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Topic 7.3 The structure of matter
Ernst Rutherford decided to probe
the atom using fast moving alpha
(α) particles.
He got his students Geiger and
Marsden t...
Most of the α particles passed‑
straight through the foil, but to
his surprise a few were scattered
back towards the sourc...
Probing matter
Rutherford soon realised that the
positive charge in the atom must
be highly concentrated to repel
the posi...
The ball is rolled towards the hill
and represents the a particle.‑
The steeper the `hill' the more
highly concentrated th...
• In 1911 Rutherford described his
nuclear model of the atom. He
said that:
• All of an atom's positive charge
and most of...
The Nuclear Model of
the atom
Can we use this model to explain the
α particle scattering?‑
The concentrated positive charge
produces an electric field w...
Most α particles are hardly‑
deflected because they are far
away from the nucleus and the
field is too weak to repel them
...
Using this model Rutherford
calculated that the diameter of the
gold nucleus could not be larger than
10-14
m.
This diagra...
History of Constituents of
Matter
AD
What makes a particle “elementary” ?
A particle is
elementary if it has
no inner structure
(i.e not “made” of
some even sm...
Which particles were considered
elementary throughout History?
1911 : Rutherford discovers the nucleus.
Transmutation reac...
This was the “simplest” elementary
particle set ever described. A small
number of particles, a small number of
interaction...
However, some problems were already
present.
1. The photon : Photoelectric effect ;
Compton scattering.
2. Antiparticles :...
3. Mesons : These particles were first postulated
by Yukawa (1935) to explain the force that binds
the nucleus together. B...
In 1947, pions (mesons) were detected in cosmic
rays. They were thought of as Yukawa’s mediator
particle for the strong in...
Moreover, some rules seemed to be missing to predict if a
decay could occur or not :
Why is π-
+ p+
 K+
+ Σ-
possible ,
W...
The standard model
The Quark Model (1964)
u
d
¯u ¯d
s
¯s
S=0
S=-1
S=1
Q=2/3Q=-1/3
Q=-2/3 Q=-1/3
υ0
0
0
1
60
28
60
27 ++→ − eNiCo
Q = -1e almost all trapped in atoms
Q= 0 all freely moving through universe
The Baryon Octet
n p
Σ+
Ξ0
Ξ-
Σ-
Σ0 ; Λ
S=0
S=-1
S=-2
Q=0
Q=1
Q=-1
The Meson Octet
K0
K+
π+
¯K0
K-
Π-
π0 ; η
S=1
S= 0
S= -1
Q=0
Q=1
Q=-1
Conservation Law – use
of…
What must be conserved in a
reaction or decay?
• Mass/Energy
• Momentum
• Charge
• Baryon numbe...
n  p + e + ν
 
p + ν  n + e
 
p + π -
 p + π0
 
p + p  p + p + π0
The Four Fundamental Forces
From www.pbs.org
Feynman diagrams
Drawing Feynman
Diagrams
Each vertex has an arrow going in and
one going out.
These represent a lepton – lepton or
quark-q...
Drawing Feynman
Diagrams
Arrows from left to right represent
particles moving forward in time.
Arrows from right to left r...
What is happening here?
Now you try to construct:
•Beta decay
•Proton-electron interaction
Confinement
Quarks CANNOT be found alone.
This is known as the “quakr
confinement” – that is quarks
cannot be observed in ...
Higgs boson
The Higgs boson is a sub-atomic particle
that acts as the intermediary between
the Higgs field and other particles. All
fi...
Finding the Higgs boson
confirmed that the Higgs field
exists, and that field enables
mass to be explained
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7.3

