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THE STANDARD MODEL Schlüsselexperimente der Elementarteilchenphysik:

Schlüsselexperimente der Elementarteilchenphysik:

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THE STANDARD MODEL

Schlüsselexperimente der Elementarteilchenphysik:

Overview

The particles of SM and their properties

Interaction forces between particles

Feynman diagrams

Interactions: more

Challanges ahead

Open questions

The Standard Model:

What elementary particles are there?

The beginning… Electron: 1897, Thomson Atoms have nuclei: 1911, Rutherford Antiparticles: 1928, Dirac Neutrons: 1932, Chadwick; positron, Anderson …lots of more particles…

m110

m1010

m1410

m1810

m1510

m1510 m1810

Standard Model Elementary particles

Ordinary matter: Fermions

Gauge bosons: Mediators

Antiparticles: Same mass, and spin all other properties reversed!

Standard Model Energy & momentum Total relativistic energy: E2 = p2c2 + m2c4

Energy of a massless particle: E = pc

Rest energy: E = mc2

An interaction is possible only if the initial total energy exceeds the rest energy of the reaction products.

All interactions conserve total relativistic momentum!

Standard Model Conservation rules Conserved quantities in all particle interactions:

Charge conservation

Lepton number (electron, muon, tau)

Baryon number

Flavour (EM & strong interaction)

Standard Model conservation rules

Examples:

1. Electromagnetic:

2. Strong:

3. Weak:

ee

)()()()( ddssuKuudpud

eepn

Quantum Electrodynamics

QuantumChromodynamics

Quantum Flavourdynamics

The Standard model:

Feynman diagrams

Visualization & mathematics

(not the paths of the particles!)

Time upwards (convention)

Particle as arrow in time-direction

Antiparticle as arrow in opposite direction

Mediators as waves, lines or spirals

EXAMPLES

Feynman diagrams

QED Electromagnetic interactions

Many Feynman diagrams of same constituents.

Energy and momentum not conserved by one vertex alone.

Possible ”violation” in 1 vertex because of virtual particles.

EM: Best known of fundamental forces!

QED Cross sections & coupling

There are infinitely many Feynman diagrams for a particular process.

Feynmans golden rules: each vertex contributes to the scattering amplitude…

The strength of the coupling in a vertex is given by:

..an infinite contribution to scattering amplitude..?

Solution:

e

1371

40

2

ce

Quantum Chromodynamics Search for patterns;

Eightfold way

1964: Quark theory (Gell-Mann,Zweig): Up, Down, Strange

The Charm quark and J/Ψ

Tau, Bottom and Top

J/Ψ: First particle with c quark.Computer reconstruction of its decay.Slac, Slide747

Finding a top quark:Proton-antiproton collision creates top quarks which decay to W and b.Nature, June 2004

…but what about Ω- & the Pauli principle?

Quantum Chromodynamics Quarks in nuclei held together by their colour

Antiquarks have anticolour.

A quark can ”be” either red, green or blue.

Gluons mediates the strong force. They have a colour and an anticolour. Self-interaction!

Only bound states of 2 or 3 quarks are observed; forming ”colourless states”.

ggbbrrMesons

rgbsAntibaryon

rgbBaryons

,,:

:

:

QCD Cross-section & Coupling

Srong coupling constant: running!

Decreasing αs with increasing number of vertices

Asymptotic freedom: Coupling less at short distances; ”free” quarks inside the nucleus.

Quark confinement: Coupling increases at distances > nuclei

Reason that quarks only detected in colorless combinations

Large separation energy: Jets

3-jet event from decaying Z0

into quark-antiquark + gluon.LEP, CERN

Experimental evidence for the 3 colours (e-e+-colliders):

QCD Cross-section & Colour

udscb , 911

udsc , 9

10

uds , 32

31

31

32

)(222

iqhadronsee

2

2

34

)(CME

ee

)(

)(

ee

hadronseeR

Quantum Flavourdynamics6 flavours of quarks, 6 flavours of

leptons. All can interact weekly.

Flavour is conserved in strong

and electromagnetic interaction.

QFD Flavour in weak interaction

Flavour is not conserved in weak interactions!

Neutron (β) decay Muon decay

Problem: Neutral interaction is rarely observed, competing with much stronger EM interaction.

QFD Observation

Weak interaction is more easily observed in flavour-changing processes…

Problem: strong interaction screen the weak; easier to observe leptonic decay!

Flavour change; for quarks also between generations

QFD Electroweak theory Why so heavy? Glashow, Weinberg, Salam: EM and weak forces are unified

at high energies!

Prediction:

Weak coupling g = e

G ~ 10-5 GeV-2

Measured:

Theory: responsible for their masses is the Higgs field,

causing spontaneous symmetry breaking. Higgs boson?

(Peter Higgs, 1964)

MW,Z

MW = 81GeV, MZ = 94 GeV

GeV 90~4

~~GG

e

Higgs field & Higgs boson 4-component field 3 components massive W, Z 1 component Higgs boson Field VEV: 246 GeV Symmetry breaking Mass to all particles

Higgs boson is the only SM particle not yet observed. Above: Simulated Higgs boson decay, ATLAS.

Four possible processes involving a Higgs boson

QFD Three important examples

1) In the sun: Transmutation pn gives deuterium, which fusionates

2) Build-up of heavy nuclei (radioactive decay + neutron capture)

3) Stability of elementary particles

ppHeHeHe

HepH

ee

eHpp

processesSolar

e

433

32

2

2

e

e

e

AZeAZ

eAZAZ

eAZAZ

decay

),1(),(

),1(),(

),1(),(

QFD A very special one…

Weak force not only breaks the

flavour conserving…

Also: Non-conservation of parity!

Parity = symmetry under inversion

of space.

Example: Neutrinos left-handed..

CP-invariance?...

…CPT-invariance?

Standard Model Elementary particles:

6 leptons, 6 quarks, 12 bosons. Each have spin, charge and mass

Fundamental forces:

Conservation rules obeyed in all interactions

EM: electric charge; photons

Strong: colour charge; gluons

Weak: charged and neutral currents; W´s and Z

Cross-sections and transition rates can be calculated and the range of forces estimated better understanding of the forces

Electromagnetic and weak interactions as one unified

Limitations of SMThe Standard Model is confirmed by many different experiments.

But fundamental questions are left open:

Free parameters. What gives mass to the elementary particles? Intensive research of the Higgs particle at CERN (LHC).

Why observed tiny asymmetry between matter and antimatter?Reason that universe still exists…?

Are known elementary particles really elementary?So far…

New elementary particles?Possible example: super-symmetric particles...

More complete theory, including

e.g. gravitational interaction?

Simulated Higgs event, ATLAS

Beyond the Standard Model

GUT: Electroweak QCD at 1016 GeV? TOE? SUSY?

Higher energies in experiments

Heavier particles may be found

Possible extension of Standard Model!

Final conclusion: Still a lot to be done!

At last…

THANK YOU FOR THE ATTENTION!