ELECTROWEAK UNIFICATION Ryan Clark, Cong Nguyen, Robert Kruse and Blake Watson PHYS-3313, Fall 2013 University of Texas Arlington December 2, 2013

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

The Standard Model of Particle Physics Fig 1. A diagram of all the particle the standard model (minus the Higgs) Source: 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)

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 Z 0 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.

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.

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

The Weak Force First proposed by Enrico Fermi to explain beta decay. Force Carriers  W ± Z 0 o Massive particles o W is charged, Z 0 is neutral o Limited Range ( m) Approximately times as strong as the strong force. Acts on both charged and uncharged particles

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 :

Electroweak Unification Developed by Weinberg, Salam, and Glashow between 1960 and Central to the unification was the necessary existence of the W and the Z bosons. ◦ m W = 80 GeV/c 2 q W = ±e ◦ m Z = 90 GeV/c 2 q Z = 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.

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