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Physics 564: Introduction to Elementary Particle Physics

• 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

First Lecture – August 20, 2007

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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 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.

• In 1905, Einstein argued that photons behave like particles.

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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

• Photographic emulsions exposed by the passage of charged particles

• In 1912, Wilson develops the cloud chamber for seeing the paths of fundamental particles

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Discoveries in Cosmic Rays• 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.• In 1912, Viktor Hess investigated

terrestrial radioactivity in balloon experiments.

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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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Organizing the Data

“Strangeness”

Spin

Electric charge

Mas

s

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1964: Quarks?• Murray Gell-Mann: • George Zweig:

Physical meson states are representations of the SU(3) symmetry group:

Physical baryon states are representations of the SU(3) symmetry group:

Hadrons are composed of more elementary objects:

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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

Hydrogen target

Analyzing magnets

Detector

People

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Elastic Scattering

Proton

Electron

Used to measure the size of the proton.

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Inelastic Scattering

Proton

Electron

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Deep Inelastic Scattering

Proton

Electron

“Partons”

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 electron-positron collider at SLAC

The Mark-I detector tracked charged particles using spark chambers.The charm quark was heavy and non-relativistic.

Charmonium behaved like a hydrogen atom made of quarks.

Simultaneously observed at Brookhaven where it was called the “J”.

To this day it is called the ...

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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:

– Electron– Photon

– Add together ALL possible Feynman diagrams

Time

Spa

ce e-

e-

e-

e-

Initial state Final state

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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+W- production 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 Z0 predicted to be of order 100 GeV/c2

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The CERN SPS• Produce W± and Z0

directly by colliding quarks and anti-quarks:

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1983: Observation of W and Z Bosons

W+ μ+ νµZ0 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 ModelQ

uarksLeptons

Gauge

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 ColliderSwiss Alps

Geneva

Airport

LEP tunnel

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ALPEH, DELPHI, L3, OPAL

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Z0 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

People

High energy collisions and high intensity beams require complex detectors.

Lots of money, lots of 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.