Download Room: PHYS 238 Time: 9:00 10:15 Monday and Wednesday

First Lecture
August 20, 2007
Room: PHYS 238
Time: 9:00 10:15 Monday and Wednesday
Text: Halzen & Martin and/or Perkins
Decide for yourself if you want to buy one
Assignments: probably 4 or 5
Project: mini-symposium
See Physics 536 Fall 2005 web page for examples
Grading: 70% assignments, 30% project
No in-class exams
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Lectures will generally not be Power Point
You will need to take notes
This lecture is the exception
Lecture notes will usually be scanned and
made available on the web page:
http://www.physics.purdue.edu/~mjones/phys564
This doesn t mean you can get by without
coming to class...
Based on past experience
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Late assignment policy:
Late assignments get 80% credit.
Essential to hand them in even if you can t get them
done on time.
Ask for help if you get stuck.
Computer access:
You need an account on the PCN cluster
Let me know if you don t have one
Office hours:
Easiest to arrange a time by e-mail or after class.
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The field of Elementary Particle Physics
has developed in a natural progression
since the turn of the last century
These first two lectures attempt to provide
the historical context and examples of the
experiments that shape our current
understanding of the most fundamental
principles of nature.
(based on a seminar given to the Purdue REU summer students, July 13, 2007)
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5
How we got here and were we re going...
Recent history
Early particle physics
Interplay between:
Accelerators
Experiments
Theory
Our present understanding
Our present lack of understanding
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A vast array of natural
phenomena can be
explained by a small
number of simple rules.
One can determine what
these rules are by
observation and
experiment.
This is how science has
progressed since the
1700 s.
7
Nils Bohr described atomic
structure using early
concepts of Quantum
Mechanics
Albert Einstein extends the laws
of classical mechanics to
describe velocities that approach
the speed of light.
All matter should obey the laws of quantum mechanics and special relativity.
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In 1896, Thompson
showed that electrons
were particles, not a
fluid.
In 1905, Einstein
argued that photons
behave like particles.
In 1907, Rutherford
showed that the mass
of an atom was
concentrated in a
nucleus.
Particles that should obey the laws of quantum mechanics and relativity.
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, , emission
Properties of neutrons
Fission of heavy elements
Nuclear chemistry
Nuclear forces
Beta decay
Neutrino postulated
Theories of beta decay
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In 1932, Cockroft and Walton
accelerated protons to 600 keV,
produced the reaction
and verified E=mc2.
From 1930-1939, Lawrence
built bigger and bigger
cyclotrons, accelerating
protons to higher and higher
energies: 80 keV
100 MeV.
11
In 1912, Wilson develops the
cloud chamber for seeing the
paths of fundamental particles
Photographic emulsions
exposed by the passage of
charged particles
12
In 1912, Viktor Hess investigated
terrestrial radioactivity in balloon
experiments.
Penetrating radiation
observed at high altitudes
Solutions to Dirac s
equations interpreted as
positive electrons
Yukawa proposed a
meson to explain the
strong nuclear force
Anderson observed
positrons in 1932 and
muons in 1936
Perkins discovered pions
photographic emulsions in
1947.
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Get your own particle data book at http://pdg.lbl.gov.
14
1952: Brookhaven 3 GeV Cosmotron
1954: Berkeley 6 GeV Bevatron
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The Berkeley 72 inch liquid
hydrogen bubble chamber
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Mass
Strangeness
Electric charge
Spin
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Murray Gell-Mann:
George Zweig:
Physical meson states are
representations of the SU(3)
symmetry group:
Hadrons are composed of
more elementary objects:
Physical baryon states are
representations of the SU(3)
symmetry group:
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-
Observed in the 80 inch bubble chamber at Brookhaven in 1964.
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2 mile long, 30 GeV
electron accelerator
Analyzing magnets
Detector
Hydrogen target
People
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Electron
Proton
Used to measure the size of the proton.
