Download The beginning of physics

The beginning of physics
IoP Physics Update 2008
Forces and fields
Particles and symmetries
Cosmology
The Large Hadron Collider
Particle detectors
A Ring-Imaging Cherenkov detector
A Cosmic Ray detector
Forces – A particle physicist’s view
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Familiar electric, magnetic and
gravitational forces are described by
separate theories
Classical picture
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Steve Wotton
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
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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.
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Unification of forces
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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.
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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?
 We’d like to but it is HARD.
p
n
 Is there anything else?
e-
W-
 Yes.
 The weak nuclear force (beta
decay).
 The strong nuclear force
(binds protons and neutrons
in nuclei).
ne
 Build on the success of QED
 New ingredients
 New interactions
 New particles
 New diagrams
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e-
Z0 , g
e-
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Particle zoo
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Steve Wotton
The quest for the elements
 A search for order
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Earth, air, fire and water
The chemical elements
 Periodic table
 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
objects.
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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
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Duplication – why?
Mass hierarchy – how?
Anti-matter – where?
Gravity – still mysterious.
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Where we are
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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.
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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
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Higgs – the solution or the problem? LHC – the kill or cure?
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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
exceed 1).
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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.
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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).
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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.
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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.
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Detection techniques
Steve Wotton
 A discussion of techniques used in particle detectors…
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Ring-Imaging Cherenkov Detectors
Steve Wotton
 Identifying particles using
Cherenkov radiation…
 Cerenkov worksheet
 RICH animator
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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
 Radio-isotopes production in accelerators
 Proton cancer therapy
 Security scanning
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Image Intensifiers
X-ray
Gamma-ray
Cosmic-ray
 Non-destructive testing
 Cosmic rays or neutrinos (“X-raying” the Pyramids)
 Fragile/valuable objects
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Paintings
Sculptures
Wine
Archaeological remains
 Engineering
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