| Quantum chromodynamics | |
|---|---|
| Name | Quantum chromodynamics |
| Caption | Schematic of gluon self-interaction |
| Field | Particle physics |
| Introduced | 1970s |
| Founders | Murray Gell-Mann; developed by Harald Fritzsch and Heinrich Leutwyler and others |
| Institutions | CERN; Fermilab; DESY; SLAC National Accelerator Laboratory |
Quantum chromodynamics
Quantum chromodynamics (QCD) is the quantum field theory describing the strong interaction, one of the four fundamental forces, responsible for binding quarks into hadrons such as protons and neutrons. QCD matters in Quantum Physics and Particle physics because it provides the non-Abelian gauge framework that explains phenomena from deep inelastic scattering to the mass budget of ordinary matter.
Quantum chromodynamics emerged in the late 1960s and early 1970s to account for the observed spectrum of hadrons and the pattern of strong interactions. The concept of color charge was introduced to resolve the spin–statistics puzzles of the quark model proposed by Murray Gell-Mann and George Zweig. The non-Abelian gauge theory with symmetry group SU(3) was formulated by theorists including Harald Fritzsch, Murray Gell-Mann, and Heinrich Leutwyler, building on earlier work on gauge theories by Chen Ning Yang and Robert Mills. The discovery of asymptotic freedom by David Gross, Frank Wilczek, and David Politzer in 1973 established QCD as the correct theory at high energies and earned them the Nobel Prize in Physics. Experimental confirmation came from deep inelastic scattering experiments at SLAC and jet observations at colliders such as CERN and Fermilab.
QCD is a renormalizable quantum field theory based on the local gauge group SU(3) that acts on quark color degrees of freedom. The fundamental fields are spin-1/2 quarks in various flavors (up, down, strange, charm, bottom, top) and eight spin-1 gluons, the gauge bosons of SU(3). The QCD Lagrangian density combines the Dirac term for quarks with the Yang–Mills field strength for gluons and quark–gluon coupling: - quark fields: psi_{f} for flavor f, - gluon field strength: G_{μν}^a with structure constants f^{abc}. Self-interaction of gluons, absent in quantum electrodynamics (QED), arises from the non-Abelian structure and leads to distinctive dynamics such as confinement and asymptotic freedom. Renormalization and running coupling are treated within perturbation theory using schemes like MS-bar.
Asymptotic freedom is the property that the QCD coupling α_s decreases at high momentum transfer, derived from the negative beta function computed by Gross–Wilczek–Politzer. This underpins the success of perturbative methods in processes at energies probed by LHC and Tevatron experiments. In contrast, at low energies the coupling grows, leading to confinement: colored states (free quarks or gluons) are not observed, only color-singlet hadrons exist. Confinement is closely connected to nonperturbative phenomena such as chiral symmetry breaking, the formation of the QCD vacuum and topological objects like instantons and monopole-inspired mechanisms. The Wilson loop criterion and area law provide formal diagnostics of confinement in gauge theories.
Perturbative QCD (pQCD) uses expansions in the small parameter α_s(Q^2) for processes with large momentum transfer Q. pQCD calculates observable quantities via Feynman diagrams, factorization theorems, and evolution equations such as the Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) equations. Applications include predictions for jet production, hard scattering cross sections, and precision observables in electron–positron annihilation (e.g., at LEP), deep inelastic scattering at HERA, and partonic subprocesses at the LHC. Higher-order computations employ techniques like dimensional regularization, renormalization group methods, and automated tools developed at collaborations such as NNPDF and software like MadGraph and PYTHIA for event simulation.
Nonperturbative QCD addresses low-energy phenomena by numerical and analytical techniques. Lattice QCD discretizes spacetime into a lattice and computes correlation functions using Monte Carlo importance sampling; major collaborations at CERN, Brookhaven National Laboratory, Riken, and national computing centers perform large-scale simulations. Lattice results provide hadron spectra, decay constants, and inputs for flavor physics and CP violation studies. Other approaches include QCD sum rules, effective field theories such as chiral perturbation theory and heavy quark effective theory (HQET), and models like the bag model and constituent quark model. Nonperturbative methods also study the QCD phase diagram, including the quark–gluon plasma investigated at RHIC and the LHC heavy-ion program.
Understanding hadron structure in QCD involves parton distribution functions (PDFs), generalized parton distributions, and fragmentation functions. PDFs encode the momentum and spin distributions of quarks and gluons inside hadrons and are extracted from global analyses combining data from deep inelastic scattering, Drell–Yan processes, and collider experiments. Factorization theorems separate perturbative hard parts from nonperturbative PDFs, enabling precision tests of the Standard Model and searches for new physics. Lattice QCD and experiments at polarized facilities (e.g., Jefferson Lab, COMPASS) contribute to mapping nucleon spin decomposition and transverse momentum distributions.
QCD has been rigorously tested across multiple experimental programs: jet substructure and strong coupling measurements at LEP, Tevatron, and the LHC; heavy-flavor production at B-factories and LHCb; and quark–gluon plasma signatures at RHIC and LHC heavy-ion runs. Precision determinations of α_s, studies of hadronization, and comparisons of lattice QCD predictions with observed hadron masses constitute ongoing validation. Phenomenological frameworks combine perturbative calculations, parton showers, and hadronization models implemented in event generators (e.g., HERWIG, PYTHIA), while global PDF fits from groups like CTEQ, MSTW/MMHT, and NNPDF underpin collider predictions. Continued interplay between theory, lattice computation, and experiment refines QCD inputs for searches of beyond-Standard Model phenomena.
Category:Quantum chromodynamics Category:Quantum field theory Category:Particle physics