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…

Elementary particles

Ordinary matter: Fermions Gauge bosons: Mediators Antiparticles: Same mass, and spin all other properties reversed!

Energy & momentum

   Total relativistic energy: E 2 = p 2 c 2 + m 2 c 4 Energy of a massless particle: E = pc Rest energy: E = mc 2 An interaction is possible only if the initial total energy exceeds the rest energy of the reaction products.

All interactions conserve total relativistic momentum!

Conservation rules

Conserved quantities in all particle interactions:  Charge conservation  Lepton number (electron, muon, tau)  Baryon number  Flavour (EM & strong interaction)

Examples:

1. Electromagnetic: 2. Strong: 3. Weak:

conservation rules

The Standard model:

Quantum Electrodynamics Quantum Chromodynamics Quantum Flavourdynamics

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

EM: Best known of fundamental forces!

Many Feynman diagrams of same constituents.

Energy and momentum not conserved by one vertex alone. Possible ”violation” in 1 vertex because of virtual particles.

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:

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”.

Cross-section & Coupling

 Srong coupling constant: running!

     Decreasing α s 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 Z 0 into quark-antiquark + gluon.

LEP, CERN

Cross-section & Colour

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

R

  (

e



e

  (

e



e

 

hadrons

)      )  (

e



e

      )  4  2 3 2

E CM

 (

e



e

 

hadrons

)  

q i

 2 3 2  1 3 2  1 3 2  2 3 , uds  10 , udsc 9  11 , udscb 9

Quantum Flavourdynamics

6 flavours of quarks, 6 flavours of leptons. All can interact weekly.

Flavour is conserved in strong and electromagnetic interaction.

Flavour in weak interaction

Flavour is not conserved in weak interactions!

Neutron ( β) decay Muon decay

Observation

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

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

Weak interaction is more easily observed in flavour changing processes … Flavour change; for quarks also between generations

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:

M W,Z

~

e G

~ 4  ~ 90 GeV

G M W

= 81GeV,

M Z

= 94 GeV

Theory: responsible for their masses is the Higgs field, causing spontaneous symmetry breaking. Higgs boson?

(Peter Higgs, 1964)

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

Three important examples

1) 2) 3) In the sun: Transmutation p  n gives deuterium, which fusionates Build-up of heavy nuclei (radioactive decay + neutron capture) Stability of elementary particles

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 SM

The 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

Reason that universe still exists…?

and

antimatter

?

 Are known

elementary

So far… particles really elementary?

 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 TOE?

SUSY?

QCD at 10 16 GeV? Higher energies in experiments ↓ Heavier particles may be found ↓ Possible extension of Standard Model!

Final conclusion: Still a lot to be done!

At last…