Electroweak theory unifies electromagnetic and weak forces, explaining their behavior as aspects of a single interaction. It introduces key particles like W and Z bosons, and predicts phenomena like neutral weak currents and the Higgs boson.

This unification revolutionized our understanding of fundamental forces. It showed how distinct interactions can be unified, paving the way for further theories and providing a framework for exploring the early universe and particle physics mysteries.

Electroweak theory development

Historical context and key contributors

• Sheldon Glashow, Abdus Salam, and Steven Weinberg developed electroweak theory in the 1960s
  • Jointly received the Nobel Prize in Physics in 1979 for their contributions
• Built upon earlier work on weak interactions by Enrico Fermi in the 1930s
• Incorporated developments in quantum electrodynamics from the 1940s and 1950s
• Gauge invariance concept from electromagnetism played crucial role in theory formulation

Experimental confirmations

• Neutral weak currents predicted by electroweak theory experimentally confirmed in 1973 at CERN
  • Provided strong support for the theory
• W and Z bosons discovered in 1983 at CERN's Super Proton Synchrotron
  • Offered direct experimental evidence for electroweak theory
• Higgs boson discovered in 2012 at CERN's Large Hadron Collider
  • Completed the Standard Model of particle physics
  • Represented final piece of electroweak puzzle

Electromagnetic and weak interaction unification

Fundamental principles and particles

• Unifies electromagnetic and weak interactions as aspects of single electroweak interaction
• Introduces four gauge bosons
  • Photon (γ) for electromagnetic interactions
  • W+, W-, and Z0 bosons for weak interactions
• At high energies (above 246 GeV electroweak scale), electromagnetic and weak forces become indistinguishable
  • Described by single SU(2) × U(1) gauge symmetry

Key concepts and relationships

• Predicts Weinberg angle (weak mixing angle)
  • Determines relationship between electromagnetic and weak coupling constants
• Explains weakness of weak interaction compared to electromagnetic interaction at low energies
  • Due to large masses of W and Z bosons
• Accounts for charged current and neutral current weak interactions
• Incorporates electromagnetic interactions within unified framework

Symmetry breaking in electroweak theory

Spontaneous symmetry breaking and the Higgs mechanism

• Spontaneous symmetry breaking explains mass acquisition of W and Z bosons while photon remains massless
• Higgs mechanism provides mathematical framework for spontaneous symmetry breaking
  • Proposed by Peter Higgs and others in 1964
• Before symmetry breaking, theory describes four massless gauge bosons
• After symmetry breaking
  • W+, W-, and Z0 bosons acquire mass
  • Photon remains massless
• Higgs field permeates all space
  • Non-zero vacuum expectation value gives mass to W and Z bosons

Consequences of symmetry breaking

• Goldstone bosons from symmetry breaking "eaten" by W and Z bosons
  • Provides longitudinal polarization states to W and Z bosons
• Remaining degree of freedom in Higgs field manifests as Higgs boson
  • Scalar particle discovered in 2012
  • Completed Standard Model of particle physics

Implications of electroweak unification

Theoretical advancements

• Demonstrates distinct forces can be aspects of single, more fundamental interaction
  • Encourages search for further unification (grand unified theories)
• Supports gauge principle as fundamental concept in particle physics
  • Guides development of theories for other interactions
• Predicts precise relationships between observable quantities
  • Allows stringent tests of Standard Model through precision measurements

Experimental and practical impacts

• Successful prediction and discovery of W and Z bosons increased confidence in theoretical particle physics
• Raises questions about hierarchy problem
  • Why electroweak scale is much smaller than Planck scale
  • Motivates theories like supersymmetry
• Spontaneous symmetry breaking concept applied to other areas of physics (condensed matter physics, cosmology)
• Provides framework for understanding early universe processes
  • Baryogenesis
  • Electroweak phase transition