schlüsselexperimente der elementarteilchenphysik:
Post on 21-Dec-2015
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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…
Standard Model Elementary particles
Ordinary matter: Fermions
Gauge bosons: Mediators
Antiparticles: Same mass, and spin all other properties reversed!
Standard Model 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!
Standard Model Conservation rules Conserved quantities in all particle interactions:
Charge conservation
Lepton number (electron, muon, tau)
Baryon number
Flavour (EM & strong interaction)
Standard Model conservation rules
Examples:
1. Electromagnetic:
2. Strong:
3. Weak:
ee
)()()()( ddssuKuudpud
eepn
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
QED Electromagnetic interactions
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!
QED 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:
e
1371
40
2
ce
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”.
ggbbrrMesons
rgbsAntibaryon
rgbBaryons
,,:
:
:
QCD 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
Large separation energy: Jets
3-jet event from decaying Z0
into quark-antiquark + gluon.LEP, CERN
Experimental evidence for the 3 colours (e-e+-colliders):
QCD Cross-section & Colour
udscb , 911
udsc , 9
10
uds , 32
31
31
32
)(222
iqhadronsee
2
2
34
)(CME
ee
)(
)(
ee
hadronseeR
Quantum Flavourdynamics6 flavours of quarks, 6 flavours of
leptons. All can interact weekly.
Flavour is conserved in strong
and electromagnetic interaction.
QFD Flavour in weak 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.
QFD Observation
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
QFD 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:
Theory: responsible for their masses is the Higgs field,
causing spontaneous symmetry breaking. Higgs boson?
(Peter Higgs, 1964)
MW,Z
MW = 81GeV, MZ = 94 GeV
GeV 90~4
~~GG
e
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
QFD 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
ppHeHeHe
HepH
ee
eHpp
processesSolar
e
433
32
2
2
e
e
e
AZeAZ
eAZAZ
eAZAZ
decay
),1(),(
),1(),(
),1(),(
QFD 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 SMThe 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!
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