Nuclear and Particle Physics 3:
Particle PhysicsLecture 1: Introduction to Particle Physics
February 5th 2007
Particle Physics (PP)a.k.a. High-Energy Physics (HEP)
1
Dr Victoria MartinJCMB room 4405
My research: Particle Physics at Colliders
• CDF at the Fermilab Tevatron, near Chicago: colliding protons and anti-protons at ~2TeV. Currently the world’s highest energy collider.
• The international linear collider (ILC), the world’s next electron-position collider. Won’t start operation until ~2018!!
2
CDF and the Tevatron
CDF
p p
1.96 TeV
3
International Linear Collider
A future project
Many elements still under design and discussion, including...
•location
•cost
•will we build it at all?
e!
e+
4
Course Contents
Lecture 1 - Introduction
The fundamental particles and forces
Lecture 2 - Tools
Natural Units
Kinematics
Antiparticles
Lecture 3 - Feynman Diagrams
The electromagnetic force
Feynman Diagrams
Lecture 4 - Interactions with matter
Particle accelerators, detectors and experiments.
Lecture 5 - Quarks
Mesons, Baryons
Isospin and strangeness
Lecture 5 - Strong Interactions
Colour, gluons
confinement, running coupling constant
Lecture 7 - Weak Interactions
Muon and tau decay
Heavy quarks, CKM mechanism
Lecture 8 - Neutrinos
Neutrino Mass and oscillations
Lecture 9 - Electroweak Theory
W and Z bosons
LEP
Spontaneous Symmetry Breaking and the Higgs Boson
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Books!
Particle Physics, second edition, by B.R. Martin & G. Shaw. 2nd edition (Wiley 1997)
7 copies in JCMB Library.
Introduction to High Energy Physics - D.H. Perkins, 4th edition (CUP 2000)
Introduction to Elementary Particles - D. Griffiths (Wiley 1987)
Quarks and Leptons –F. Halzen & A.D. Martin (Wiley 1984)
For more information that you could ever need on every particle ever: http://durpdg.dur.ac.uk/lbl/
• In conjunction with attending the lectures you will need to read around the subject to fully understand the material.
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Particle physics collisions recreate conditions just after the big bang; the closest we’ll ever get to the big bang on earth.
Matter and anti-matter were made in equal amounts: so why does the universe look like
Motivation
To be able to explain what’s correct and
what’s not in this book
What’s the interest in particle physics?
THIS NOTTHIS
& ?
Technology spin-offs CERN: where the web was born!
Medical applications: e.g. PETMagnetic resonance imaging (MRI)
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Particle Physics still deals with the last four of these questions!
The Questions of Particle Physics
8
•How does these particles interact?
• What are electromagnetic forces?
• What holds nucleus together?
• What causes radioactivity?
• How does gravity act at such large distances?
atoms
nucleus &electrons
protons &neutrons
• What is matter made from?
quarks and electrons
particle physics
9
The simplest classification of the fundamental particles and their interactions
Matter - the fundamental constituents of the universe
• All mater elementary particles are fermions, spin !"
• leptons e#, !
• quarks u, d proton = uud
• Every particle has an antiparticle e.g. positron(e+), antiproton (p ")
Forces - the interactions between elementary particles
• Interactions between quarks and leptons are mediated by exchange of gauge bosons–spin 1"
• Electromagnetic interaction - photons (!)
• Strong force - gluons (g)
• Weak force - W and Z
• Gravity - graviton
• Every force couples to a property of the fermions, e.g. electromagnetic force acts on the electric charge
Standard Model of Particle Physics
Nuclear and Particle Physics Franz Muheim 6
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The Electron
Electron - elementary particle
• Point-like: size < 10#6 fm (10#21m)
• Stable: lifetime > 4.6$1026 yrs
• Electric charge,
• Mass,
me = 0.510998918(44) MeV/c2
= 9.11$10#31 kg
• Spin, intrinsic property: !"
The electron is a fermion i.e. electrons satisfy Pauli exclusion principle.
Discovered in 1897 by J. J. Thomson
qe = !e
= !1.60217653(14)" 10!19 C
1916 Noble Prize in Physics
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Compton scattering:
Photon - elementary particle
• pointlike: size < 10#6 fm (10#21m)
• stable
No electric charge
Massless
Energy
Spin intrinsic property spin-1"
The Photon
m! ! 6 " 10!17 eV/c2 # 10!52 kg
q! ! 5 " 10!30
qe
E! = h! = hc/"
1927 Noble Prize in Physics
Nuclear and Particle Physics Franz Muheim 8
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Particulate nature of light was confirmed in 1924 by A. H. Compton.
! e! ! ! e!
photons are bosons
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Antiparticles
Compared its matter partner, an antiparticle has:
• equal mass
• opposite electric charge
• opposite “additive” quantum numbers
Example: positron (e+) antiparticle of the electron (“anti-electron”)Discovered in 1931 by Carl Anderson
Notation: bar over symbol or minus↔plus
e.g. e!
! e+
u ! u !e ! !e
Every particle has a corresponding antiparticle
Antiparticles of the SM particles are antimatterPositron “track”
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Elementary Particles
Particle SymbolElectric Charge
Type
Electron e! !1 lepton
Neutrino #e 0 lepton
Up-quark u +2/3 quark
Down-quark d !1/3 quark
Basic Constituents of MatterFour spin-! fermions
The particles that you know already; describes (most) of nuclear physics.
LeptonsElectron and neutrino
QuarksNucleons are bounds states of up- and down-quarks
Beta Decay
n ! p + e! + !e
d ! u + e! + !e
dd
d
uuue!W#e
d$W#u % e !"e
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GenerationsNature replicates itself: there are three generations of quarks and leptons
1st Generation 2nd Generation 3rd Generation charge
electron neutrino #e
muonneutrino #& tau neutrino #' 0
electron e! muon &! tau '! !1
up quark u charm quark c top quark t +(
down quark d strange quark s bottom quark b !)
