properties and there are other Higgs bosons?
• Why do the fundamental interactions as different coupling strengths and what about the gravity?
• The CP violation alone can not explain the observed matter-antimatter asymmetry in the Universe.
• Why are there just three generations ( each with two flavors ) of fundamental fermions?
• The standard model includes at least 18 free parameters that must be previously determined by measurement. Let this be predicted from a more general theory?