# 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
e
e− → ν
e
e− elastic scattering amplitude

$\mathfrak{M}^{\mathrm{NC}} \propto J_{\mu}^{\mathrm{(NC)}}(\nu_{\mathrm{e}}) \; J^{\mathrm{(NC)}\mu}(\mathrm{e^{-}})$

where the neutral currents describing the flow of the neutrino and of the electron are given by

$J^{\mathrm{(NC)}\mu}(f) = \bar{u}_{f}\gamma^{\mu}\frac{1}{2}\left(g^{f}_{V}-g^{f}_{A}\gamma^{5}\right)u_{f},$

and $g^{f}_{V}$ and $g^{f}_{A}$ are the vector and axial vector couplings for fermion $f$.

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 ( $~(E/M_Z)^2$ ) 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.

## References

1. "The Nobel Prize in Physics 1979". Nobel Foundation. Retrieved 2008-09-10.
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