= One-loop QCD antiscreening
{title2=$\beta_0=11-2n_f/3$}
The $+11$ contribution to $\beta_0$ is the net <Non-Abelian gauge theory> contribution; the $-2n_f/3$ term comes from <quark> loops. Thus <quarks> screen <color charge>, opposing <asymptotic freedom>. It is the net <gluon> contribution that produces antiscreening: the inferred <QCD coupling> decreases toward shorter distances. Unlike the electrically neutral <photon>, a <gluon> carries <color charge> and has self-interactions. Describing this as <vacuum polarization> must not imply that all vacuum contributions have the screening sign of <quantum electrodynamics>.<The discovery of asymptotic freedom and the emergence of QCD by David J. Gross>[2005 reprint, pp. 843–844, magnetic-response explanation; Eqs. 1–3]{ref}
Individual gauge-dependent <Feynman diagrams> do not separately define observable screening clouds. A proof of the coefficient sums the <gauge fixing>, <Faddeev-Popov ghost> and matter contributions and extracts the <charge renormalization>; the final one-<loop order> coefficient is independent of the gauge choice. The <beta function (physics)> is negative near zero for integer $0\leq n_f\leq16$. This is a statement about <weak coupling>, not a determination of the low-<energy> <particle> spectrum.<Reliable Perturbative Results for Strong Interactions? by Hugh David Politzer>[1973, pp. 1347–1348, Eqs. 3–7; warning below Eq. 7]{ref}
\Image[/qcd-fixed-flavor-running-coupling.png]
{title=One-loop running couplings with a common chosen calibration}
{description=Original fixed-<quark flavor> calculations with $\alpha_s(20\,\mathrm{GeV})=0.16$ in each theory. More <quark flavors> reduce $\beta_0$ and slow the decrease at higher <energy>. The three curves have different <strong-coupling scales>; they are not successive threshold intervals of one physical theory.}
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