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Transcript
Introduction to Particle Physics
All matter in the universe is
made from FUNdamental particles
™ searching for the ultimate building blocks of matter
and the forces of Nature that govern their behaviour
m
Cosmology
1021
Astrophysics
1015
Astronomy
Geophysics
109
103
Sociology ?
Biology - Chemistry
Copyright Chris Madden
Atomic Physics
Nuclear Physics
Particle Physics
10-3
10-9
10-15
Travel back in time
the universe
- much smaller
- much hotter
matter constituents
not frozen
in structure
E=kT
k Boltzman’s
constant
Energy Scales of the Universe
1021
1018
ZeV
(Zeta)
EeV
(Exa)
PeV
(Peta)
1012
TeV
(Tera)
109
GeV
(Giga)
106
MeV
(Mega)
1015
103
1
KeV
eV
(Kilo)
???
Active Galactic Nuclei
Gamma Ray Bursts
Supernova
Limit of human
technology
Early accelerators
Lab X-ray
Battery
How can we learn what matter is made of ?
™ look ?
– not so easy to see details
limit to what we can see at ~105 nm
our eyes detect only electromagnetic radiation
in the wavelength range 380 to 750 nm
… but nucleons, electrons at <10-6 nm
the optical wavelengths >> than the objects we would like to see !
™ need resolution AND different detectors!
Historical interlude
… not much progress
till end of
18th century
1802 John Dalton formulates
a ”modern” atomic theory
• all known substances are made from
<100 different fundamental elements
• each element is made of a different
kind of atom
• atoms are indivisible
• the atoms of different elements have
different weights
1869 Dimitri Mendeleev
presents the periodic table
- systematics
- substructure ???
Experiments show that atoms are NOT indivisible!
1897 J.J. Thomson demonstrates that ”cathode
rays” are negatively charged particles i.e. electrons
- first ”modern” model of the atom
1909 Ernest Rutherford directs experiments to confirm
the ”plum-pudding model” of the atom
- finds that most α’s go straight through a gold foil
- but a few are scattered at large angle
Is there anything ”inside” the nucleons?
p,n discovered by ~1930
bombarding nuclei with α’s
Rutherford probed at
a wavelength of ~40 fm
In the first half of the 20th century: study CR reactions with emulsion
Balloons were launched from
the top of the Bristol University
Physics Building by Powell et al.
Photo courtesy University of
Bristol.
New particles detected
using photo emulsion
π
μ
e
The pion
discovered 1947.
Accelerators and new particle detectors
Construction of the bevatron at Berkeley
Æ Discovery of the anti-proton 1955
Bubble chambers
SURPRISE
large energy is not sufficient to reveal the nucleon constituents!
At large beam particle energy
ƒ the target does not break up into constituents!
ƒ new particles are created!
ƒ mass is not conserved!
Are all these particles
fundamental?
The particle ZOO
Matter constituents
THE STANDARD MODEL
Anti-particles
THE STANDARD MODEL
Leptons and quarks interact through
electromagnetic or weak interactions.
Quarks also have strong interactions.
In quantum mechanics each force field has a corresponding
”field quantum” – a force mediator.
129th meeting of the American Association of
Physics Teachers, 2004
Feynman diagrams
• a pictorial technique treating particles and anti-particles
on equal footing
• a way of calculating cross-sections etc given a set of rules
• basic ideas illustrated in electromagnetic interactions
The fundamental building block of the Feynman diagram –
the vertex
• a point where a quark or lepton interact with a force carrier
• characterized by a ”coupling constant” – strength of interaction
arrow of time
before
after
destruction of a particle
destruction of an anti-particle
creation of a particle
creation of an anti- particle
Arrows DO NOT represent direction of motion (or momentum).
Vertex rules
• electric charge
• baryon number
• lepton number
are conserved
Energy need NOT be conserved!
ΔE ⋅ Δt ≈ h
• solid lines indicate fermions (anti-fermions)
labels at the end of the lines
force carriers (exchange particles) are indicated by
• dashed – wavy – or curly lines
coupling strength ~ electric charge
q
’
q
n (udd ) → p (uud ) + e − + ν e
μ − → ν μ + e− +ν e
e+ + e− → γ + γ
e+ + γ → e+ + γ
e+
e+
Conservation of
• lepton number
• el. charge
at each vertex!
γ
γ
Pictures make computations easy!
each vertex and each line
correspond to a math term
Feynman diagrams from
http://teachers.web.cern.ch/teachers/archiv/HST2002/feynman/Feynman.pdf