Constants library

4 values, each with its units, its uncertainty, and where it came from.

Universal & Atomic 4

m_{\mu} = 1.883531627 \times 10^{-28}\ \text{kg}

kgThe rest mass of the muon, 1.883 531 627 × 10⁻²⁸ kg or 105.658 MeV/c² — 207 electrons in one unstable package.

m_{\tau} = 3.16754 \times 10^{-27}\ \text{kg}

kgThe rest mass of the tau lepton, 3.167 54 × 10⁻²⁷ kg or 1776.86 MeV/c² — heavier than a proton, and the shortest-lived lepton.

m_{\mu}/m_{\mathrm{e}} = 206.7682827

dimensionlessThe muon is 206.768 times heavier than the electron — the same particle in every respect except mass, and nobody knows why.

G_{\mathrm{F}}/(\hbar c)^{3} = 0.000011663787\ \text{GeV}^{-2}

GeV⁻²The strength of the weak interaction at low energy, 1.166 378 7 × 10⁻⁵ GeV⁻² — measured from the muon's 2.2 microsecond lifetime.