The beginning of physics
IoP Physics Update 2008
Forces and fields
Particles and symmetries
The Large Hadron Collider
A Ring-Imaging Cherenkov detector
A Cosmic Ray detector
Forces – A particle physicist’s view Steve Wotton
Familiar electric, magnetic and
gravitational forces are described by
Action at a distance – two bodies feel a
mutually attractive or repulsive force even
though separated by large distances
Introduce abstract concept of a field – the
strength and direction of the force felt by a
test body is uniquely defined at every
point in space
Strength of force from an idealised point
body decreases according to an inverse
We draw pictures of the fields as if they
had a physical existence (a prejudice
reinforced by e.g. iron-filings aligning
along magnetic field lines)
Quantum field theory view
Forces are due to the exchange of (virtual)
particles that carry momentum
Strength of the force is determined by the
coupling to the force carrying particle. The
strength is (very) different for the known
A consistent mathematical description is
formulated by combining quantum
mechanics and relativity.
14-15th December 2008 2
Unification of forces Steve Wotton
Electricity and magnetism once
considered separate phenomena.
Now unified and enshrined in
Electric and magnetic forces are
merely different manifestations of the
same underlying mechanism.
By changing our viewpoint, an electric
field can become a magnetic field (and
vice versa). Relativity at work.
Add an extra ingredient – Quantum
Mechanics – we get Quantum
Electrodynamics. The photon
(quantum of light) is the force
A very, very, very, very, very, very,
very, very, well-tested theory.
Feynman expressed the calculations
in pictures (Feynman diagrams)
Each diagram represents a
mathematical term in the solution to a
14-15th December 2008 3
Force unification – the next step Steve Wotton
Can we include gravity? p
We’d like to but it is HARD. n
Is there anything else?
The weak nuclear force (beta
The strong nuclear force ne
(binds protons and neutrons
Build on the success of QED e-
New interactions Z0 , g
14-15th December 2008 4
Particle zoo Steve Wotton
The quest for the elements
A search for order
Earth, air, fire and water
The chemical elements
Patterns are due to a set of
quantisation rules that must be
followed when adding more
electrons to an atom.
A complicated picture (many
elements with different properties)
simplified by applying a set of rules
to build elements from a small
number of more fundamental
The chart of the nucleides
A periodic table for nuclei
Patterns also due to quantisation
A complicated picture simplified…
14-15th December 2008 5
Particle zoo 2 Steve Wotton
Many different particles can be
created in the lab.
A complicated picture but we can
Must be due to an underlying
theory that combines a smaller
number of more fundamental
particles using a set of rules.
The fundamental particles
All ordinary matter made of up
quark, down quark, electrons and
All forces (except gravity) mediated
by photon, Z boson, W boson and
But there are problems
Duplication – why?
Mass hierarchy – how?
Anti-matter – where?
Gravity – still mysterious.
14-15th December 2008 6
Where we are Steve Wotton
The Standard Model.
Describes all known particles and their
electroweak and strong interactions.
No significant deviations from SM observed
Observed differences between forces due to
Possibly the best physical theory in the
history of physics.
We are still waiting for the Higgs boson
We don’t understand the origin of mass
We don’t know how to solve the hierarchy
We don’t know how to include gravity
We don’t know the origin of symmetry
We do know that The Standard Model must
break down at TeV energies
14-15th December 2008 7
Higgs – the solution or the problem? LHC – the kill or cure? Steve Wotton
If the Higgs particle is not found in the
mass range accessible to the LHC…
Breakdown of Standard Model.
Violation of unitarity in WW scattering for
large mH (sum of probabilities cannot
If the Higgs particle is found at the LHC…
Low mass (compared to Planck mass)
implies a convenient cancellation of large
Or there must be new physics.
Supersymmetry is a favourite candidate
for new physics (symmetry is good, more
is better) but...
Requires new particles to exist (Who
ordered that?, Rabi).
Properties must explain why they haven’t
been observed already (heavy, or weakly
14-15th December 2008 8
Cosmology 1 Steve Wotton
Looking back in time
Universe contains fixed amount of energy (mass and radiation)
Space is expanding, universe is cooling.
kT = hc/λ = eV relates temperature (T), length (λ), accelerating potential
(V) through fundamental constants k, h, c, e.
E.g 14TeV = 10-19m = 1017K (Note: size of proton = 10-15m)
A time machine
that sees objects
smaller than can
be seen with
14-15th December 2008 9
Cosmology 2 Steve Wotton
The LHC will recreate the conditions
of the early Universe.
Provides evidence of the processes
that are assumed to operate.
Mechanisms driving evolution of
The origin of mass.
The unification of all known forces.
14-15th December 2008 10
Detection techniques Steve Wotton
A discussion of techniques used in particle detectors…
14-15th December 2008 11
Ring-Imaging Cherenkov Detectors Steve Wotton
Identifying particles using
14-15th December 2008 12
Cosmic Ray detection Steve Wotton
A practical demonstration
of detection of high
energy particles in the
14-15th December 2008 13
Other connections Steve Wotton
Radio-isotopes production in accelerators
Proton cancer therapy
Cosmic rays or neutrinos (“X-raying” the Pyramids)
14-15th December 2008 14