Neutral current

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 is

${\displaystyle {\mathfrak{M}}^{\mathsf{N}\mathsf{C}}\propto J_{\mu }^{\mathsf{(}\mathsf{N}\mathsf{C}\mathsf{)}}({\nu }_{\mathrm{e}})J^{\mathsf{(}\mathsf{N}\mathsf{C}\mathsf{)}\mu }({\mathrm{e}}^{-}),}$

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

${\displaystyle J^{\mathsf{(}\mathsf{N}\mathsf{C}\mathsf{)}\mu }(f)={\overline{u}}_f{\gamma }^{\mu }\frac{1}{2}\left(g_{\mathsf{V}}^f-g_{\mathsf{A}}^f{\gamma }^5\right)u_f,}$

where:

${\displaystyle g_{\mathsf{V}}^f=T_3(f)-2{\sin }^2{\theta }_{\mathsf{W}}Q(f)=\frac{1}{2}{Q}_{\mathsf{W}}(f)}$

and ${\displaystyle g_{\mathsf{A}}^f=T_3(f)}$ are the vector and axial couplings for fermion ${\displaystyle f.}$ ${\displaystyle T_3}$ denotes the weak isospin of the fermions, Q their electric charge and ${\displaystyle Q_{\mathsf{W}}}$ their weak charge. These couplings amount to essentially left chiral for neutrinos and axial for charged leptons.

The Z boson can couple to any Standard Model particle, except gluons and photons (sterile neutrinos would also be an exception, if they were found to exist). 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, ${\displaystyle (E/M_{\mathrm{Z}}{)}^2,}$ 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), of major experimental significance, in, e.g. , the Sudbury Neutrino Observatory experiment.

Weak neutral currents were predicted by electroweak theory developed mainly by Abdus Salam, John Clive Ward, Sheldon Glashow and Steven Weinberg, and confirmed shortly thereafter in 1973, in a neutrino experiment in the Gargamelle bubble chamber at CERN.