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Contents
Main topic
Sub topic
Page
1a. Elementary particles
1b. Three classes of observed particles
1c. Identifying elementary particles
1d. Mass and quantum numbers of particles
1e. Spin
1f. Antiparticles
1g. Pauli’s Exclusion Principle
1h. The four fundamental interactions
1i. Exchange particles
1j. Heisenberg’s Uncertainty Principle
1k. Feynman diagrams
1l. Virtual particles
1m. Interaction range
1n. Pair production and annihilation
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3
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2a. High energy for mass
2b. High energy for size
2c. Linear accelerator
2d. Cyclotron
2e. Synchrotron
2f. Bremsstrahlung braking radiation
2g. Pros and cons of accelerators
2h. Photomultiplier
2i. Bubble chamber
2j. Wire chamber
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3a. Structure in terms of quarks
3b. The need for colour
3c. Strangeness
3d. Quark confinement
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4a. The three classes of fundamental particles
4b. Conservation laws
4c. The Higgs boson
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24
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5a. The eight-fold way
5b. Deep inelastic scattering
5c. Asymptotic freedom
5d. Neutral current
5e. Evidence for the standard model
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26
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6a. Temperature and the Big Bang
6b. Early particle interactions
6c. Matter vs antimatter
6d. String theory
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1. Particles and interactions
2. Particle accelerators and detectors
3. Quarks
4. Leptons and the standard model
5. Experimental evidence
6. Cosmology and strings
7. Common exam mistakes
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8. Questions
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9. Answers
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2
Option J Particle physics
J1 Particles and interactions
1a. Elementary particles
-an elementary or fundamental particle is one that is not composed of anything smaller.
-there are three types of elementary particles, leptons, quarks and exchange bosons.
-leptons are particles that feel the weak force but do not feel the strong (colour) force.
-the electron is the most well known lepton.
-there are six types of leptons: electrons, muons and tauons and each has its own
neutrino, the electron-neutrino, mu-neutrino and tau-neutrino.
-each lepton has its antiparticle (see below), the positron, anti-muon, anti-tauon and the
antiparticle of each neutrino (anti-neutrino).
-quarks feel the strong force, or now more correctly named the colour force.
-there are six types of quarks; up, down, strange, charm, top and bottom.
-each quark has an antiparticle.
-leptons and quarks split into three generations, electron, muon and tauon.
-each generation contains two quarks and two leptons (the lepton and its neutrino).
-particles in each generation have a counterpart in the other generations.
-each counterpart has the same charge.
-the masses of counterpart particles increase down the generations.
-all fundamental particles are shown in the table below:
name of particle
electron
electron neutrino
down
up
muon
muon neutrino
strange
charm
tauon
tauon neutrino
bottom
top
type of
particle
lepton
lepton
quark
quark
lepton
lepton
quark
quark
lepton
lepton
quark
quark
symbol
charge
e
νe
d
u
µ
νµ
s
c
τ
ντ
b
t
-1
0
-1/3
+2/3
-1
0
-1/3
+2/3
-1
0
-1/3
+2/3
Lepton
number
Le = 1
Le = 1
0
0
Lµ = 1
Lµ = 1
0
0
Lτ = 1
Lτ = 1
0
0
generation
electron
electron
electron
electron
muon
muon
muon
muon
tauon
tauon
tauon
tauon
Mass
(approx)
511 keV
0.03 eV ?
0.35GeV
0.35GeV
106 MeV
< .27MeV
100 MeV
1.2 GeV
1.78GeV
< 31MeV
4 GeV
170 GeV
-note masses for quarks are approximate.
-note the antiparticles have the opposite charges and lepton numbers.
-however they will all have the same masses as their matter counterparts.
-gauge bosons or exchange particles (or force carriers) carry the fundamental forces.
-they include the photon in the Coulomb force, gravitons in gravity, the W and Z particles
in the weak force and gluons in the strong force.
-note the graviton is predicted by theory, but has yet to be discovered, though there is
indirect evidence (eg the binary pulsar).
-note there is another particle called the Higgs boson that might also be fundamental.
-for clarity these particles will be referred to mainly as gauge bosons.
Task 1 : Okay let’s see if you’re sober enough to read from the above table. Give
the name for each of these fundamental particles:
1. it has a charge of -1/3 and a mass of 4GeV ……………………
2. it is from the muon generation and has no charge ………………
3. it has a charge of +1 and mass of 511keV ………………………..
4. it is a quark with a mass 1.2GeV………………………………..
answers shown below
3
ms-1 (but ignoring relativity), it would not travel far before disappearing: about 2x10-13m,
which is barely a fraction the distance across an atom!
-note the Uncertainty Principle can also be applied to the momentum and position of a
particle. If the uncertainty in the momentum is ∆mv and the uncertainty in position is ∆r
then
h
4π
∆mv. ∆r ≥
-this variation of the Principle is useful when finding how far a short half life particle will
travel before it decays, ∆r being the range and the velocity is taken as the speed of light
c. (See later).
Key points
-two non-commuting operators of a particle cannot be both known
with perfect accuracy.
-this is Heisenberg’s Uncertainty Principle.
-this is a consequence of quantum mechanics.
1k. Feynman Diagrams
-particle interactions are shown in Feynman diagrams.
-time is taken as the horizontal axis, increasing left to the right.
-leptons, quarks or hadrons are shown as arrows to the right.
-antimatter particles are shown as arrows pointing to the left
-note: not all books show the backward pointing arrow.
-exchange particles (gauge bosons) are shown as wavy lines between them.
-e.g. take two electrons approaching each other.
-they are first shown as two arrows converging.
-a photon is exchanged; the two arrows are then shown diverging.
-suppose a proton and electron are considered.
-a photon is passed between them causing attraction.
-both situations are shown in the diagrams below.
charge repulsion
charge attraction
proton
electron
photon
photon
electron
electron
time
time
-gravity is attractive so a Feynman diagram resembles charge attraction.
-the graviton is the exchange particle between masses (the gauge boson).
-this is shown in the diagram below left. Note the quantum theory of gravity
is still incomplete, but the reaction is expected to be like this.
gravitational attraction
the weak nuclear interaction
p+
graviton
eW-
two masses
time
n
time
-for the weak interaction, there are three exchange particles:
9
⎯ν
Task 3 answers
1. elementary/
fundamental.
2. 3. quarks,
leptons
4.5. exchange
particles or gauge
bosons
6. fundamental
7.8.9.10. gravity,
electromagnetic,
weak, strong.
11.12. weak,
electromagnetic.
13. electroweak.
14. gravitons.
15. photons.
16. W and Z.
17. meson but at
a deeper level
gluons.