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The Standard Model

Schlüsselexperimente der Elementarteilchenphysik :. The Standard Model. Overview. The particles of SM and their properties Interaction forces between particles Feynman diagrams Interactions : more Challanges ahead Open questions. The Standard Model :.

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The Standard Model

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  1. Schlüsselexperimente der Elementarteilchenphysik: The Standard Model

  2. Overview • The particles of SM and theirproperties • Interaction forcesbetweenparticles • Feynman diagrams • Interactions: more • Challangesahead • Open questions

  3. The Standard Model: Whatelementaryparticles are there? The beginning… • Electron: 1897, Thomson • Atoms havenuclei: 1911, Rutherford • Antiparticles: 1928, Dirac • Neutrons: 1932, Chadwick; positron, Anderson • …lots of moreparticles…

  4. Standard ModelElementaryparticles Ordinarymatter: Fermions Gauge bosons: Mediators Antiparticles: Same mass, and spin all otherpropertiesreversed!

  5. Standard ModelEnergy & momentum • Total relativisticenergy: E2 = p2c2 + m2c4 • Energy of a massless particle: E = pc • Rest energy: E = mc2 An interaction is possibleonlyif the initial total energyexceeds the rest energy of the reactionproducts. All interactionsconserve total relativisticmomentum!

  6. Standard ModelConservationrules Conservedquantities in all particleinteractions: • Charge conservation • Leptonnumber (electron, muon, tau) • Baryonnumber • Flavour (EM & strong interaction)

  7. Standard Modelconservationrules Examples: 1. Electromagnetic: 2. Strong: 3. Weak:

  8. The Standard model: Quantum Electrodynamics Quantum Chromodynamics Quantum Flavourdynamics

  9. Feynman diagrams • Visualization & mathematics (not the paths of the particles!) • Time upwards (convention) • Particle as arrow in time-direction • Antiparticle as arrow in oppositedirection • Mediators as waves, lines or spirals • EXAMPLES 

  10. Feynman diagrams

  11. QEDElectromagneticinteractions EM: Best known of fundamental forces! ManyFeynman diagrams of same constituents. Energy and momentumnotconserved by onevertexalone. Possible ”violation” in 1 vertexbecause of virtualparticles.

  12. QED Cross sections & coupling There are infinitelymanyFeynman diagrams for a particular process. Feynmansgoldenrules: eachvertexcontributes to the scatteringamplitude… The strength of the coupling in a vertex is given by: ..an infinite contribution to scatteringamplitude..? Solution:

  13. Quantum Chromodynamics • Search for patterns; Eightfoldway • 1964: Quarktheory (Gell-Mann,Zweig): Up, Down, Strange • The Charm quark and J/Ψ • Tau, Bottom and Top

  14. J/Ψ: First particle with c quark. Computer reconstruction of itsdecay. Slac, Slide747 Finding a topquark: Proton-antiprotoncollisioncreatestopquarkswhichdecay to W and b. Nature, June 2004 …butwhataboutΩ- & the Pauli principle?

  15. Quantum Chromodynamics • Quarks in nucleiheldtogether by theircolour • Antiquarkshaveanticolour. • A quarkcan ”be” either red, green or blue. • Gluonsmediates the strong force. Theyhave a colour and an anticolour. Self-interaction! Onlyboundstates of 2 or 3 quarks are observed; forming ”colourlessstates”.

  16. QCD Cross-section & Coupling • Srongcouplingconstant: running! • Decreasingαs with increasingnumber of vertices • Asymptotic freedom: Couplingless at shortdistances; ”free” quarksinside the nucleus. • Quarkconfinement: Couplingincreases at distances > nuclei • Reason that quarksonlydetected in colorless combinations • Large separation energy: Jets 3-jet event from decaying Z0 intoquark-antiquark + gluon. LEP, CERN

  17. QCD Cross-section & Colour Experimental evidence for the 3 colours (e-e+-colliders):

  18. Quantum Flavourdynamics 6 flavours of quarks, 6 flavours of leptons. All caninteractweekly. Flavour is conserved in strong and electromagnetic interaction.

  19. QFDFlavour in weak interaction Flavour is not conserved in weakinteractions! Neutron (β) decay Muondecay

  20. QFD Observation Problem: strong interaction screen the weak;easier to observeleptonicdecay! Problem: Neutral interaction is rarelyobserved, competing with muchstronger EM interaction. Weak interaction is moreeasilyobserved in flavour-changingprocesses… Flavourchange; for quarksalsobetween generations

  21. QFDElectroweaktheory • Why so heavy? • Glashow, Weinberg, Salam: EM and weakforces are unified at high energies! Prediction: Weakcoupling g = e G ~ 10-5 GeV-2 Measured: Theory: responsible for theirmasses is the Higgsfield, causingspontaneoussymmetrybreaking. Higgsboson? (Peter Higgs, 1964) MW,Z MW = 81GeV, MZ = 94 GeV

  22. Higgsfield & Higgsboson • 4-component field • 3 components  massive W, Z • 1 component  Higgsboson • Field VEV: 246 GeV  • Symmetrybreaking • Mass to all particles Higgsboson is the only SM particle not yetobserved. Above: SimulatedHiggsbosondecay, ATLAS. Four possibleprocessesinvolving a Higgsboson

  23. QFD Three importantexamples • In the sun: Transmutation pn gives deuterium, whichfusionates • Build-up of heavy nuclei (radioactivedecay + neutron capture) • Stability of elementaryparticles

  24. QFD A very special one… Weak force not only breaks the flavourconserving… Also: Non-conservation of parity! Parity = symmetry under inversion of space. Example: Neutrinos left-handed.. CP-invariance?... …CPT-invariance?

  25. Standard Model • Elementaryparticles: 6 leptons, 6 quarks, 12 bosons. Eachhave spin, charge and mass • Fundamental forces: Conservationrulesobeyed 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 forcesestimated betterunderstanding of the forces • Electromagnetic and weakinteractions as oneunified

  26. 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…?

  27. Are known elementary particles really elementary?So far… • New elementary particles?Possible example: super-symmetric particles... • More complete theory, including e.g. gravitational interaction? SimulatedHiggs event, ATLAS

  28. Beyond the Standard Model • GUT: Electroweak QCD at 1016 GeV? • TOE? • SUSY? Higherenergies in experiments ↓ Heavierparticlesmay be found ↓ Possible extension of Standard Model! Final conclusion: Still a lot to be done!

  29. At last… Thank you for the attention!

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