Electroweak interaction

The electroweak interactions (abbr. ESW ) forms the basis of a unified theory of quantum electrodynamics and weak interactions. With this theory of the phenomenology forth quite different phenomena can be described uniformly. For example: radio waves (electromagnetic wave, abbreviated EM wave ), light ( EM wave ), electric motor ( EM field ), atomic spectrum ( quantum physics), Beta radiation ( weak interaction ), the production of particle-antiparticle pairs light ( quantum electrodynamics), or the formation of neutron stars ( weak interaction under gravity ). In addition to these phenomena, which are already produced by the individual theories, the ESW also describes phenomena such as increasing the reaction probability in the collision of electron and positron with finite energy ( when these particles carry just enough energy to produce a Z boson ). The reaction product sometimes behaves like heavy light (Weinberg angle). Furthermore, all phenomena of neutrino physics ( ghostly ), so -called weak interactions, by the ESW treated as other, more well-known electromagnetic phenomena in nature. The theory of electroweak interactions is in addition to the quantum chromodynamics a pillar of the Standard Model of elementary particle physics.

Nobel Prizes

The quantization of the electromagnetic radiation is ultimately to the explanation of blackbody radiation by Max Planck in 1900 back ( cal planck radiation law ). For the interpretation of the photoelectric effect in terms of the light -quantum hypothesis in 1905, Albert Einstein was awarded the Nobel Prize in Physics in 1921. These light quanta were found later than photons in quantum physics again. The photon is the most famous exchange boson of the electroweak interaction.

1957 succeeded Chien- Shiung Wu in Wu- experiment named after her ( carried out at the National Bureau of Standards ) proof of parity violation in weak interactions and thus the empirical evidence for the hypothesis of Tsung- Dao Lee and Chen Ning Yang. These were published in 1956, the theory that a permutation of the left and right can make a difference in elementary particle physics, ie at a spatial reflection original and mirror image do not always have to be indistinguishable ( parity violation ).

When Lee and Yang were awarded the Nobel Prize in Physics that same year, many experts thought that Chien- Shiung Wu was missed out wrongly. The reason was seen in the traditional disregard of the experimental vs. theoretical physics.

The unification of the electromagnetic and weak interaction was first of Sheldon Glashow, Abdus Salam and Steven Weinberg 1967 theoretically described (GWS - theory) has been experimentally the theory in 1973 indirectly by the discovery of neutral currents ( see below) and 1983 directly through the detection the W ± and Z0 gauge bosons ( exchange bosons ) confirmed. A special feature is the violation of parity by the electroweak interaction. For their theory the above in 1979 received the Nobel Prize for Physics.

As spokesperson for the international research team at the UA1 detector and the particle accelerator SPS at CERN received Carlo Rubbia and Simon van der Meer in 1984 the Nobel Prize in Physics, "for their decisive contributions to the large project, which the discovery of the field particles W and Z, mediators of weak interactions, has led ".

For the physical description, it is necessary to summarize a doublet for the left - chiral particles and singlets for right - chiral particles, the leptons and quarks of one generation (or family). The electro- weak interaction acts on the following Teilchendubletts and singlets of fermions:

The up -type fermions are each listed above. Their electrical charge to one greater than that of the down - like particles below it, the corresponding.

The electric charge is understood in units of the elementary charge e. The bar at d, s and b should point to the CKM - mixing (see CKM matrix ).

The electroweak interaction also acts on the associated antiparticles and particles composed of these systems. In addition to the electric charge Q, the above enumerated particles carry a weak hypercharge YW. The electric charge is connected to this and the third component of the weak isospin associated, the following applies:.

Gauge bosons

As with all quantum field theoretical gauge theories, the interactions are mediated by gauge bosons in the electroweak theory. In the electroweak theory initially occur four massless gauge bosons:

• A B0- boson (weak isospin singlet with coupling strength g '),
• Three W bosons W0, W1, W2 (weak isospin triplet with coupling strength g).

After spontaneous symmetry breaking one obtains four bosons, which can be represented as a mixture of massless bosons:

• A photon is massless, not loaded
• A Z0- boson mass 91.18 ( 7) GeV, not loaded
• Two W bosons W ±, mass 80 (41 ) GeV, charge ± 1

The linear combinations of describing these bosons are:

The Z0 boson is not like the W bosons maximum parity- violating, as it contains a fraction of the B0- boson. It is said that the states of the photon and the Z0 boson are rotated about the so-called Weinberg angle.

The photon behaves as described in the framework of QED.

Z and W bosons

The uncharged gauge boson Z0 acts on all left-handed shares listed in the table above, and by the vineyard mixture to a part on the right-handed components. Since the Z boson has no electric charge, it is called in these processes also by neutral currents (English neutral currents, NC), see Figures 1 and 2 For both processes, in part, a violation of parity takes place.

Bear the W ± bosons, in contrast to the Z boson, an electric charge. The associated Teilchenprozesse referred to so as " charged currents " (english charged currents, CC), see Figures 3 and 4, since these two charged currents couple only to the left-handed doublets, there occurs a maximum violation of parity in both processes.

For quarks in the context of the two W bosons, the CKM mixing is in addition (named after Nicola Cabibbo, Makoto Kobayashi and Toshihide Maskawa ) must be observed. For example, a u-quark with a W - are transformed not only into a d-quark. It is less likely the possibility an s- quark or b- quark to obtain. The W bosons can therefore also change the flavor. This behavior is caused by the mass eigenstates not correspond to the so-called interaction eigenstates.

Interaction and mass

Fused mass gauge bosons can be described in quantum field theory only with the help of a scalar field that gives the involved gauge bosons mass. In the electroweak theory, this field is the Higgs field (named after Peter Higgs ). Here one assumes that the scalar Higgs field had only a minimum in the early universe.

Due to the continuous cooling gave rise to spontaneous symmetry breaking and the Higgs field fell to a new minimum. The gauge bosons of the electroweak interaction obtained by the coupling to the Higgs field finite masses. Direct evidence of the Higgs particle has not yet succeeded. A proof is hoped that since 2009 from the experiments at the LHC, the large particle accelerator at CERN. On 4 July 2012, the discovery of a boson with a mass of about 125 GeV / c ² was announced by CERN, in which it could be either the Higgs particle.

Extensions

It tries in turn to unite with other interactions, the electroweak interaction. The obvious is the addition of the strong interaction ( QCD ) to a GOOD.

Credentials

• Particle Physics
• Nuclear physics