= Light-neutrino counting from the Z width
{title2=$N_\nu=\Gamma_{\mathrm{inv}}/\Gamma_\nu,\quad\Gamma_\nu=\frac{G_FM_Z^3}{12\sqrt2\pi}$}
Each effectively massless active <neutrino> species contributes the same invisible <Z boson> partial <decay width>. Subtracting calculated or measured visible widths from the total width therefore measures the displayed number, assuming no additional invisible channels. The inference concerns particles lighter than $M_Z/2$ with ordinary weak couplings. It does not exclude heavier neutrinos or <right-handed neutrinos> that are <gauge singlets>. Early LEP resonance measurements were consistent with three light active species, with statistical uncertainty. See the https://inspirehep.net/files/afa87f449c4b3cb87894b53955b564da[OPAL report on its 1989 data] for the original resonance-width comparison.
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