Transcript An Overview of the Field of High Energy Physics

Physics 564: Introduction to
Elementary Particle Physics
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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Physics 564 Overview
• 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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Physics 564 Overview
• 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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Physics 564 – Historical Overview
• 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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HEP Laboratories
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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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Recent History
• 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.
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More Recent History
• 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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The Birth of Particle Physics
•
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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Nuclear Physics
•
•
•
•
•
•
•
•
α, β, ɣ emission
Properties of neutrons
Fission of heavy elements
Nuclear “chemistry”
Nuclear forces
Beta decay
Neutrino postulated
Theories of beta decay
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Particle Accelerators
• 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.
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Particle Detectors
• In 1912, Wilson develops the
cloud chamber for seeing the
paths of fundamental particles
• Photographic emulsions
exposed by the passage of
charged particles
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Discoveries in Cosmic Rays
• 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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The Known Particles in 1950
• Get your own particle data book at http://pdg.lbl.gov.
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New Accelerators: Synchrotrons
1952: Brookhaven 3 GeV “Cosmotron”
1954: Berkeley 6 GeV “Bevatron”
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New Detectors: Bubble Chambers
The Berkeley 72 inch liquid
hydrogen bubble chamber
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Known Particles in 1957
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Strongly Interacting Particles: 1961
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Strongly Interacting Particles: 1963
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Mass
Organizing the Data
“Strangeness”
Electric charge
Spin
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1964: Quarks?
• Murray Gell-Mann:
Physical meson states are
representations of the SU(3)
symmetry group:
• George Zweig:
Hadrons are composed of
more elementary objects:
Physical baryon states are
representations of the SU(3)
symmetry group:
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1964: Observation of the Ω-
Observed in the 80 inch bubble chamber at Brookhaven in 1964.
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1968: Deep Inelastic Scattering
2 mile long, 30 GeV
electron accelerator
Analyzing magnets
Detector
Hydrogen target
People
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Elastic Scattering
Electron
Proton
Used to measure the size of the proton.
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Inelastic Scattering
Electron
Proton
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Deep Inelastic Scattering
“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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1974: The November Revolution
The SPEAR synchtrotron: 8 GeV
Simultaneously
electron-positron collider
at SLAC observed
at Brookhaven where it
was called the “J”.
To this
dayMark-I
it is called
the tracked
... charged
The
detector
particles
usingquark
sparkwas
chambers.
The charm
heavy and non-relativistic.
Charmonium behaved like a hydrogen atom made of quarks.
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Bottom Quarks
• 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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Fundamental Particles of Matter
?
?
• 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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Returning to the 1950’s:
Quantum Electrodynamics
• 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.
Symmetry
Forces
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Quantum Electrodynamics
• Electron-electron scattering:
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Quantum Electrodynamics
• Feynman Rules:
Space
– Electron
– Photon
e-
e-
e-
e-
Time
Initial state
Final state
– Add together ALL possible
Feynman diagrams
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Weak Interactions
• Beta decay described by Fermi (1930’s):
• Predicted that the probability of elastic
neutrino scattering would exceed unity at
energies of around 100 GeV.
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Weak Interactions
• A significant improvement:
• A very massive W boson would explain
why the interaction is weak.
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Weak Interactions
• But hypothetically, at least:
• At high energies, the probability for W+Wproduction by “neutrino-neutrino”
scattering would exceed unity.
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Weak Interactions
• This combination worked:
• But it required adding a new, neutral
boson to the theory.
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Observation of Neutral Currents
• 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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The CERN SPS
0
• Produce
and Z
directly by colliding
quarks and antiquarks:
W±
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1983: Observation of W and Z Bosons
+  μ+ ν
W
Z0  μ+ µ-µ
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But there’s more...
• 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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The Standard Model
Quarks
Gauge
Leptons bosons
Higgs boson
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SLAC and LEP
• 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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1987: Stanford Linear Collider
SLD detector
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Large Electron Positron Collider
Swiss Alps
Geneva
Airport
LEP tunnel
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ALPEH, DELPHI, L3, OPAL
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0
Z
Production at LEP
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Physics from LEP
• Only 3 generations of quarks and leptons.
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1997: LEP II – W+W- Production
We
WW
WWZ
All Feynman diagrams are needed to
explain the observed W+W- production
cross section.
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The Higgs Boson?
• Direct searches at LEP did not find it.
• Although not directly observed, it should
influence precision measurements:
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The Higgs Boson
Could it be just
around the corner?
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The Large Hadron Collider
• Replace LEP with a proton-proton collider
• Seven-fold increase in energy – 14 TeV
• Turn-on scheduled in 2008!
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CMS and ATLAS
High energy collisions and high
intensity beams require
complex detectors.
Lots of money, lots of people.
People
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What we still don’t understand
• 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...
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Summary
• 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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