Transcript The Standard Model - KIT
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…