Three Neutrinos

Plate 54 speaks for itself: “The Z counts three light neutrinos.”

The Physics

The Z boson's line shape at LEP measures its total width; subtracting the visible decays leaves the invisible width, which counts the light neutrino species.

N_ν = 2.984 ± 0.008

• N_ν: number of light neutrino species, dimensionless
• √s: collision energy near the Z mass 91.19 GeV, GeV

The Z width counts invisible decays. 2- and 4-neutrino curves miss the data.

Honest caveat: It counts neutrinos lighter than half the Z mass that couple to the Z; it does not rule out sterile or very heavy ones.

Sources

• ALEPH, DELPHI, L3, OPAL, SLD, Phys. Rep. 427, 257 (2006).

On the poster

Plate 054, “Three Neutrinos”: the pixel-art plate with its slogan “The Z counts three light neutrinos.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.