Standard model

The standard model of elementary particle physics (SM) summarizes the essential knowledge of particle physics according to the current state (beginning of the 21st century). It describes all known elementary particles and the important interactions between them: the strong interaction , the weak interaction and the electromagnetic interaction . Only the (comparatively very weak) gravity is not taken into account.

In theoretical terms, the Standard Model is a quantum field theory . Their fundamental objects are fields that can only be changed in discrete packets ; the discrete packets correspond to the observed particles in a suitable representation. The standard model is built in such a way that the particles and fields it describes fulfill the laws of special relativity . At the same time it contains the statements of quantum mechanics and quantum chromodynamics .

Many of the predictions of the Standard Model have been confirmed by experiments in particle physics. In particular, the existence of those elementary particles of the model that were first predicted by the theory has been proven. The measured quantitative properties of the particles agree very well with the predictions of the Standard Model. A particularly clear example of this is the g-factor of the electron .

There are, however, reasons to believe that the Standard Model is just one aspect of an even broader theory. Dark matter and dark energy are not described in the standard model. His statements lead to contradictions with the general theory of relativity at high energies, such as those that occurred during the Big Bang . In addition, 18 parameters , the values of which cannot be derived from theory, have to be determined on the basis of experimental results. This makes it quite “flexible” and can, within a certain framework, adapt to the observations actually made. There are also numerous efforts to expand or replace the standard model.

Interactions

In the standard model, the interaction of the matter fields is described by abstract (mathematical) gauge symmetries, whereby the standard model is also a gauge theory . The calibration groups of the SM are , and . The respective charges of these symmetries are the (weak) hypercharge , the (weak) isospin and the color charge . The three interactions usually listed as interactions of the SM (the electromagnetic interaction, the weak interaction and the strong interaction) result from these calibration groups: ${\ displaystyle U (1) _ {Y}}$ ${\ displaystyle SU (2) _ {L}}$ ${\ displaystyle SU (3) _ {c}}$

The Higgs mechanism leads to the electroweak symmetry breaking. The groups and the particle representation result in 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 are the massive exchange particle of the weak interaction. ${\ displaystyle U (1) _ {Y}}$ ${\ displaystyle SU (2) _ {L}}$

The local calibration group forces the existence of the gluon fields, which mediate the color interaction between the quarks and each other. The color interaction enables the exchange of bound quark-antiquark states ( pions ) between the building blocks of an atomic nucleus ( nucleons ). Depending on the nomenclature, the term strong interaction is used as follows: Either it is understood to mean the effective interaction between the nucleons that can be described by pion exchange, or the color interaction itself is directly referred to as a strong interaction . ${\ displaystyle SU (3) _ {c}}$

Elementary particles

Elementary particles of the Standard Model

! Quarks	! Exchange particles
! Leptons	! Higgs boson

Fermions: matter particles

The Standard Model fermions and non-elemental particles made up of them are by convention the particles called "matter". Fermions that are subject to the color interaction are called " quarks "; the other fermions are " leptons " ( light particles). For practical reasons, both leptons and quarks are divided into three “ generations ”, each with a pair of particles. The particles of a pair differ in their behavior with respect to the -Eich group and thus in their electroweak interaction - their different electrical charges are particularly noteworthy. Equivalent particles of different generations have almost identical properties, the most obvious difference being the mass that increases with the generation. ${\ displaystyle SU (2) _ {L}}$

Vector bosons: interaction particles

The bosonic elementary particles of the Standard Model differ in their spin; the vector bosons (photon, W, Z, gluon) have the spin quantum number 1, the Higgs boson the spin quantum number 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 in principle they can also appear as independent particles (especially the photon, which, as an elementary particle, represents a “quantum quantity” of electromagnetic waves).

