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 νμ 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 νμ, remains an outgoing neutral νμ. See 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 Elementary particle, Fundamental interactions, Symmetry laws (physics), Weak nuclear interactions