Glashow–Weinberg–Salam model

Glashow–Weinberg–Salam model
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Name	Glashow–Weinberg–Salam model
Field	Particle physics
Discovered by	Sheldon Glashow; Steven Weinberg; Abdus Salam
Introduced	1961; 1967
Components	Electroweak interaction; Weak interaction; Electromagnetism; Higgs mechanism

Glashow–Weinberg–Salam model. The Glashow–Weinberg–Salam model unifies the Electromagnetism and Weak interaction into a single electroweak framework introduced by Sheldon Glashow, Steven Weinberg, and Abdus Salam and formulated using tools from Quantum field theory, Gauge theory, and Spontaneous symmetry breaking. It provided a renormalizable description that linked observables measured at facilities such as CERN, Fermilab, and SLAC National Accelerator Laboratory to parameters constrained by experiments at Brookhaven National Laboratory and DESY. The model underpins the electroweak sector of the Standard Model and informed searches culminating in discoveries at the Large Hadron Collider and earlier accelerator programs like LEP.

Introduction

The model synthesizes ideas articulated by Sheldon Glashow in 1961, formalized by Steven Weinberg in 1967 and presented by Abdus Salam in contemporaneous work, embedding SU(2)×U(1) gauge symmetry within a quantum framework drawing on concepts from Yang–Mills theory, Noether's theorem, and the renormalization program advanced by Gerard 't Hooft and Martinus Veltman. It predicts mediators of the weak force—the charged W± and neutral Z0 bosons—and preserves the photon as the massless gauge boson of electromagnetism, linking to precision tests performed by collaborations at ALEPH, OPAL, DELPHI, and L3.

Historical development and contributors

Early roots trace to symmetry ideas in works by Paul Dirac, Enrico Fermi, and the Fermi theory of beta decay, while the gauge formalism built on Yang–Mills theory by Chen Ning Yang and Robert Mills. Sheldon Glashow proposed partial unification in 1961; Steven Weinberg incorporated spontaneous symmetry breaking and the Higgs mechanism in 1967 drawing on concepts from Peter Higgs, François Englert, and Robert Brout, and Abdus Salam independently presented similar synthesis at conferences involving participants from Imperial College London and CERN. The proof of renormalizability by Gerard 't Hooft and Martinus Veltman vindicated the theoretical program and influenced experiments at CERN SPS, Tevatron, and later Large Hadron Collider collaborations including ATLAS and CMS.

Theoretical framework

The model is built on local gauge invariance under the group SU(2)_L×U(1)_Y with left-handed fermions assigned to SU(2) doublets and right-handed fermions to singlets, using representations familiar from Group theory and the classification introduced in Murray Gell-Mann's work. The Lagrangian combines kinetic terms for gauge fields, Yukawa couplings connecting fermions to a scalar doublet, and a scalar potential whose vacuum expectation value breaks symmetry, employing techniques from Renormalization central to work by Ken Wilson and perturbative expansions developed by Richard Feynman and Julian Schwinger. The electroweak mixing angle introduced by Steven Weinberg (the Weinberg angle) parametrizes the rotation between gauge eigenstates, determining couplings measured in scattering at SLAC National Accelerator Laboratory and decay widths explored by experiments at Fermilab.

Electroweak symmetry breaking and the Higgs mechanism

Symmetry breaking is implemented by a complex scalar doublet acquiring a nonzero vacuum expectation value through the Higgs potential originally proposed by Peter Higgs and elaborated by François Englert and Robert Brout, producing a massive scalar boson and giving mass to W± and Z0 via the Brout–Englert–Higgs mechanism. The mechanism preserves gauge invariance while generating longitudinal polarization states for massive gauge bosons, concepts central to analyses by Cornwall, Levin, and Tiktopoulos and formalized in the context of unitarity constraints studied by Lee, Quigg, and Thacker. The discovery of a scalar resonance near 125 GeV at CERN by the ATLAS and CMS collaborations provided the decisive signature consistent with the Higgs field predicted in the model.

Predictions and experimental confirmation

The model predicted masses, couplings, and neutral-current processes, including neutral-current neutrino scattering observed in experiments at Gargamelle and cross-section relations verified at SLAC and CERN. Precision electroweak observables measured at LEP and SLC tested radiative corrections computed using techniques from Dimensional regularization and the renormalization-group methods of Wolfgang Pauli's successors, constraining the top quark mass later found at Fermilab by the CDF and DØ collaborations and anticipating the Higgs mass explored at Tevatron and finally observed by ATLAS and CMS at Large Hadron Collider. Measurements of parity violation in atomic systems and polarized electron scattering at SLAC supplied complementary tests of weak mixing parameters predicted by the model.

Extensions, limitations, and open questions

While the model successfully unifies electroweak phenomena, it leaves open questions addressed by extensions such as Grand Unified Theory proposals by Georgi and Glashow, supersymmetric frameworks developed by Hugh Everett's intellectual successors like Howard Georgi and Hitoshi Murayama, and neutrino mass models inspired by oscillation results from Super-Kamiokande and SNO. It does not incorporate Gravity as described by Albert Einstein's general relativity nor explain the hierarchy problem emphasized by analyses invoking Naturalness and explored in models by Giudice and Dimopoulos. Cosmological implications intersect with observations by Planck (spacecraft), WMAP, and baryogenesis scenarios studied by Andrei Sakharov and A.D. Sakharov's community, motivating searches for physics beyond the Standard Model at LHCb, future colliders like the proposed International Linear Collider, and nonaccelerator experiments including KATRIN and neutrinoless double beta decay collaborations.

Category:Particle physics