Electroweak Unification

1. Ryan Clark, Cong Nguyen, Robert Kruse and Blake Watson PHYS-3313, Fall 2013 University of Texas Arlington December 2, 2013 Electroweak Unification

2. Introduction • The Standard Model • The Four Forces • History of Unification • Electromagnetism • The Weak Force • Spontaneous Symmetry Breaking • Electroweak Unification • Conclusion

3. The Standard Model of Particle Physics • Fermions(Half integer spin) • Quarks • Strongly interact • Make up hadrons (includes proton and neutron) • Leptons • Electron, Muon, Tau and Neutrinos • Bosons (Whole Integer Spin) • Force Carriers • Photon, Gluon, and W and Z • (Higgs) Fig 1. A diagram of all the particle the standard model (minus the Higgs) Source: http://www.controlyourcash.com/

4. The Four Forces of the Standard Model • The Strong Force • Holds quarks together inside proton and neutron. • Force carried by gluons. • Strongest known force. • The Weak Force • Responsible for beta radiation and neutrino interactions. • Force carried by W± and Z0 bosons. • Electromagnetism • Unified theory of the electric and magnetic forces. • Responsible for attraction and repulsion between charges. • Gravity • Responsible for attraction between masses. • Weakest of all known forces.

5. History of Unification • Newton’s Theory of Universal Gravitation • Unified the motion of the planets with the motion of earthly projectiles. • Maxwell’s Theory of Electromagnetism • Unified electricity and magnetism in four fundamental equations • Explained light as an electromagnetic wave.

6. Electromagnetism • The most well-understood force of nature. • Force Carrier  Photon • Stable • Massless • Infinite range • Approximately 10-2 times the strength of the strong force • Acts only on matter with non-zero charge and/or a non-zero magnetic moment.

7. The Weak Force • First proposed by Enrico Fermi to explain beta decay. • Force Carriers  W± Z0 • Massive particles • W is charged, Z0 is neutral • Limited Range (10-18m) • Approximately 10-6 times as strong as the strong force. • Acts on both charged and uncharged particles n p W— νe — e— Fig. 2 Feynman Diagram showing beta decay via the weak interaction

8. Spontaneous Symmetry Breaking • An Analogy: • Crystal State = High Symmetry • Liquid State = Low Symmetry • The early universe existed in a different state from today. • Particles were indistinguishable • The four forces were unified. • As the universe cooled, this symmetry was spontaneously broken by the Higgs mechanism. Fig 3. An analogy for Spontaneous Symmetry Breaking using ice crystals. Source: http://beyondpenguins.ehe.osu.edu/

9. Electroweak Unification • Developed by Weinberg, Salam, and Glashow between 1960 and 1979. • Central to the unification was the necessary existence of the W and the Z bosons. • mW = 80 GeV/c2qW = ±e • mZ = 90 GeV/c2qZ = 0 • The observation of the Z boson in 1983 confirmed the theoretical predictions. • At high enough energies (100 GeV), the coupling strength of electromagnetism and and the weak force becomes the same and the W and Z bosons are indistinguishable from the photon. • Spontaneous symmetry breaking explains why the two forces appear different today.

10. Conclusion • The unification of the electromagnetic and weak forces stands as one of the great theoretical achievements of the 21st century. • The theory is a monumental step on the path towards a Grand Unified Theory of physics and a complete understanding of reality.