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…

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

 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!

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:

Quantum Electrodynamics Quantum Chromodynamics 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

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!

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”.

 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

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

Quantum Flavourdynamics 6 flavours of quarks, 6 flavours of leptons. All can interact weekly. Flavour is conserved in strong and electromagnetic 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. 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

 Why so heavy?  Glashow, Weinberg, Salam: EM and weak forces are unified at high energies! Prediction: Weak coupling g = e G ~ GeV -2 Measured: Theory: responsible for their masses is the Higgs field, causing spontaneous symmetry breaking. Higgs boson? (Peter Higgs, 1964) M W,Z M W = 81GeV, M Z = 94 GeV

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

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

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 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 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!

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