= Neutral current
{wiki}
= Weak neutral current
{synonym}
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.
The <neutral current> transfers no <electric charge> at a <fermion> vertex; “neutral” does not mean that every participating <particle> is uncharged. For example, $\nu_\mu e^-\to\nu_\mu e^-$ transfers <energy> and <momentum> to an <electron> through <Z boson> exchange. By contrast, a <charged current> can convert a <muon neutrino> into a <muon>, as in $\nu_\mu n\to\mu^-p$. Both processes conserve total <electric charge>. A <photon> also carries no <electric charge>, but an active <neutrino> has no electromagnetic charge coupling: its <neutral current> is a distinct <weak interaction>.
The experimentally useful contrast on a <nucleon> target is
$$
\nu_\mu+N\longrightarrow\mu^-+X\quad\text{(charged current)},\qquad
\nu_\mu+N\longrightarrow\nu_\mu+X\quad\text{(neutral current)},
$$
where $N$ is a <nucleon> and $X$ is an allowed final state of <hadrons>. The <charged current> makes a charged <lepton> available for identification; the outgoing <neutrino> in a <neutral current> leaves no <bubble-chamber track>. A sample of hadronic events without an identified <muon> therefore poses an experimental question: how many could instead be <neutron> interactions, or <charged current> events whose <muon> was missed? The answer requires a detector-response and background analysis, not merely an inspection for an absent line.
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