The weak neutral current couples the Z boson to a fermion without changing its electric charge. Its vector and axial-vector coefficients depend on weak isospin, electric charge, and the weak mixing angle.
If a neutral-current vertex has prefactor and chiral coefficients , then , with and . For one Standard Model charged lepton, ; for an active neutrino, . A prefactor instead requires doubled coefficients. Mixing these conventions creates a factor-of-four error in squared amplitudes.
A flavor-changing neutral current changes fermion flavor without changing electric charge. Such currents are absent at tree level in the Standard Model and are strongly suppressed at loop level by the Glashow-Iliopoulos-Maiani mechanism.
Unitarity of the CKM matrix cancels flavour-independent contributions to weak flavour-changing neutral amplitudes. In a quark loop, equal internal masses would give a common contribution whose off-diagonal CKM sum vanishes; unequal masses leave a suppressed nonzero amplitude. This explains why neutral kaon mixing occurs through loops rather than a tree neutral-current vertex.
The Glashow-Iliopoulos-Maiani mechanism uses the unitarity of the quark mixing matrix to cancel flavor-changing neutral-current amplitudes when equal-charge quark masses are degenerate. The remaining loop amplitudes are suppressed by quark-mass differences.
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Neutral current is a term that refers to the current that flows in the neutral conductor of an electrical system, particularly in alternating current (AC) systems. The neutral wire serves as a return path for current in a balanced system. In a three-phase system, for example, it helps to ensure that the load is evenly distributed among the phases.