Weak Photons and Strong Glue

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Lecture 20

• Weak Photons and Strong Glue

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Fundamental Particles (circa 1930s)

• Photon (particle of light)
• Electron, proton, neutron (making up atoms)
• Positron (antiparticle of electron)

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Neutrino

• Predicted by Pauli in 1931 to explain the decay
  of the neutron neutron ? proton electron
  antineutrino
• Almost massless and interacts very weakly with
  other particles (via the weak force)
• Discovered only in 1956

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Particle Explosion(1940s to 60s)

• Leptons (light) electron, muon, neutrino
• Mesons (middle) pion, kaon, ...
• Baryons (heavy) proton, neutron, L, W, ?, ...

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Quark Hypothesis

• Proposed independently by Murray Gell-Mann and
  George Zweig in 1961
• Even more fundamental particles called quarks
  carrying charge 2/3 or -1/3
• Mesons are composed of two quarks while baryons
  are composed of three quarks

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Six Quarks of Nature
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Examples of Baryons
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From Atoms to Quarks ...
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Properties of Fermions
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The Four Fundamental Interactions
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Properties of the Four Fundamental Forces
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Two skaters exchanging a ball
The skaters will be pushed apart
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Quantum Field Theory Explanation of Forces

• Forces are due to the exchange of virtual
  particles
• Replaces the classical concept of fields (e.g.,
  electric or magnetic fields)

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Range of Forces

• The range of the force depends on the mass of the
  virtual particle
• The more massive the particle, the shorter the
  range of the force, since the particle can only
  exist for a shorter time
• This is due to the energy-time Heisenberg
  Uncertainty Principle

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Particles Mediating the Forces are Bosons

• Quantum electrodynamics (QED) describes
  electro-magnetism in terms of exchange of photons
• Weak force W, Z0 bosons
• Quantum chromodynamics (QCD) describes the strong
  force in terms of exchange of eight different
  gluons carrying colour charge

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Properties of Bosons
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Stanford Linear Accelerator (SLAC)
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European Centre for Nuclear Research (CERN)
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LEP Detector
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Discovery of Top Quark
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Unification of the Four Forces

• Electricity Magnetism Electromagnetism
• Electromagnetism Weak Electroweak
• Electroweak Strong Grand Unified Theory
  (GUT)
• GUT Gravity Theory of Everything (TOE)??

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Gravity

• Due to geometry rather than exchange of particles
• Remains impossible to incorporate within the
  framework of quantum field theory like the other
  forces

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Quantum Gravity?

• Does space-time fluctuate ina quantum fashion at
  very small scales?
• Called space-time foam
• How to describe this mathematically?

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Superstring Theory

• Best candidate for a TOE is superstring theory
• Assumes all fundamental particles are actually
  string-like objects rather than point-like
  objects
• Different particles correspond to different
  vibrational modes of the same string

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Point particles and strings interact differently
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Extra Dimensions?

• Superstring theory predicts that space-time is
  10-dimensional (9 space 1 time)
• The extra 6 dimensions are thought to be rolled
  up to a very small size