ELECTROWEAK UNIFICATION Ryan Clark Cong Nguyen Robert Kruse
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 Fermions (Half integer spin) � Quarks � Strongly interact � Make up hadrons (includes proton and neutron) � Leptons � Electron, Muon, Tau and Neutrinos Bosons (Whole Integer Spin) Fig 1. A diagram of all the particle the standard model (minus the Higgs) Source: http: //www. controlyourcash. com/ � 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 10 -2 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 o Carriers W± Z 0 n p Massive particles Z 0 o W is charged, is neutral o Limited Range (10 -18 m) � Approximately 10 -6 times as strong as the strong force. � Acts on both charged and uncharged particles W— — νe e— Fig. 2 Feynman Diagram showing beta decay via the weak interaction
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 Fig 3. An analogy for Spontaneous Symmetry Breaking using ice crystals. Source: http: //beyondpenguins. ehe. osu. edu/
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. ◦ m. W = 80 Ge. V/c 2 q. W = ±e ◦ m. Z = 90 Ge. V/c 2 q. Z = 0 � The observation of the Z boson in 1983 confirmed theoretical predictions. � At high enough energies (100 Ge. V), the coupling strength of electromagnetism 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.
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