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Particles & Waves
The Standard Model
Orders of Magnitude
Human Scale
Distance
10-3 ~ 102 m
Time
100
Mass
100 ~ 102 kg
~
102 s
Measurable without additional
technology
Orders of Magnitude
Universal Scale
Distance
1026 m
Distance to furthest know celestial object
Time
1017 s
Time since big bang
Mass
1032 kg
Hyper star R136a1 (most massive known star)
1050 ~ 1060 kg
Estimated mass of the universe
Orders of Magnitude
Sub-Atomic Scale
10-10 m
Typical atom diameter
10-14 m
Typical nucleus diameter
10-15 m
Proton/neutron diameter
10-18 m
Electron diameter
Time
10-22 s
Time for photon to cross the nucleus
Mass
10-27 kg
Mass of proton/neutron
10-31 kg
Mass of electron
Distance
Particle Physics
1897 – J.J. Thompson discovers the electron.
Early 1900’s – Structure of the atom was probed with the
aid of newly discovered radiations (α, β and γ).
Thompson’s plum pudding
model – where both positive
and negative charges were
evenly distributed throughout
atom was eventually
disproved by Rutherford.
1909 – Rutherfords α scattering experiment
NOTE
Particle Physics
1909 – Rutherfords α scattering experiment
Rutherford bombarded a thin gold leaf with a beam of alpha particles.
The experimental set-up is shown in the diagram below. A collimated beam of
alpha particles from a radium source is fired at a thin film of gold.
Particle Physics
When an alpha particle strikes the zinc sulphide screen a
flash of light is produced. The number of flashes, and
hence the number of alpha particles, can be counted by
observing the screen through a microscope.
Most of the beam travelled straight through but some of
the alpha particles were deflected through various angles
and a few were actually deflected through large angles,
i.e., back the way they had come.
From these results, Rutherford suggested that although
the atom occupied a certain volume, most of the volume
was space and all the mass of the atom was concentrated
in a small centre core or nucleus which was positively
charged. Spinning around this nucleus at the extremity of
the atom were the electrons.
When an alpha particle came very close to a nucleus, the
repulsion between the positively charged alpha particle
and the positively charged nucleus caused the alpha
particle to be deflected. The closer the alpha particle is to
the nucleus, the bigger the deflection of the beam. An
alpha particle is deflected through a large angle when it
makes a head-on collision with a nucleus.
Particle Physics
1928 - 1932 – Discovery of Anti-matter (Paul Dirac)
Positron discovered which is identical to electron but with
opposite charge.
Evidence for anti-matter
CLOUD CHAMBER
with magnetic field
Positron has the
exact same path
but curves in the
opposite
direction
Electrons will
display a
characteristic
‘curl’ in a cloud
chamber
This shows these two particles have the
same momentum but have equal and
opposite charges
young, hot, energetic
old, cool, less energetic
Particles discoveries take off…
Standard Model
The standard model is the extent of our current
understanding of the nature of matter.
Standard Model Diagram
Standard Model Diagram
‘Particle Zoo’
Fermions are matter particles and are in two classes:
Quarks: fundamental “heavy” particles.
Leptons: fundamental “light” particles.
Bosons are force mediating particles
Matter Particles
These are what
protons and
neutrons are
made of
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Matter Particles
These form a
cloud around
atomic nucleii
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Matter Particles
Produced by
cosmic rays
A few hundred of these pass through your body every second
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Matter Particles
These come from
nuclear reactions in
the sun, radioactive
decays, etc.
A few billion of these pass through
your body every second
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Matter Particles
First generation: these are
the only particles needed to
make all the matter we see;
all chemical elements
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Matter Particles
But we see three generations
• Undergoing similar interactions
• Mass hierarchy
• Each has an antiparticle
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Matter Particles
• Are there only three
generations?
• And if so why?
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Hadrons
Hadrons are composite particles made of quarks. There are two
types of hadrons:
Baryons – are made up of three quarks.
Proton
Neutron
2u + 1d
(+2/3) + (+2/3) + (-1/3) = +1
1u + 2d
(+2/3) + (-1/3) + (-1/3) = 0
Mesons – are made up of two quarks.
All hadrons MUST have an integer value of charge. (i.e. the sum
of the charges of the quarks must be an integer.)
Forces
In the Standard Model, we depict (and calculate) forces as the exchange of a
force-carrier boson, between particles
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Forces
In the Standard Model, we depict (and calculate) forces as the exchange of a
force-carrier boson, between particles
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Forces
Force
Range
(m)
Relative
strength
Guageboson
Example effects
Strong
10–15
1
gluon
Holding neutrons in
the nucleus
Weak
10–18
10-6
W+ , W - , Z
bosons
Beta decay; decay of
unstable hadrons
Electromagnetic
∞
1/137
photon
Holding electrons in
atoms
Gravitational
∞
10-39
Undiscovere
d
Holding matter in
planets, stars and
galaxies