QCD running coupling: quarks become free at high energy (Nobel 2004)
Asymptotic freedom, discovered by Gross, Politzer, and Wilczek in 1973 (Nobel Prize 2004), is the remarkable property that the QCD strong coupling αs decreases at high energies (short distances). This is the opposite of QED. The one-loop beta function β(αs) = −(b₀/2π)αs² with b₀ = 11 − 2Nf/3 is positive for Nf < 17 quark flavors, driving the coupling to zero. At the LHC (~1 TeV), αs ≈ 0.09, while at the proton scale (~1 GeV), αs ≈ 0.4.