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Neutral current
Weak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the Z boson. The discovery of weak neutral currents was a significant step toward the unification of electromagnetism and the weak force into the electroweak force, and led to the discovery of the W and Z bosons.
Definition
The neutral current that gives the interaction its name is that of the interacting particles. For example, the neutral-current contribution to the ν
ee− → ν
ee− elastic scattering amplitude
- <math>\mathfrak{M}^{\mathrm{NC}} \propto J_{\mu}^{\mathrm{(NC)}}(\nu_{\mathrm{e}}) \; J^{\mathrm{(NC)}\mu}(\mathrm{e^{-}})</math>
where the neutral currents describing the flow of the neutrino and of the electron are given by
- <math>J^{\mathrm{(NC)}\mu}(f) = \bar{u}_{f}\gamma^{\mu}\frac{1}{2}\left(g^{f}_{V}-g^{f}_{A}\gamma^{5}\right)u_{f},</math>
and <math>g^{f}_{V}</math> and <math>g^{f}_{A}</math> are the vector and axial vector couplings for fermion <math>f</math>.
The Z boson can couple to any Standard Model particle, except gluons and photons. However, any interaction between two charged particles that can occur via the exchange of a virtual Z boson can also occur via the exchange of a virtual photon. Unless the interacting particles have energies on the order of the Z boson mass (91 GeV) or higher, the virtual Z boson exchange has an effect of a tiny correction ( <math>~(E/M_Z)^2</math> ) to the amplitude of the electromagnetic process. Particle accelerators with energies necessary to observe neutral current interactions and to measure the mass of Z boson weren't available until 1983.
On the other hand, Z boson interactions involving neutrinos have distinctive signatures: They provide the only known mechanism for elastic scattering of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via Z boson exchange) as inelastically (via W boson exchange). Weak neutral currents were predicted in 1973 by Abdus Salam, Sheldon Glashow and Steven Weinberg,^{[1]} and confirmed shortly thereafter in 1973, in a neutrino experiment in the Gargamelle bubble chamber at CERN.
See also
References
- ^ "The Nobel Prize in Physics 1979". Nobel Foundation. Retrieved 2008-09-10.
External links
- THE DISCOVERY OF WEAK NEUTRAL CURRENTS, CERN Courier
- public web CERN
- britannica
- R Nave
- http://www.actaphys.uj.edu.pl/vol37/pdf/v37p2295.pdf
- http://www.symmetrymagazine.org/breaking/2009/07/07/gargamelle/
- cerncourier
- Padilla, Antonio (Tony). "Gargamelle and Neutral Currents". Sixty Symbols. Brady Haran for the University of Nottingham.
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