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7.3

  1. 1. Topic 7.3 The structure of matter
  2. 2. Ernst Rutherford decided to probe the atom using fast moving alpha (α) particles. He got his students Geiger and Marsden to fire the positively charged‑ α particles at‑ very thin gold foil and observe how they were scattered. The diagram summarises his results
  3. 3. Most of the α particles passed‑ straight through the foil, but to his surprise a few were scattered back towards the source. Rutherford said that this was rather like firing a gun at tissue paper and finding that some bullets bounce back towards you!
  4. 4. Probing matter Rutherford soon realised that the positive charge in the atom must be highly concentrated to repel the positive a particles in this‑ way. The diagram shows a simple analogy:
  5. 5. The ball is rolled towards the hill and represents the a particle.‑ The steeper the `hill' the more highly concentrated the charge. The closer the approach of the steel ball to the hill, the greater its angle of deflection. TOK
  6. 6. • In 1911 Rutherford described his nuclear model of the atom. He said that: • All of an atom's positive charge and most of its mass is concentrated in a tiny core. • Rutherford called this the nucleus. • The electrons surround the nucleus, but they are at relatively large distances from it. • The atom is mainly empty space!
  7. 7. The Nuclear Model of the atom
  8. 8. Can we use this model to explain the α particle scattering?‑ The concentrated positive charge produces an electric field which is very strong close to the nucleus. The closer the path of the α particle to‑ the nucleus, the greater the electrostatic repulsion and the greater the deflection. TOK
  9. 9. Most α particles are hardly‑ deflected because they are far away from the nucleus and the field is too weak to repel them much. The electrons do not deflect the α particles because the effect of‑ their negative charge is spread thinly throughout the atom.
  10. 10. Using this model Rutherford calculated that the diameter of the gold nucleus could not be larger than 10-14 m. This diagram is not to scale. With a 1 mm diameter nucleus the diameter of the atom would have to be 10 000 mm or 10 m!
  11. 11. History of Constituents of Matter AD
  12. 12. What makes a particle “elementary” ? A particle is elementary if it has no inner structure (i.e not “made” of some even smaller entities).
  13. 13. Which particles were considered elementary throughout History? 1911 : Rutherford discovers the nucleus. Transmutation reactions showed that the hydrogen nucleus played a specific role (4 2He + 14 7N --> 18 9F --> 17 8O + 1 1p) . Rutherford named it proton (protos = first) 1932 : Chadwick discovers the neutron, which is not stable when isolated, and decays as follows : n  p + e- (+ ¯νe). The proton, electron and neutron account for all the atoms of all the elements in the Universe.
  14. 14. This was the “simplest” elementary particle set ever described. A small number of particles, a small number of interactions. LEPTON (leptos = light) : e- BARYONS (baryos = heavy) : p , n
  15. 15. However, some problems were already present. 1. The photon : Photoelectric effect ; Compton scattering. 2. Antiparticles : Discovery of the positron by Anderson (1932), studying cosmic rays. Many more particles would be discovered in cosmic rays…
  16. 16. 3. Mesons : These particles were first postulated by Yukawa (1935) to explain the force that binds the nucleus together. Being of intermediate masses, they were called mesons (mesos = middle). 4. Neutrinos : Necessary to preserve E conservation in β decay From the particle garden to the jungle : In 1937, Anderson discovered the muon μ. The μ proved to be some sort of heavier electron (lepton). The muon decays into through β decay: μ νμ + e- +¯νe Who ordered THAT ? I.I Rabi, Nobel 1944
  17. 17. In 1947, pions (mesons) were detected in cosmic rays. They were thought of as Yukawa’s mediator particle for the strong interaction. The Universe was in order again, except for the muon, which played no visible role. In December 1947, new mesons were found : the kaons. The place got crowded again… With the use of particle accelerators in the 50’s, many new particles were discovered. Some of them were « strange » because they were produced by the strong force but decayed through the weak force.
  18. 18. Moreover, some rules seemed to be missing to predict if a decay could occur or not : Why is π- + p+  K+ + Σ- possible , When π- + p+  K0 + n is impossible ? In 1953, Gell-Mann and Nishijima came with a simple and elegant idea. Each particle was to be assigned a «strangeness », and the overall strangeness had to be conserved during a collision (not through decay). There were then THREE laws of conservations for reactions : Charge Baryonic number (proton like particles) Strangeness
  19. 19. The standard model
  20. 20. The Quark Model (1964) u d ¯u ¯d s ¯s S=0 S=-1 S=1 Q=2/3Q=-1/3 Q=-2/3 Q=-1/3
  21. 21. υ0 0 0 1 60 28 60 27 ++→ − eNiCo Q = -1e almost all trapped in atoms Q= 0 all freely moving through universe
  22. 22. The Baryon Octet n p Σ+ Ξ0 Ξ- Σ- Σ0 ; Λ S=0 S=-1 S=-2 Q=0 Q=1 Q=-1
  23. 23. The Meson Octet K0 K+ π+ ¯K0 K- Π- π0 ; η S=1 S= 0 S= -1 Q=0 Q=1 Q=-1
  24. 24. Conservation Law – use of… What must be conserved in a reaction or decay? • Mass/Energy • Momentum • Charge • Baryon number • Lepton number • Strangeness • colour
  25. 25. n  p + e + ν   p + ν  n + e   p + π -  p + π0   p + p  p + p + π0
  26. 26. The Four Fundamental Forces From www.pbs.org
  27. 27. Feynman diagrams
  28. 28. Drawing Feynman Diagrams Each vertex has an arrow going in and one going out. These represent a lepton – lepton or quark-quark transition. Quarks or leptons are solid straight lines Exchange particles are either wavy (Photons, W, Z) or curly (gluons). Time usually flows from left to right.
  29. 29. Drawing Feynman Diagrams Arrows from left to right represent particles moving forward in time. Arrows from right to left represent antiparticles moving forward in time. (think of them as moving left to right). Vertices are linked by a line representing an exchange particle Charge and colour are conserved at each vertex.
  30. 30. What is happening here? Now you try to construct: •Beta decay •Proton-electron interaction
  31. 31. Confinement Quarks CANNOT be found alone. This is known as the “quakr confinement” – that is quarks cannot be observed in isolation.
  32. 32. Higgs boson
  33. 33. The Higgs boson is a sub-atomic particle that acts as the intermediary between the Higgs field and other particles. All fields are mediated by bosons, some of which pop into and out of existence depending on the state of the field, sort of like how rain drops emerge out of a cloud when it reaches a certain point.
  34. 34. Finding the Higgs boson confirmed that the Higgs field exists, and that field enables mass to be explained

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