Introduction to QED

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Introduction toQED

• Quantum Electrodynamics
• Part IV

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Applications of QED

• Through the use of atom smashers, over 400
  particles have been discovered.
• This multitude of particles has created a need to
  explain their number and the nature of their
  interactions.
• QED deals specifically with electrons and
  photons, but its form and function can be applied
  to other particle interactions.

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QCD

• A great number of these newly-discovered
  particles are simply combinations of quarks.
• The quantum theory of the interactions of quarks
  via the strong force is called Quantum
  Chromodynamics (QCD).
• 3 make a baryon and 2 make a meson

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Elementary Particles
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Baryons
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Gluons

• Quarks interact via the gluon.
• The gluon functions in many ways like a photon.
• The probability of a coupling occurring is the
  constant g (similar in function to j for
  electrons)
• Diagrams of interactions will look very similar
  to that of the electron and photon.

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Elementary Particles
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Quark-Gluon Coupling
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Color

• Quarks and gluons have a property called color.
• Quarks can change color by coupling with a gluon.
• Colors are red, green, and blue.

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Change of Color
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Gluon Coupling
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Rules of Color

• All particles created by quarks must be
  colorless.
• Baryons have one of each color and mesons have a
  quark-anti-quark pair.
• Impossible to have a single quark.

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Change of Flavor

• A down quark can change into an up quark.
• This is done by emitting a W particle, which then
  decays into an electron-anti-neutrino pair.
• This process is called beta decay.

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Beta Decay
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Elementary Particles
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Neutral Currents

• There is a particle Z0 that is a neutral W boson.
• Z0 has no charge.
• Couplings with Z0 result in no change in a
  particles charge. (Neutral Currents)

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Z0 Couplings
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W-Anti-W Coupling
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Electro-Weak Force

• The observed coupling constant is almost
  identical as that of the photon.
• The three Ws and the photon would then appear to
  be somehow interconnected.
• Electrodynamics and the weak force were
  successfully combined by Stephen Weinberg and
  Abdus Salam

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Redundant Particles

• As nuclei have been bombarded by protons of
  higher and higher energy, new particles have
  appeared.
• These particles seem to mimic lower-energy
  versions and differ only by their higher mass.
• E.g. The Muon is identical to an electron, except
  its about 200 times heavier.

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Muon Interactions
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Beta Decay with a Muon
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More Elementary Particles
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Even More Elementary Particles
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• Diagrams
• Feynman, Richard P. QED The Strange Theory of
  Light and Matter. Princeton University Press.
  Princeton, NJ, 1988.

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Questions?