Neutral current

Exchange currents which carry no electric charge and mediate certain types of electroweak interactions. The discovery of the neutral-current weak interactions and the agreement of their experimentally measured properties with the theoretical predictions were of great significance in establishing the validity of the Weinberg-Salam model of the electroweak forces.

The electroweak forces come in three subclasses: the electromagnetic interactions, the charged-current weak interactions, and the neutral-current weak interactions. The electromagnetic interaction is mediated by an exchanged photon γ. Since the photon carries no electric charge, there is no change in charge between the incoming and the outgoing particles. The charged-current weak interaction is mediated by the exchange of a charged intermediate boson, the W⁺, and thus, for example, an incoming neutral lepton such as the ν_μ is changed into a charged lepton, the μ⁻. In the neutral-current weak interactions, the exchanged intermediate boson, the Z⁰, carries no electric charge (hence the name neutral-current interaction), and thus for example, an incident neutral lepton, such as the ν_μ, remains an outgoing neutral ν_μ. See also Electron; Intermediate vector boson; Lepton; Neutrino; Photon.

The neutral-current interactions were experimentally discovered in 1973, and have since been extensively studied, in neutrino scattering processes. Very important information about the properties of the neutral currents have been obtained by studying the interference effects between the electromagnetic and the neutral-current weak interactions in the scattering of polarized electrons on deuterium. Parity violating effects in atomic physics processes due to the neutral weak currents have been observed, and predicted parity-violating nuclear effects have been searched for. See also Elementary particle; Fundamental interactions; Parity (quantum mechanics); Symmetry laws (physics); Weak nuclear interactions.

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, and the interaction is called 'neutral' because the Z has no electric charge. 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.

The Z boson can couple to any Standard Model particle, except gluons. 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 ( ) 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 1974, in a neutrino experiment in the Gargamelle bubble chamber at CERN.

See also

• Charged current
• Flavor changing neutral current

References

1. ^ "The Nobel Prize in Physics 1979". Nobel Foundation. http://www.nobel.se/physics/laureates/1979. Retrieved on 2008-09-10. 

• The discovery of the weak neutral currents, CERN Courier

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