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| strong coupling constant | |
|---|---|
| Name | Strong coupling constant |
| Quantity | Dimensionless coupling |
| Theory | Quantum Chromodynamics |
| Related | Quantum Electrodynamics, Quantum Field Theory, Renormalization Group |
strong coupling constant The strong coupling constant is the dimensionless parameter that sets the strength of the Quantum Chromodynamics interaction among quarks and gluons. It governs processes in high-energy particle accelerator experiments such as those at the Large Hadron Collider and shapes phenomena ranging from hadron formation to early-universe dynamics near the Big Bang. Precise knowledge of the constant is central to tests of the Standard Model and searches for Beyond the Standard Model physics at facilities like CERN and Fermilab.
In Quantum Chromodynamics, the strong coupling constant appears in the QCD Lagrangian multiplying the gauge-field interaction terms and determines the force between quarks mediated by gluons. It directly affects observables such as jet rates at the Large Electron–Positron Collider, decay widths measured at the SLAC National Accelerator Laboratory, and cross sections studied by the ATLAS experiment and CMS experiment. The magnitude of the constant influences confinement leading to bound states like protons, neutrons, pions, and heavier meson and baryon resonances cataloged by collaborations at the Brookhaven National Laboratory and Jefferson Lab. Understanding its value is crucial for interpreting results from the Tevatron and planning future facilities such as the proposed Future Circular Collider and International Linear Collider.
The strong coupling constant is embedded in Quantum Chromodynamics, a non-Abelian gauge theory based on the gauge group SU(3) that was developed by researchers including Murray Gell-Mann and Georgi-style model builders. Within the framework of Renormalization Group equations, αs runs with the renormalization scale μ and enters perturbative expansions for processes computed using techniques pioneered at institutions like MIT, Harvard University, and University of Cambridge. Calculations of higher-order corrections employ methods from Perturbation theory and are carried out by collaborations associated with journals such as Physical Review Letters and Journal of High Energy Physics. Theoretical advances from groups at CERN Theory Division and SLAC have produced multi-loop results that are used in precision fits performed by committees like the Particle Data Group.
A defining feature of the coupling is its scale dependence: αs decreases at high energies (short distances) — a property known as asymptotic freedom established by David Gross, Frank Wilczek, and David Politzer and recognized with the Nobel Prize in Physics. Conversely, αs increases at low energies leading to confinement and hadronization studied in experiments at DESY and theoretical work at Princeton University. The running is governed by the QCD beta function calculated in multi-loop orders by collaborations from institutions like Yale University and Oxford University and implemented in tools such as those developed at SLAC and CERN. This behavior underlies interpretations of deep inelastic scattering results from experiments like HERA and neutrino scattering data from Super-Kamiokande.
Measurements of αs come from a variety of observables: event shapes and jet rates at LEP and SLC, scaling violations in structure functions from HERA, hadronic decays of the Z boson measured at LEP, and lattice determinations from groups at Brookhaven National Laboratory and Riken. Collider experiments including ATLAS, CMS, ALEPH, DELPHI, OPAL, and L3 provide high-precision inputs; fixed-target programs at CERN SPS and Fermilab contribute complementary constraints. Global analyses are coordinated by consortia associated with the Particle Data Group and theoretical efforts at IPPP Durham and DESY that combine data from BaBar and Belle as well as heavy-flavor results from LHCb.
Nonperturbative determinations of the coupling employ Lattice QCD simulations developed by collaborations like MILC, UKQCD, ETM Collaboration, and groups at CERN and Brookhaven National Laboratory. Lattice results are cross-checked using techniques from Schrödinger functional studies by teams at CERN and ALPHA Collaboration, potential models inspired by work at Cornell University, and sum-rule approaches associated with researchers at Princeton University and Yale University. These methods are essential to bridge the perturbative regime studied at SLAC and DESY with low-energy phenomena explored at Jefferson Lab and heavy-ion programs at RHIC and the ALICE experiment.
Global fits of αs integrate results from experimental collaborations and theoretical inputs compiled by the Particle Data Group and analysis groups at CERN, IHEP, and INFN. These fits combine perturbative calculations up to high loop orders from teams at KIT and IPPP Durham with lattice inputs from MILC and ETM Collaboration to produce world-average values used by the Large Hadron Collider community. Precision determinations test the consistency of the Standard Model in conjunction with electroweak fits from groups at SLAC and DESY and probe potential signals of Supersymmetry or other Beyond the Standard Model scenarios advocated by researchers at CERN Theory Division and Perimeter Institute.
The value and running of the strong coupling constant affect predictions for processes at the Large Hadron Collider, rates of Big Bang Nucleosynthesis reactions in the early Universe, and phase transitions in the Quark–Gluon Plasma studied at RHIC and CERN. It plays a role in modeling cosmic microwave background related processes explored by teams behind Planck (spacecraft) and impacts constraints on theories proposed by scientists at Institute for Advanced Study and Perimeter Institute. Accurate αs inputs are necessary for interpreting searches for dark matter at LUX-ZEPLIN and XENON collaborations and for assessing unification scenarios studied in grand unified theories developed by physicists at University of Chicago and Harvard University.