Asymptotic freedom (Gross, Politzer, Wilczek 1973 — Nobel 2004): the QCD coupling αs decreases logarithmically at high energy scales μ. The one-loop beta function gives αs(μ) = αs(MZ) / [1 + (αs(MZ)/2π)(11 − 2Nf/3)ln(μ/MZ)], where Nf is active quark flavors. At low energies αs → ~1, signaling confinement; at MZ = 91.2 GeV, αs(MZ) ≈ 0.118.