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The Standard Model

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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… Elementary particles Ordinary matter: Fermions Gauge bosons: Mediators Antiparticles: Same mass, and spin all other properties reversed! 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! Conservation rules Conserved quantities in all particle interactions: Charge conservation Lepton number (electron, muon, tau) Baryon number Flavour (EM & strong interaction) conservation rules Examples: 1. Electromagnetic: 2. Strong: 3. Weak: 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 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 3-jet event from decaying Z0 into quark-antiquark + gluon. Large separation energy: Jets LEP, CERN Cross-section & Colour Experimental evidence for the 3 colours (e-e+-colliders): (e e hadrons) 4 2 R (e e ) 2 (e e ) 3ECM (e e hadrons) qi 2 2 1 2 1 2 2 , uds 3 3 3 3 10 11 , udsc , udscb 9 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: e 4 Weak coupling g = e MW,Z ~ ~ ~ 90 GeV G ~ 10-5 GeV-2 G G M = 81GeV, M = 94 GeV Measured: W Z 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) In the sun: Transmutation pn gives deuterium, which fusionates 2) Build-up of heavy nuclei (radioactive decay + neutron capture) 3) 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 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….
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