# PU The beginning of physics IoP Physics Update 2008

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```					The beginning of physics

IoP Physics Update 2008

Forces and fields
Particles and symmetries
Cosmology
Particle detectors
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
separate theories
   Classical picture
   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
square law.
   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
forces.
   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
Maxwell’s equations.
 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
carrying particle.
 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
problem

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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?
W-                e-
 Yes.
 The weak nuclear force (beta
decay).
 The strong nuclear force                                       ne
(binds protons and neutrons
in nuclei).
 Build on the success of QED                       e-

 New ingredients
 New interactions                                       Z0 , g

 New particles
e-
 New diagrams

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
 Periodic table
 Patterns are due to a set of
quantisation rules that must be
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
objects.
   The chart of the nucleides
 A periodic table for nuclei
 Patterns also due to quantisation
rules
 A complicated picture simplified…

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Particle zoo 2                                  Steve Wotton

 Many different particles can be
created in the lab.
 A complicated picture but we can
discern patterns.
 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
electron neutrinos.
 All forces (except gravity) mediated
by photon, Z boson, W boson and
gluon.
 But there are problems
   Duplication – why?
   Mass hierarchy – how?
   Anti-matter – where?
   Gravity – still mysterious.
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Where we are                                          Steve Wotton

   The Standard Model.
 Describes all known particles and their
electroweak and strong interactions.
 No significant deviations from SM observed
to date.
 Observed differences between forces due to
non-exact symmetry.
 Possibly the best physical theory in the
history of physics.
   But…
 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
problem
 Supersymmetry
 Extra dimensions
 We don’t know how to include gravity
 We don’t know the origin of symmetry
breaking
 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

mass range accessible to the LHC…
 Breakdown of Standard Model.
 Violation of unitarity in WW scattering for
large mH (sum of probabilities cannot
exceed 1).
   If the Higgs particle is found at the LHC…
 Low mass (compared to Planck mass)
implies a convenient cancellation of large
terms.
 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
interacting).

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)

LHC analogies:
A time machine
that recreates
conditions of
early universe.
A microscope
that sees objects
smaller than can
be seen with
light.

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.
 May explain:
 Dark matter.
 Matter-antimatter asymmetry.
 Mechanisms driving evolution of
early universe.
 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
 Cerenkov worksheet
 RICH animator

14-15th December 2008                       12
Cosmic Ray detection            Steve Wotton

 A practical demonstration
of detection of high
energy particles in the
classroom…

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Other connections                                             Steve Wotton

 Medical imaging
 Photomultiplier tubes
 MRI
 Proton cancer therapy
 Security scanning
   Image Intensifiers
   X-ray
   Gamma-ray
   Cosmic-ray
 Non-destructive testing
 Cosmic rays or neutrinos (“X-raying” the Pyramids)
 Fragile/valuable objects
   Paintings
   Sculptures
   Wine
   Archaeological remains
 Engineering

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