Ordinary Matter: built from the 1st generation
Higher Generations:
• copies of (%e, e#, u, d)
• undergo identical interactions
• only difference is mass of particles
• generations are successively heavier
Why 3 generations?symmetry/structure not understood!
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LeptonsLeptons are particles that do not interact via the strong force.
Fermions - spin !"
There are six distinct flavours:
• electron and its electron neutrino partner
• muon
• tau: even heavier 3500 $ me
Unstable: lifetime is 291 fs
Lepton Symbol Charge Mass Lepton Family Numbere (MeV/c2) Le Lµ L!
Electron Neutrino !e 0 < 0.001 +1 0 0Electron e! !1 0.511 +1 0 0Muon Neutrino !µ 0 < 0.001 0 +1 0Muon µ!
!1 105.7 0 +1 0Tau Neutrino !! 0 < 0.001 0 0 +1Tau "!
!1 1777 0 0 +1
discovered by Anderson in 1936 in cosmic rays.
Very similar to electrons but ~200 times heavier
Unstable: lifetime is 2.2µs
The additive quantum numbers are charge, Q, and Lepton Family Number, Le, Lµ, L!
• muon neutrino partner
• tau neutrino partner
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Charge and lepton family number are conserved in all interactions.
Question: how does a muon decay?
Charged leptons interact via electromagnetic and weak forces.
Neutrinos interact only via weak force.
• neutrinos are almost massless
• !e and !!e are distinct particles with different quantum numbers
Leptons Properties
µ+ ! e+!e !µµ! ! e!!e !µ
µ+ ! e+!
Le(!e) = +1 Le(!e) = !1
FORBIDDEN
ALLOWED
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Quarks• Quarks are fundamental fermions with spin-!"
• Six distinct flavours: u, d, s, c, t, b. Fractional charge: +2/3 e or #1/3 e
• Quark carry isospin, quark flavour and electric charge quantum numbers
Quark Symbol Charge Mass Isospin Quark Flavoure (I, Iz) quantum number
up u +2/3 1.5 - 4.5 MeV/c2 (1/2, +1/2) -down d !1/3 5 - 8.5 MeV/c2 (1/2, !1/2) -charm c +2/3 1 - 1.4 GeV/c2 - C = +1strange s !1/3 80 - 155 MeV/c2 - S = !1top t +2/3 171.4 ± 2.1 GeV/c2 - T = +1bottom b !1/3 4 - 4.5 GeV/c2 - B = !1
• Quarks interact via the strong, electromagnetic and weak forces
• Quarks are never found isolated, they always combine into bound states called “hadrons” - confinement
• Quarks carry a new quantum number known as colour -
• each quark has either red, blue or green colour charge
• anti-quarks have anti-red, anti-blue or anti-green colour charge
m(proton) = 938 MeV/c2
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Mesons = qq"
Bound states of quark anti-quark pair
Bosons: spin 0, 1", 2"
e.g. pions
Baryons = qqq
Three quark bound states
Fermions: spin 1/2", 3/2" ...
e.g. proton (uud), neutron (ddd)
anti-baryons e.g. anti-proton
•
• Free quarks have never been observed - quarks are locked inside hadrons
• Hadrons are bound states of quarks: either (qqq) or (qq ")
• Charge of hadron is always integer multiple of electric charge, e
• Colour charge of hadron is always neutral
• Two types of hadrons – mesons and baryons
Hadrons: Mesons & Baryons
!+ = (ud)!! = (ud)!0 = 1
"
2(uu ! dd)
p = (uud)n = (udd)p = (uud)
q"
q
q
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Interactions: Classical & QuantumClassical Interactions
electromagnetism and gravity - action at a distance"...that one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that, I believe no man, who has in philosophic matters a competent faculty of thinking, could ever fall into it."
(Newton in a letter to Richard Bentley, 25 Feb. 1693)
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Interactions and Forces in Particle Physicsquarks and leptons interact via exchange of spin-1" gauge bosonsgravity does not (yet) fit into this framework
Force Gauge Symbol Charge Mass Relativeboson e GeV/c2 strength
Strong (8) gluons g 0 0 1Electro- photon ! 0 0 ! 10!2
magentic
Weak ‘intermediate Z0 0 91.188 ! 10!7
vector bosons’ W ± ±1 80.40Gravity graviton 0 0 ! 10!40
20
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The Fundamental Forces
Strong Force (gluon, g)
• Acts on colour charge i.e. only on quarks
• Holds hadrons together and nuclei together
Weak Force (W & Z)
• Acts on “weak hypercharge” i.e. on all quarks and leptons
• Responsible for fission, fusion and radioactive decays
Electromagnetic force (photon, ")
• Acts on electric charge i.e. on all charged particles
• Hold atoms and molecules together
Gravity (graviton)
• Acts on mass ... i.e. on all particles except the photon and gluons
• v. weak on subatomic scales
• Responsible for large scale structure of the universe
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• fermions
• 3 generations of quarks & leptons
• Antimatter
• Quarks form into hadrons - mesons and baryons
Summary
An elegant theory that describes accurately (almost) all measurements in particle physics
The Standard Model of Particle Physics
Quarks and Leptons Charge, e
%e
e%µ
µ%&
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0#1
ud
cs
tb
+2/3#1/3
InteractionGauge Bosons
Charge, e
Strong gluons 0
Electro-magnetic
Photon 0
Weak W, Z 0, ±1
Gravity graviton 0
Matter Forces
• mediated by the exchange of gauge bosons
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