Quantum ChromoDynamics

(Nobel Prize 2004)

Chris McLauchlin

Outline

The Four Fundamental ForcesThe Strong Force

History of the Strong Force

What These People Did

Experimental Support

1

Fundamental Forces

2

Gravity Weak

Electromagnetic Strong

Fundamental Coupling

3

αg ≈ 5.9 × 10-39 αW ≈ 10-6

α≈137-1 αS ≈ 1

Development of QCD

4

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Proton Neutron

Proton Proton

NeutronNeutron

Quantum Field Theory

5

Time

Space

[[theideagirlsays.wordpress.com]]

Electron Exchange Neutron and Proton

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Electron ExchangeNeutron and Neutron

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Electron Exchange?Proton and Proton

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??

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? ?

??

Possibly Pions?

9

E t

Mesons

Mu-MesonAnderson and Neddermeyer 1937

Street and Stevenson

Bubble Chambers- Cosmic Rays

PionPowel 1947

Bubble chamber at high altitude

10

Bubble Chambers

Mixture of Ne-H

Superheating

Ionization by particles

Can find q/m

3-D tracks of Particles

11

Murray Gell-Mann

Developed the Eightfold Way in 1961

Geometric ordering of baryons and mesons

Organization by strangeness and charge

12

1

-1

0

Gell-Man and Zweig

1964

QuarksComprise Hadrons

Flavors

Quark Confinement

Color ChargePauli’s Exclusion Principle

13

ud 0 uu,dd du

So What Bonds the Quarks?

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u u

d

Quantum ChromoDynamicsStrong Force = Colour Force

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u u

d

Quantum ChromoDynamicsStrong Force = Colour Force

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u u

d

Quantum ChromoDynamicsStrong Force = Colour Force

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u

d

u

Fundamental Forces

18 [[theideagirlsays.wordpress.com]]

Nuclear Force

19

The Nobel Prize

Dr’s David J. Gross, H. David Politzer and Frank Wilczek

Asymptotic Freedom

20

(g) g3

16 2

113

C2 43

T (R)

O(g5 )

Asymptotic Freedom

Opposite of QED

CouplingIncreases with distance

Quark Confinement

Scaling

21

Asymptotic Freedom Shown by the β-Function

22

(g) g3

16 2

113

C2 43

T (R)

O(g5 )

This is why these guys got a Nobel Prize

QED Screening

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+-

+-

+-

+-

+-

+-

+-

+-

QED Screening

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QCD Screening

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QCD Screening

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Quark Confinement

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u u

d

Quark Confinement

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u u

d

Quark Confinement

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u u

d

u

ū

d

đ

Standard Model

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Summary

QuarksFlavor

Color Charge

Hadrons

Pions

Asymptotic FreedomQuark Confinement

ScalingLimits to three constituents

31

Thank youAny Questions?

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Experimental Evidence

Hadron Production, Muon-pair Production, e- e+ ProductionADONE accelerator

Consistent with factor of 2 implied by three coloursAs opposed to 2/3 without colours

Coupling StrengthCoupling Constant αShould decrease with an increase in Energy of bombardment

33

(E) 12

(33 2n)ln E2

2

Experimental Evidence

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Scaling

The cross-section of a lepton scattering off of nucleon to depend on a dimensionless quantity

Implies multiple components of nucleon

This is governed by a particle’s β-FunctionMust be negative to imply scaling

Theories using this refer to it as “Asymptotic Freedom”

35

q22p

What is Alpha?

36

(E) 12

(33 2n)ln E2

2

Quarks are Surrounded by a Gluon Cloud

Penetration of the cloud lowers Coupling

The β Function

37

(g) g

QED vs. QCD

Positive β-Function Negative β-Function

Decays with Distance Increases with Distance

Shielding by other Fermions

No Shielding *

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Strong Theory Development

Hideki Yukawa (1935)1935

Mediated by a particle like E&M, but with mass

Richard FeynmanQED

Yang-Mills (1954)Three field particles: B+, B-, B0

QED principles applied to Strong ForceNon-Abelian Gauge Theory

Not Renormalizable/ zeroth order said massless particles

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E t 2

Strong Theory Development

Gell-Mann (1959)Quarks!

Make up Protons and Neutrons (three per each)Needed another degree of freedom

Colour introduced

Nambu (1960)Discovered Spontaneous Gauge Symmetry Breaking

Gauge Theories can have Short Distances

40

Strong Theory Development

Bjorken (1968)Asymptotic Freedom

ScalingCross Section of particles related to charge and momentum

Governed by the β-function

t’ Hooft discovered this, but never said anything

41

Strong Theory Development

t’ Hooft (1971)Proved Non-Abelian Gauge Theories to be renormalizable!

Weak Interaction fixed And mixed with Electromagnetic

Also got the negative, but didn’t know it was right and forgot about it

New PeopleFound it again and got Nobel Prize

42

Renormalization

Any Technique to deal with Infinities in calculated quantitiesSuch as the electron

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Non-Abelian Gauge Theory

Non-Commutative

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a*b b*avsa*b b*a

Strong Force

Short Range

Very Strong

Decreases at close distances

45 [[http://hadron.physics.fsu.edu/~crede/forces.html]]cc

Quantum Number Conservation

Baryon Number

Lepton Number

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Bosons and Fermions

Fermion: ½ integer spinsProton, Neutron, Electron

Matter Particles

Boson: Integer spinsForce Carrying Particles

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Pictures

[[1] http://theideagirlsays.wordpress.com/2012/08/18/higgs-boson-borexino-geo-neutrino-particles-weak-nuclear-force-yukawa-gluons-neutron-decays-5g-wow-seti/electromagnetic-weak-between-quarks-nucelons-strong-interaction-diagram/

[[2]] http://hadron.physics.fsu.edu/~crede/forces.html

[[3]] http://www.dichotomistic.com/matter_infinoverse_two.html

[[4]] http://www.particleadventure.org/residualstrong.html

[[5]]

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References

http://lss.fnal.gov/archive/other/ift-p-036-93.pdf

Book: The Quantum Story

Book: Introduction to Elementary Particle PhysicsGriffiths

A Modern Introduction to Quantum Field TheoryMaggiore

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