quantum chromodynamics
/ KWON-tum kroh-moh-dy-NAM-iks /
Quantum chromodynamics, QCD, is the theory of the strong force, the rulebook for how quarks and gluons interact and how they lock themselves into protons, neutrons and every other hadron. The 'chromo' is Greek for color, a whimsical name for the strong-force charge that comes in three varieties nicknamed red, green and blue. It has nothing to do with visible color; it is simply the label for the property that plays the role electric charge plays in electromagnetism. QCD explains the deepest layer of ordinary matter, why nuclei exist at all.
QCD is a non-abelian gauge theory based on the color symmetry group SU(3). Quarks transform as a color triplet, the eight gluons as a color octet, and the interaction strength is set by the strong coupling constant alpha_s. Its signature feature is that this coupling runs strongly with energy: alpha_s decreases at high energies or short distances, asymptotic freedom, for which Gross, Wilczek and Politzer shared the 2004 Nobel Prize, and grows large at low energies. This means that at high energy quarks behave almost like free particles and calculations can be done perturbatively, while at low energy the coupling is too large for perturbation theory and the quarks are bound so tightly they can never escape.
This two-faced behavior is why QCD is at once beautifully predictive and stubbornly hard. High-energy processes like jet production at colliders are computed to great accuracy from Feynman diagrams; but the mass of the proton, the spectrum of hadrons, and the nature of confinement require solving the theory non-perturbatively, most successfully by lattice QCD, which simulates the fields on a discretized spacetime grid. A crucial honesty: most of the mass of the visible universe is QCD binding energy, not Higgs-given quark mass, the proton weighs far more than the sum of its quarks because of the energy stored in the gluon field.
When an electron and positron annihilate at high energy and produce a quark and antiquark, you never detect the quarks themselves; QCD confinement turns them into two back-to-back sprays of hadrons called jets, whose energy and direction faithfully trace the original quarks.
Jets are QCD's fingerprint: quarks and gluons announce themselves only as collimated showers of hadrons.
Proving mathematically that QCD has a mass gap and confines quarks is an open problem, one of the Clay Millennium Prize problems, so confinement, though overwhelmingly supported by experiment and simulation, is not yet a theorem.