In elementary particle physics, gauge bosons are the particles that convey the basic forces . They are bosons that are sent out by one particle and received by another. That is why they are also referred to as exchange bosons, exchange particles, messenger particles, carrier particles, force particles or interaction particles .
The gauge bosons result from the requirement of local gauge invariance on a field theory if this is made into a quantum field theory by quantization . The requirement states that in this field theory the physical effect should be independent of a gauge transformation . For this purpose, an additional gauge field must generally be introduced into the Lagrangian of the theory. After the transition to a quantum field theory, the calibration field includes field quanta with an integer spin , i.e. of the boson type. These are called calibration bosons. In the standard model , each of the gauge bosons has spin 1 and is therefore a vector particle .
The photon is the best known calibration boson. It mediates the electromagnetic interaction . The other gauge bosons of the Standard Model are the eight gluons of the strong interaction and the W ± bosons and Z bosons of the weak nuclear force .
|Eichboson (s)||number||interaction||Particles of matter||Calibration group|
|Gluons||8th||Strong interaction||Quarks||SU (3)|
|W + - W - - and Z 0 boson||3||Weak interaction||Quarks , leptons||SU (2)|
|photon||1||Electromagnetic interaction||Quarks , leptons (without neutrinos)||U (1)|
In a quantized gauge theory, gauge bosons are quanta of the gauge fields. There are as many calibration bosons as there are generators in the calibration group. In quantum electrodynamics , the gauge group U (1) is one-dimensional, so there is only one gauge boson. The calibration group of quantum chromodynamics , SU (3) , has eight generators, correspondingly there are eight gluons. The unified theory of the electro-weak interaction (GSW) is the group SU (2) x U (1) based on this ultimately leads to the 4 bosons photon, W + - W - -, and Z 0 boson.
Gauge bosons are adjoint representations of the underlying symmetry group. For the SU (N) groups of the Standard Model , this representation is (N 2 −1) -dimensional. Therefore there are 8 gluons and 4 (= 3 + 1) gauge bosons in the electroweak theory.
The gauge invariance condition requires that all gauge bosons are massless, since a mass term in the Lagrangian function is not gauge invariant . The W + and Z bosons, however, have mass. This is an effect of the Higgs mechanism by which the SU (2) × U (1) symmetry of the electroweak interaction is broken spontaneously. It is not the original SU (2) × U (1) calibration bosons that are measured, but linear combinations thereof. The associated Higgs boson was the last experimentally confirmed particle of the Standard Model of elementary particle physics. It was found on the Large Hadron Collider (LHC) in 2012 . François Englert and Peter Higgs were awarded the 2013 Nobel Prize in Physics for the theoretical development of the Higgs mechanism .
Beyond the standard model
Many theories that go beyond the standard model of elementary particle physics introduce new interactions and thus new gauge bosons. However, none of these particles has yet been measured in an experiment. Strictly speaking, the graviton is also such a hypothetical particle, since no quantum gravity theory has yet been confirmed by experiments.
Great Unified Theory
In Great Unified Theories (GUTs), additional gauge bosons are predicted as X and Y. These would mediate interactions between quarks and leptons , thus violating the conservation of the baryon number and could thus cause proton decay . These bosons would be extremely massive due to symmetry breaking (even heavier than the W and Z bosons), their spins 0 or 1.
In contrast to the others, the gravitational interaction is not an object of the Standard Model , as is the hypothetical carrier particle, the graviton . This is also an exception because as a spin-2 particle it is a tensor boson , which is in accordance with the attractive effect between masses (as “gravitational charges ”).
W ′ and Z ′ bosons
By expanding the standard model by at least one further U (1) -Eichgruppe, a Z′-boson can be generated, but not a W′-boson. Another possible extension is to assume n SU (2) -Eich groups, one of which generates the usual W and Z bosons, the other n − 1 the W ′ and Z ′ bosons.
Super symmetrical partners
- Eight gluinos as super partners of the gluons.
- The electroweak Gaugino fields mix according to the minimal supersymmetric standard model (MSSM) with the Higgsino fields to form two pairs of electrically charged Charginos and four electrically neutral neutralinos as hypothetically observable particles. The Higgsinos are the super partners of the hypothetical Higgs fields, of which there are several in the MSSM.
- According to the theory of supergravity (SUGRA), a gravitino as a supersymmetrical partner of the graviton is not part of the MSSM, just as the graviton is not part of the SM.
Gauge bosons are covered in most introductory books on modern elementary particle physics . Examples are:
- David J. Griffiths: Introduction to Elementary Particles . Wiley, John & Sons, Inc, 1987, ISBN 0-471-60386-4 . (English). For physics students in the middle semesters and interested laypeople.
- Michael E. Peskin, Daniel V. Schroeder: An Introduction to Quantum Fields . Westview Press, 1995, ISBN 0-201-50397-2 . (English). For physics students with a penchant for theoretical physics (course in quantum field theory, gauge theories are only dealt with in the third part).
- Klaus Bethge , Ulrich E. Schröder : Elementary particles and their interactions - an overview. WILEY-VCH, Weinheim 2006, ISBN 3-527-40587-9 .
- Harald Fritzsch : Elementary Particles. Building blocks of matter. Beck, Munich 2004, ISBN 3-406-50846-4 .
- Henning Genz: Elementary Particles. Fischer, Frankfurt a. M. 2003, ISBN 3-596-15354-9 .
- Particle Data Group is the standard reference for current experimental findings on elementary particles.
- Michael E. Peskin, Daniel V. Schroeder: An Introduction to Quantum Fields . Westview Press, 1995, ISBN 0-201-50397-2 .
- David J. Griffiths: Introduction to Elementary Particles . Wiley, John & Sons, Inc, 1987, ISBN 0-471-60386-4 .
- CERN experiments observe particle consistent with long-sought Higgs boson . Press release from CERN. July 4, 2012. Retrieved November 28, 2015.
- Particle confirmed as a Higgs boson. April 14, 2013. Retrieved July 12, 2017 .