0. Introduction / What’s the story?
1. Structure of the atom
2. Probing deeper / cosmic rays
and particle accelerators
0. Introduction / What’s the
story?
In search of giants with Brian
Cox j.mp/WgjY6h>>
Fundamental particles
What holds the nucleus together?
Quarks
Quarks have an attribute arbitrarily named colour that feels the strong
force. Virtual gluons carry colour between quarks so that the quark
colours are continuously changing. All observed particles are colourless so
in a baryon there must be three continuously changing but different colours
and in a meson a quark and an antiquark so that a colour and its anticolour again give a colourless particle.
Colourlessness or containment means that isolated quarks and gluons
should never be observed.
Although pentaquarks [𝒒𝒒𝒒𝒒𝒒] and glueballs [confined gluons]
might seem possible they have not been reliably observed.
~10-10m
~10-14
~10-15m
Strong force range ~10-15m
The strong force acts only
over very short distances
that separate the quarks in
a nucleon or in the adjacent
nucleons of a nucleus.
The four fundamental forces
Virtual particles mediate the fundamental forces
1. Structure of the atom
Rutherford scattering
High energy alpha particles
from a natural radioisotope
bombard a gold foil [only a few
atoms thick].
Most particles pass through
gold with little or no deflection.
Very small number undergo
huge deflections [>90O]
Rutherford scattering [cont.]
Provides strong evidence for a
nuclear atom [1911].
Electrons had been discovered
by JJ Thompson [1896].
The neutron was confirmed by
Chadwick [1932].
2. Probing deeper / cosmic rays and particle
accelerators
Electron ray gun
Electrons “boiled off” the
cathode and accelerated
toward the anode. Can only
be explained in terms of
negatively charged
electrons.
Moving e’s can be deflected
by E- and/or B- field.
Electron diffraction
Electrons behave as waves.
De Broglie…
𝑝
𝜆=
ℎ
Electrons diffract as they pass
though solids revealing the
atomic/molecular spacing and
arrangements.
Linac [linear accelerator]
Charged particles
accelerated in E-fields
in gaps between drift
tubes.
Polarity of tubes must
alternate. Why?
Tubes get longer. Why?
𝑛𝑉𝑄 = ΔΕΚ
𝐹 = 𝐸𝑄
𝑉
𝐸≈
𝑑
Relativistic effects
At high speeds relativistic effects are significant and must be
taken into account.
Cyclotron
E-field accelerates charged
particles across gap between
dees.
B-field provides centripetal
acceleration…
2
𝑚𝑣
𝐹 = 𝐵𝑄𝑣 =
𝑟
𝑝
𝐵𝑄 =
𝑟
𝑝
𝑟=
𝐵𝑄
Cosmic rays
Cosmic rays contain particles that were not just protons,
neutrons or electrons. Soon there was a zoo of so-called
fundamental particles.
This is somewhat like the situation in the 19th century when the
number of elements in the periodic table was >80. Rather than
having >80 different atoms surely there was something more
fundamental than the atom.
The standard model
Each generation of particles have
the same attributes [or quantum
numbers] except increasing mass.
The t and b quarks are only created
in extremely high energy events and
were predicted for reasons of
symmetry, i.e. so that the quarks
have three generations as do the
leptons.
Each particle has a corresponding
antiparticle.
The large hadron collider [LHC] / CERN
E-fields increase speed of charged
particles. B-fields accelerate particles to
keep them moving in a circle of
constant r.
In collisions…
ΔE
𝑚= 2
𝑐
Colliding beams of particles moving in
opposite directions transfer more
energy to create particles than a single
beam hitting a stationary target. Why?
Conservation laws
In all interactions…
• charge
• mass-energy
• momentum
• colour
• baryon number
• lepton number
…conserved
In the weak interaction quark
flavour is not conserved.
Non SI units / convenience
1 electron-volt [eV] is the work done to a particle with a charge
of 1e [1.6x10-19C] passing through a pd of 1V.
E = QV = 1.6x10-19C x 1V = 1.6x10-19J
1keV =
1MeV =
1GeV =
103 x 1.6x10-19J =
106 x 1.6x10-19J =
109 x 1.6x10-19J =
1.6x10-16J
1.6x10-13J
1.6x10-10J