Scattering Experiments

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Scattering Experiments

• Rutherfords first critical experiments showed
  that atoms consisted of a massive, tiny nucleus
  surrounded by some electrons
• Almost all the mass of the atom was concentrated
  in a region of size 10-15 m
• His probing particles were alphas from
  radioactive decay of radium

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Scattering Experiments

• The detail provided by smashing a probe into
  microscopic particles became the standard for
  unraveling the mysteries of the nucleus
• Using radioactive elements provided a range of
  probe particles, but the energies of the probe
  were limited
• Cosmic rays provided another source of energetic
  particles to be used as probes, but available
  numbers of particles were limited

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Scattering Experiments

• It was time to build machines to allow
  experimenters to accelerate whatever probe one
  chose to whatever energy one wanted
• Thus was born the arena of high energy physics

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High Energy Particles

• We want to probe the nucleus
• Since our probes are particles, we need to know
  their wavelengths to determine how finely we can
  resolve structures
• Recall that resolution cannot be better than
  about a wavelength of the probe

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High Energy Particles

• Consider electrons as a probe
• The Stanford Linac generates 50 GeV electrons
• This is 100,000 times greater than rest mass
  energy of an electron

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High Energy Particles

• Thus the electron speed is essentially c
• Compute its wavelength

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High Energy Particles

• So the probes wavelength is smaller than a
  nucleus, so we can probe the nucleus
• Need mechanisms to accelerate particles
• Van de Graff
• Cyclotrton
• Synchrotron
• Linear Accelerators
• Colliding Beams

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Van de Graff Accelerator
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Cyclotron
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Cyclotron
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Synchrocyclotron

• As the particles go faster, their mass increases
  and the frequency must be reduced to stay in
  synch
• This is a synchrocyclotron
• Alternative is to increase B as the particles
  speed up
• This is called a synchrotron and is the current
  big machine favorite

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Linacs

• Alternate voltages on the tubes so particle is
  accelerated across the gap
• Increase tube length as particles go faster

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Colliding Beams

• Available energy is increased over fixed target
  and moving probe
• Look at center of mass of system and conservation
  laws

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Scattering Experiments

• Assumption was that atoms were composed of a
  nucleus (neutrons and protons) and electrons
• Send in a high energy probe (often protons) and
  see what happens
• Started seeing all kinds of new particles with
  differing masses and properties
• Also saw these from cosmic ray studies

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Elementary Particles

• So, what are the elementary particles from
  which all matter is made?
• How to explain what finally became a list of
  dozens of particles?
• What is the strong nuclear force and how does it
  work?
• What is the weak nuclear force?

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Strong Force

• In 1935, Yukawa proposed a mechanism for
  explaining the interaction between nucleons
• Kind of modeled on simple chemical bonds, but
  somewhat different
• Consider a hydrogen molecule, H2
• Electrons move from one proton to the other
• The atoms exchange electrons and this exchange
  results in creating the bond

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Strong Force

• A similar mechanism can explain the interaction
  between two charged particles
• Simply postulate the particles exchange photons!

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Strong Force

• Yukawa postulated that the interaction between
  nucleons occurred by exchanging a particle
• He knew that the range of the strong nuclear
  force only extended about 10-15 meters
• Where does the energy come from to create the
  mass of the exchange particle?

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Strong Force

• We have to borrow the energy using the
  uncertainty principle!!!
• That means the particle cant exist for very long
  since
• The maximum distance the particle can travel in
  this time is
• So lets put this together

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Strong Force
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Strong Force

• These particles (pions) were observed in cosmic
  ray studies and came in three different charge
  states, ?, ?-, ?0
• The plus and minus versions had mass 139.6 MeV
  and the neutral version had 135.0 MeV
• Close to the Yukawa prediction

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Strong Force