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 photons, on the other hand, do not directly represent the degrees of freedom of the rest of the gauge group , but are nevertheless sometimes referred to as gauge bosons. The vector bosons of the Standard Model are also called “messenger particles” or “exchange particles”. ${\ displaystyle SU (3)}$ ${\ displaystyle SU (2) _ {L} \ times U (1)}$

Higgs boson

The Higgs boson is not a direct consequence of a gauge symmetry and therefore does not convey any interaction in the sense of the Standard Model and is therefore not regarded as an exchange particle. However, the Higgs boson is “needed” to break the electroweak symmetry ${\ displaystyle SU (2) \ times U (1)}$ and thus give mass to both the Z and W bosons. On July 4, 2012, it was announced in a seminar at CERN that experiments at the Large Hadron Collider revealed a boson that corresponds to the Higgs boson in all the properties investigated so far, which further measurements were able to confirm.

Physics beyond the standard model

The standard model of particle physics can explain almost all particle physics observations that have been made so far. However, it is incomplete because it does not describe the gravitational interaction at all. In addition, there are some open questions within particle physics that the Standard Model cannot solve, such as: B. the hierarchy problem and the union of the three basic forces . The now confirmed, non-zero mass of the neutrinos also goes beyond the theory of the Standard Model.

There are a number of alternative models, on the basis of which the established standard model is merely expanded to include further approaches in order to be able to describe some problems better without changing its foundation. The best-known approaches for new models are attempts to combine the three interactions occurring in the Standard Model in a large unified theory (GUT). Such models often include supersymmetry , a symmetry between bosons and fermions . These theories postulate partner particles for each particle of the Standard Model with a different spin from the original particle, none of which has yet been proven. Another approach to extending the Standard Model gives rise to theories of quantum gravity . Such approaches include, for example, the string theories , which also contain GUT models, and loop quantum gravity .

Fundamental interactions and their descriptions
	Strong interaction	Electromagnetic interaction	Weak interaction	Gravity
classic		Electrostatics & magnetostatics , electrodynamics		Newton's law of gravitation , general relativity
quantum theory	Quantum ( standard model )	Quantum electrodynamics	Fermi theory	Quantum gravity ?
		Electroweak Interaction ( Standard Model )
	Big Unified Theory ?
	World formula ("theory of everything")?
Theories at an early stage of development are grayed out.

In summary, there are still the following open questions in the standard model:

Does the Higgs boson found have the predicted properties and are there other Higgs bosons?
Why do the fundamental interactions have so different coupling strengths and what about gravity ?
The CP violation alone can antimatter-matter asymmetry not explain the observed in the universe.
Why are there just three generations (each with two flavors ) of fundamental fermions?
The standard model contains at least 18 free parameters that previously had to be determined by measurement. Can these be predicted from a more general theory?

literature

John F. Donoghue: Dynamics of the Standard Model . New edition. Cambridge University Press, 1994, ISBN 978-0-521-47652-2 .
Gernot Münster : From quantum field theory to the standard model . New edition. de Gruyter, 2019, ISBN 978-3-11-063853-0 .

Web links

arxiv.org (PDF, page 43 ff; 711 kB): A complete description of the standard model in terms of the Lagrange function .
Leonard Susskind : Particle Physics: Standard Model. Lecture series, video recordings (English), 2010.

Individual evidence

↑ Brockhaus Encyclopedia, 21st edition, 2006.
↑ CERN experiments observe particle consistent with long-sought Higgs boson. Retrieved November 12, 2016 . CERN press release of July 4th, 2012.
↑ New results indicate that particle discovered at CERN is a Higgs boson. Retrieved November 12, 2016 . CERN press release of March 14, 2013.

[1] Brockhaus Encyclopedia, 21st edition, 2006.

[CERN20120704-2] CERN experiments observe particle consistent with long-sought Higgs boson. Retrieved November 12, 2016 . CERN press release of July 4th, 2012.

[CERN20130314-3] New results indicate that particle discovered at CERN is a Higgs boson. Retrieved November 12, 2016 . CERN press release of March 14, 2013.