W boson
|
W boson |
|
|---|---|
| classification | |
|
Elementary particle boson gauge boson |
|
| properties | |
| electric charge | ± 1 e (± 1.602 10 −19 C ) |
| Dimensions | (1.433 · 10 −25 ) kg |
| Resting energy | (80 379 ± 12) MeV |
| Spin | 1 |
| average lifespan | 3 · 10 −25 s |
| Decay width | (2 085 ± 42) MeV |
| Interactions |
weak electromagnetic gravitation |
The W boson is a gauge boson and therefore an elementary particle . Like its related Z boson, it mediates the weak interaction , one of the fundamental forces in physics . While the Z boson is electrically neutral, the W boson carries an electrical charge ; one differentiates and , they are mutually antiparticles . The W boson is responsible for the charged currents of the weak interaction .
W boson is W = weak (Engl.) = Weak and Boson for a particle with integer spin. The W bosons were and are also called intermediate vector bosons.
properties
Since the W boson is quite heavy with its 80 times the proton mass and very short-lived with a lifetime of 3 · 10 −25 s, its range and thus also the range of the weak interaction is very small (approx. 10 −18 m, about a thousandth of a proton diameter ). It only contributes to many processes as a virtual particle . As a result, it can also occur in processes that do not have the necessary energy to generate a real W boson.
As a real particle, the W boson can be generated at particle accelerators , e.g. B. via the reaction e + e - → W + W - . The W bosons generated in this way are extremely short-lived particles ( decay width 2.085 ± 0.042 G eV , this corresponds to a lifetime of 3 · 10 −25 s), about 32% of which decay into leptons and 68% into hadrons and are detected via these decay products become. This was first achieved experimentally in January 1983 through UA1 and UA2 at CERN .
Mediation of interaction
W bosons can mediate the weak interaction both between leptons and between quarks . The type of interacting particles is changed (their electrical charge and weak isospin ).
For example, the electron (a negatively charged lepton) can be converted into the associated, electrically neutral electron neutrino by emitting a W - boson .
In quarks, the W bosons mediate the conversion of different flavors into one another. Such a process takes place, for example, in radioactive beta decay , in which a down quark (charge ) is converted into an up quark (charge ) in a neutron in the atomic nucleus . This turns the neutron into a proton and the atomic number increases by one. The W boson emitted during this process is - in accordance with the charge retention - simply negatively charged ( ), i.e. a W - boson.
Since the W boson is a virtual particle in this case, it cannot be observed itself, only its decay products, an electron and an electron antineutrino. While only a small proportion of neutrinos can be detected and only with great effort, electrons make themselves felt as ionizing beta radiation.
The weak interaction is also mediated by the Z boson, which, however, is not electrically charged. Since flavor changing neutral currents (FCNC for short) do not exist as an elementary interaction in the standard model of particle physics , the Z boson could not contribute to the conversion of quarks even if it were not associated with a change in charge.
literature
- Bogdan Povh , K. Rith , C. Scholz, F. Zetsche: Particles and Cores: An Introduction to Physical Concepts . 6th edition. Springer-Verlag, 2004. ISBN 3-540-21065-2 .
Web links
- What are W&Z bosons? from the alpha-Centauri television series(approx. 15 minutes). First broadcast on Aug 2, 2006.
- Particle Data Group 2010, Review 2011
- Particle Data Group - Particle Listings - W-Boson (PDF file; 184 kB)
Individual evidence
- ↑ M. Tanabashi et al .: 2018 Review of Particle Physics. In: Phys. Rev. D 98, 030001 (2018). Particle Data Group, accessed June 9, 2018 .
- ^ A b Chris Quigg , Elementary particles and forces , Scientific American, April 1985, p. 91