Transcript Rinormalizzazione, interazioni forti, sezione d'urto, decadimenti e risonanze (Cap. 2 - 2)

Interaction of particles
with matter
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Particle detection
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Heavy charged particles
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Mass thickness
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Energy loss by ionization:
Bethe-Bloch formula
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Bethe-Bloch
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Penetration Depth
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Range
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Tracking Detectors
Fitted Track
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Fast electrons
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Interactions of fast electrons:
Bremsstrahlung
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Energy loss by radiation
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Bremsstrahlung
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Interactions of photons
with matter
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Photon properties
Relation between particle and wave properties of light
E  h
Energy and frequency
Also have relation between momentum and wavelength
E  p c m c
Relativistic formula relating
energy and momentum
For light
2
E  pc
and
2 2
2 4
c  
h
p 

c
h
Also commonly write these as
E 
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p k
  2
angular frequency
wavevector
k
2

h

2
hbar
Photoelectric Effect
When UV light is shot on a metal plate in a
vacuum, it emits charged particles (Hertz 1887),
which were later shown to be electrons by J.J.
Thomson (1899).
Hertz
J.J. Thomson
Classical expectations
Light, frequency ν
Vacuum
chamber
Collecting
plate
Metal
plate
Electric field E of light exerts force F=eE on electrons. As intensity of light
increases, force increases, so KE of
ejected electrons should increase.
Electrons should be emitted whatever
the frequency ν of the light, so long as
E is sufficiently large
I
Ammeter
Potentiostat
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For very low intensities, expect a time
lag between light exposure and
emission, while electrons absorb enough
energy to escape from material
Einstein
Actual results:
Maximum KE of ejected electrons is
independent of intensity, but dependent
on ν
For ν<ν0 (i.e. for frequencies below a
cut-off frequency) no electrons are
emitted
There is no time lag. However, rate
of ejection of electrons depends on
light intensity.
Einstein’s interpretation
(1905):
Light comes in packets of
energy (photons)
E  h
Millikan
An electron absorbs
a single photon to
leave the material
The maximum KE of an emitted electron is then
K max  h  W
Planck constant: universal
constant of nature
h  6.63 1034 Js
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Work function: minimum
energy needed for electron to
escape from metal (depends
on material, but usually 2-5eV)
Verified in detail
through
subsequent
experiments by
Millikan
Compton Scattering
Compton
Compton (1923) measured intensity of
scattered X-rays from solid target, as function
of wavelength for different angles. He won
the 1927 Nobel prize.
X-ray source
Collimator
(selects angle)
Crystal
(selects
wavelength)
θ
Target
Result: peak in scattered radiation
shifts to longer wavelength than source.
Amount depends on θ (but not on the
target material).
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Detector
A.H. Compton, Phys. Rev. 22 409 (1923)
Classical picture: oscillating electromagnetic field causes
oscillations in positions of charged particles, which re-radiate in all
directions at same frequency and wavelength as incident radiation.
Change in wavelength of scattered light is completely
unexpected classically
Incident light wave
Oscillating electron
Emitted light wave
Compton’s explanation: “billiard ball” collisions between
particles of light (X-ray photons) and electrons in the material
Before
After
p 
scattered photon
Incoming photon
p
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θ
Electron
pe
scattered electron
Before
After
p 
scattered photon
Incoming photon
θ
p
Electron
pe
Conservation of energy
h  me c  h    p c  m c
2
2 2
e

2 4 1/ 2
e
scattered electron
Conservation of momentum
hˆ
p  i  p   p e

From this Compton derived the change in wavelength
h
   
1  cos 
me c
 c 1  cos    0
h
c  Compton wavelength 
 2.4  1012 m
me c
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Pair production
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A collider experiment
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Z  e e 
Z   
Z  qq
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