= Solution
\b[Theory.] <Quantum chromodynamics> is a local, non-Abelian <gauge theory> with gauge group <SU(3)> acting on <color charge>. Its matter fields are spin-half <quarks> of six flavours, each in three colors; its eight massless spin-one <gluons> are the gauge fields. Unlike the neutral <photon>, <gluons> themselves carry gauge charge. Consequently there are three-gluon and four-gluon vertices as well as the quark-gluon vertex. A compact <Lagrangian density> is
$$
\mathcal L_{\rm QCD}=\sum_f\bar q_f(i\gamma^\mu D_\mu-m_f)q_f-\frac14G_{\mu\nu}^aG^{a\mu\nu},
$$
where $D_\mu$ is the gauge <covariant derivative> and the field strength contains the nonlinear term $g_sf^{abc}G_\mu^bG_\nu^c$.
The <running coupling> is weak at high momentum scales. At one loop, for $n_f$ active flavours,
$$
\alpha_s(Q^2)\simeq\frac{4\pi}{(11-2n_f/3)\ln(Q^2/\Lambda_{\rm QCD}^2)}.
$$
The positive coefficient $11-2n_f/3$ produces <asymptotic freedom>: short-distance, large-$Q$ processes admit <perturbation theory>. The gluon self-interaction is crucial to its sign. At low scales the coupling grows and isolated colored particles are not observed; the physical spectrum consists of color-singlet <hadrons>, the phenomenon of <confinement>. A hard-produced <quark> or <gluon> radiates and undergoes <hadronization>, giving a collimated <jet>. The one-loop formula is not valid at $Q\sim\Lambda_{\rm QCD}$ and is not, by itself, a mathematical proof of <confinement>.
\b[Experimental tests.] Different measurements test different ingredients of this account:
* <Deep inelastic scattering> resolves approximately pointlike spin-half constituents. Approximate <Bjorken scaling> supplies the starting parton picture, while the measured logarithmic departures from scaling agree with QCD evolution of <parton distribution functions>.
* The <hadronic R ratio> in <electron-positron annihilation> counts squared quark charges multiplied by three colors, with calculable strong corrections. It tests color multiplicity as well as the quark charge assignments.
* Two-jet events identify a quark-antiquark hard topology. Three-jet events test hard gluon radiation, $e^+e^-\to q\bar qg$; their angular correlations support a spin-one <gluon>. Four-jet correlations and radiation patterns further probe the non-Abelian color factors and gluon self-interactions.
* Determinations of $\alpha_s$ from processes at widely separated scales follow the same <running coupling>, rather than independent interaction strengths. Jet rates, event shapes, hadronic weak-boson decays and high-energy hadron collisions provide complementary tests.
* <Lattice QCD> calculations reproduce many hadronic masses and other low-energy quantities from the same underlying theory, connecting its strongly coupled regime to the observed spectrum.
The convincing evidence is the quantitative consistency across these probes, with a common color group and coupling evolution. A jet by itself is evidence for a hard parton; its rates and correlations supply the more specific tests of <Quantum chromodynamics>.<Asymptotic Freedom: From Paradox to Paradigm by Frank Wilczek>{ref}
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