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Neutral-current interaction

In neutral current interactions, a quark or a lepton (e.g., an electron or a muon) emits or absorbs a neutral Z boson. For example:

Like the W bosons, the Z boson also decays rapidly, for example:

Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, and / or weak isospin, the neutral-current Z interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs.

The quantum number weak charge (Q) serves the same role in the neutral current interaction with the Z that electric charge (Q, with no subscript) does in the electromagnetic interaction: It quantifies the vector part of the interaction. Its value is given by:

Since the weak mixing angle θ w ≈ 29 ∘ {\displaystyle \theta _{\mathsf {w}}\approx 29^{\circ }}, the parenthetic expression (1 − 4 sin 2 θ w) ≈ 0.060 {\displaystyle (1-4\,\sin ^{2}\theta _{\mathsf {w}})\approx 0.060}, with its value varying slightly with the momentum difference (called " running ") between the particles involved. Hence

since by convention sgn T 3 ≡ sgn Q {\displaystyle \operatorname {sgn} T_{3}\equiv \operatorname {sgn} Q}, and for all fermions involved in the weak interaction T 3 = ± 1 2 {\displaystyle T_{3}=\pm {\tfrac {1}{2}}}. The weak charge of charged leptons is then close to zero, so these mostly interact with the Z boson through the axial coupling.