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| Fermi coupling constant | |
|---|---|
| Name | Fermi coupling constant |
| Quantity | weak interaction strength |
| Value | 1.1663787×10^−5 GeV^−2 (approx.) |
| Uncertainties | see electroweak fits |
| Units | GeV^−2 |
Fermi coupling constant
The Fermi coupling constant quantifies the low-energy strength of charged-current weak interactions and appears in descriptions of beta decay, muon decay, and electroweak processes. It links precision measurements from experiments at facilities such as the CERN Large Electron–Positron Collider, the Fermilab Tevatron and KEK with theoretical calculations developed by figures like Enrico Fermi, Richard Feynman, and Murray Gell-Mann. By connecting observables measured by collaborations such as Muon g-2, LEP and SLAC with predictions from the Standard Model and the Glashow–Weinberg–Salam model, it plays a central role in tests of electroweak theory, CKM matrix unitarity, and searches for physics beyond new physics.
In low-energy effective field descriptions the constant appears as the coupling of a four-fermion contact interaction originally used to describe beta decay and is extracted from muon lifetime measurements performed by collaborations like MuLan, TWIST, and MEG. The parameter translates between matrix elements computed in frameworks developed by theorists such as Steven Weinberg, Gerard 't Hooft, and Abdus Salam and decay rates measured at accelerator facilities like TRIUMF, J-PARC, and Brookhaven National Laboratory. Its numerical value, often quoted with inputs from electroweak fits by groups such as the Particle Data Group, sets the scale for charged-current processes in nuclei studied at institutes like Los Alamos National Laboratory and CERN experiments including NA48.
The concept emerged from Enrico Fermi's 1933 theory of beta decay and was refined through experimental programs at institutions such as Rutherford Appleton Laboratory, Cavendish Laboratory, and Lawrence Berkeley National Laboratory. Subsequent theoretical advances by Hideki Yukawa, Richard Feynman, and Sin-Itiro Tomonaga led to modern quantum field theoretic interpretations incorporated into the electroweak unification proposed by Sheldon Glashow, Steven Weinberg, and Abdus Salam. Precision experimental determinations advanced with muon physics experiments at CERN and Fermilab and with lifetime measurements from collaborations such as MuLan and TWIST.
In the low-energy limit of the Glashow–Weinberg–Salam model, the four-fermion interaction constant arises when integrating out the heavy W boson in the electroweak theory formulated by Sheldon Glashow, Steven Weinberg, and Abdus Salam. It is related to the SU(2) gauge coupling, the Weinberg angle introduced by Steven Weinberg, and the Higgs mechanism developed by Peter Higgs and others, with inputs from renormalization studies by Gerard 't Hooft and Martinus Veltman. The constant appears in effective Lagrangians used in calculations by theorists from groups at CERN, DESY, and Institute for Advanced Study.
The most precise determinations come from muon lifetime experiments such as MuLan at Paul Scherrer Institute, earlier measurements at TRIUMF, and analyses by the Particle Data Group. Results are corroborated by weak-process measurements in neutron decay experiments at facilities like Institut Laue–Langevin and nuclear beta-decay studies at laboratories including Argonne National Laboratory. Global electroweak fits incorporating data from LEP, SLAC, Tevatron, and LHC experiments by collaborations such as ATLAS and CMS constrain the value alongside inputs from Z boson and W boson observables.
Within the Standard Model, the constant is expressible in terms of the SU(2) coupling g and the W boson mass through relations developed in the electroweak framework by Sheldon Glashow, Steven Weinberg, and Abdus Salam. It provides a bridge between low-energy phenomenology measured in muon decay by experiments like MuLan and high-energy parameters probed by ATLAS and CMS at the Large Hadron Collider. Constraints on the constant influence precision tests of the CKM matrix unitarity investigated by collaborations such as Belle II and BaBar, and feed into global fits performed by groups including the LEP Electroweak Working Group.
Radiative corrections to the muon decay rate computed by quantum field theorists such as Alberto Sirlin and Kenneth G. Wilson require inclusion of electroweak and QED loop effects, with inputs from perturbative calculations by researchers at institutes like CERN and SLAC. Renormalization-group analyses developed by Kenneth G. Wilson and others describe how effective four-fermion interactions match onto the full theory with running couplings used in fits by the Particle Data Group. Precision determinations thus rely on higher-order calculations from collaborations and theory groups across Institute for Advanced Study, Perimeter Institute, and university groups worldwide.
The constant underpins predictions for beta decay rates studied in nuclear experiments at Oak Ridge National Laboratory and GANIL, informs neutrino interaction cross-section modeling for experiments like Super-Kamiokande, SNO, and DUNE, and constrains models of physics beyond the Standard Model proposed at seminars in institutions such as CERN and Princeton University. It is essential in tests of fundamental symmetries in experiments by collaborations like nEDM searches and plays a role in interpreting results from flavor factories such as Belle II and LHCb.