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Electron
Proton
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Partons
Electron
Proton
Angular distribution consistent with
scattering from point-like spin ½ particles
inside the proton
Exactly the same as the Rutherford scattering experiment
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The SPEAR synchtrotron: 8 GeV
Simultaneously
electron-positron collider
at SLAC observed
at Brookhaven where it
was called the J .
To this
day
it is called
the tracked
... charged
The
Mark-I
detector
particles
usingquark
sparkwas
chambers.
The charm
heavy and non-relativistic.
Charmonium behaved like a hydrogen atom made of quarks.
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Discovered in 1977 at a 400
GeV fixed target experiment,
Fermilab E-288.
Studied in detail with the
ARGUS detector at Hamburg
and CLEO at Cornell in the
80 s and 90 s.
B-factories now operate at
SLAC (BaBar) and in Japan
(Belle)
Detailed studies of how the
weak force interacts with
quarks.
Belle
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?
?
In 1994 the top quark was discovered by the CDF
and DØ experiments at Fermilab
In 2000 the tau neutrino was observed by the
DONUT experiment at Fermilab
The top quark is very heavy (174 GeV/c2) and it
decays directly via
....
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A complete description of electrons, positrons and
photons using relativistic quantum mechanics.
In quantum mechanics, observable quantities are
calculated using the wavefunction for a particle.
The definition of the wavefunction is not unique... it
could be arbitrarily re-defined at each point in space
without changing any observables.
This works, provided the electron interacts with the
photon.
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Electron-electron scattering:
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Feynman Rules:
Space
Electron
Photon
e-
e-
e-
eTime
Initial state
Final state
Add together ALL possible
Feynman diagrams
32
Beta decay described by Fermi (1930 s):
Predicted that the probability of elastic
neutrino scattering would exceed unity at
energies of around 100 GeV.
33
A significant improvement:
A very massive W boson would explain
why the interaction is weak.
34
But hypothetically, at least:
At high energies, the probability for W+Wproduction by neutrino-neutrino
scattering would exceed unity.
35
This combination worked:
But it required adding a new, neutral
boson to the theory.
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Observed in 1973 at
CERN in a liquid
freon bubble
chamber.
Masses of the W± and
0
Z predicted to be of
order 100 GeV/c2
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W±
0
Produce
and Z
directly by colliding
quarks and antiquarks:
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+
W
Z0
+
+ µ -µ
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A theory with explicit mass for W s and Z s
is Non-renormalizable
A theory with massless W s and Z s is
renormalizable...
By introducing the Higgs mechanism we
get the best of both worlds.
But we ve added a new particle to the
theory... one that hasn t yet been
observed.
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Higgs boson
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Masses of the W± and Z0 were known
Build electron-positron colliders to produce
them in large numbers
Make precision measurements tests of the
Standard Model
SLAC upgraded their linear accelerator...
CERN dug a BIG tunnel...
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SLD detector
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Swiss Alps
Geneva
Airport
LEP tunnel
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45
0
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Only 3 generations of quarks and leptons.
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+
-
We
WW
WWZ
All Feynman diagrams are needed to
explain the observed W+W- production
cross section.
48
Direct searches at LEP did not find it.
Although not directly observed, it should
influence precision measurements:
49
Could it be just
around the corner?
50
Replace LEP with a proton-proton collider
Seven-fold increase in energy 14 TeV
Turn-on scheduled in 2008!
51
High energy collisions and high
intensity beams require
complex detectors.
Lots of money, lots of people.
People
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Why is the Higgs mass finite?
Supersymmetry would fix this problem but would
introduce hundreds of new particles.
Neutrinos have mass! That breaks the standard
model.
Why are there only three generations of quarks
and leptons?
Are there only 4 space-time dimensions?
No easy way to incorporate gravity...
53
Matter is composed of fundamental, elementary
particles.
We can describe their properties with exquisite
precision using Quantum Field Theory.
We know our knowledge is incomplete.
The LHC will give us new (and badly needed)
experimental results.
We could witness another revolution in our
understanding of Nature over the next decade.
Be prepared to understand the basic
phenomenology of at least the standard model.
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