Overview of Particle Physics
-- the path to the Standard Model
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Topics
 historical flashback over development of the
field
o
o
o
o
o
“prehistory” 19th century
electron, radioactivity, nucleus
cosmic rays
spectroscopy era
collider era
 standard model of particle physics
2
A Century of Particle Physics
J.J Thomson
Top quark
1995
Electron – 1897
3
Sizes and
distance scales
 visible light:
wavelength
≈5∙10-7m
 virus 10-7m
 molecule 10-9m
 atom 10-10m
 nucleus 10-14m
 nucleon 10-15m
 quark <10-18m
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The Building Blocks of a Dew Drop
 dew drop: 1021 molecules
of water.
 Each molecule = one
oxygen atom and two
hydrogen atoms (H2O).
 Atom: nucleus
surrounded by electrons.
 Electrons bound to the
nucleus by photons
 nucleus of a hydrogen
atom = single proton.
 Proton: three quarks,
held together by gluons
just as photons hold the
electron to the nucleus
in the atom
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Very early era (19th century)
 chemistry, electromagnetism
 discharge tubes, “canal rays”, “cathode rays”
 photoelectric effect (Hertz, 1887)
 radioactivity (Becquerel, 1895)
 X-rays (Röntgen, 1895)

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Atoms, Nucleus
 electron: first hint that atom not indivisible
 natural radioactivity  understanding of
composition of atom, nucleus
 atom = nucleus surrounded by electrons (Geiger,
Marsden, Rutherford, 1906 -1911)
 hydrogen nucleus = proton, is component of all
nuclei (1920)
 neutron (Bothe, Becker, Joliot-Curie, Chadwick,
1930 – 1932)
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8
Cosmic rays
 Discovered by Victor Hess (1912)
 Observations on mountains and in balloon: intensity of cosmic
radiation increases with height above surface of Earth – must come
from “outer space”
 Much of cosmic radiation from sun (rather low energy protons)
 Very high energy radiation from outside solar system, but probably
from within galaxy
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Cosmic rays -- “elementary” particles
 new detectors (cloud chambers, emulsions)
exposed to cosmic rays  discovery of many new
particles
positron (anti-electron) : predicted by Dirac
(1928), discovered by Anderson 1932
 muon (μ): 1937 Nedermeyer
 pion (π) predicted by Yukawa (1935), observed
1947 (Lattes, Occhialini, Powell)
 strange particles (K, Λ, Σ,…..
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Particle Zoo
 1940’s to 1960’s :
 Plethora of new particles discovered
(mainly in cosmic rays):
 e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,….
 question:
 Can nature be so messy?
 are all these particles really intrinsically
different?
 or can we recognize patterns or
symmetries in their nature (charge, mass,
flavor) or the way they behave (decays)?
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The Particle Zoo!
±
,
0
,
±
,
e,
±
0
0
K , K S, K L,
0
+
 , p, n,  ,
0
 , , , …
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Particle spectroscopy era
 1950’s – 1960’s: accelerators, better detectors
 even more new particles are found, many of them
extremely short-lived (decay after 10-21 sec)
 1962: “eightfold way”, “flavor SU(3)” symmetry
(Gell-Mann, Ne’eman)
 allows classification of particles into “multiplets”
 Mass formula relating masses of particles in same
multiplet
 quark model – three different kinds of quarks
(u, d, s)
 Allows prediction of new particle Ω- , with all of its
properties (mass, spin, expected decay modes,..)
 subsequent observation of Ω- with expected
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properties at BNL (1964)
ΩBNL
1964
http://www.bnl.gov/bnlweb/history/Omega-minus.asp
 eight-fold way  quark model – particles
made up of three different “quarks” – u, d, s
 p = uud, n = udd,… Ω- = sss
 refinement of these ideas, more quarks,
“color”, gauge field theory
 Standard Model
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Standard Model
 A theoretical model of
interactions of elementary
particles, based on quantum field
theory
 Symmetry:
 SU(3) x SU(2) x U(1)
 “Matter particles”
 Quarks: up, down,
charm,strange, top, bottom
 Leptons: electron, muon, tau,
neutrinos
 “Force particles”
 Gauge Bosons
o  (electromagnetic force)
o W, Z (weak, electromagnetic)
o g gluons (strong force)
 Higgs boson
 spontaneous symmetry
breaking of SU(2)
 mass
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Contemporary
Physics
Education
Project17
Particles of Standard Model
Leptons
-1/3
-1
0
u
u
u
d
d
d
e
e
c
c
c
s
s
s


t
t
t
b
b
b
t
t
g
g
g
g
g
g
g
g
I
II
III

Z
W±
Bosons Fermions
+2/3
Quarks
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“every-day” matter
Proton
Neutron
d
u
u
u
d
Photon
d

Electron
e
Electron Neutrino
e
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Electromagnetic interaction
Proton
q1
Photon
q1q2
F k 2
r
Electron
q2
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Weak interaction Beta decay
Neutron
u
d
Mean lifetime of a free
neutron ~ 10.3 minutes
Proton
d
u
d
Mean lifetime of a free
proton > 1031 years!
u
W-
Anti-electron Neutrino
Electron
e
e
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The Strong Force
d
u
g
u
Strong force caused by
the exchange of gluons
d
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Forces (interactions)
 Strong interaction
1
 Binds protons and neutrons to form
nuclei
 Electromagnetic interaction 10-2
 Binds electrons and nuclei to form
atoms
 Binds atoms to form molecules etc.
 Weak interaction
10-10
 Causes radioactivity
 Gravitational interaction
10-39
 Binds matter on large scales
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What holds the world together?
interaction
strong
electromagnetic
weak
gravity
participants
quarks
charged
particles
all
particles
all
particles
1
10-2
10-10
10-39
g
gluon

relative strength
field quantum
(boson)
photon
W±
Z0
G
graviton
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The Discovery of Top Quark
1977 – 1992
Many null results
1992 – 1993
A few interesting
events show up
1994, CDF
First evidence
mt ~ 170 GeV/c2
1995 – CDF, DØ
Discovery!
1994, DØ
mt > 131 GeV/c2
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Creating Top Anti-Top Quark pairs
b
P
t

t
b
e  t uc

-1/ 3
2 / 3
W



 e   t d s
P
-2 / 3
1/ 3
e  t uc
-
W
-
-
-
 e   t d s
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-
Artist’s impression of a top event
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What do we actually “see”
_
t t e jets
Muon
Jet-1
Jet-2
Missing energy
Electron
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“event display” of a DØ
top event
t t  e   jets
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Ωb (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html
 2008 DØ experiment
at Fermilab:
 discover brother
of Ω- , the Ωb
 Ω- = sss,
Ωb = ssb,
 theory predicts
properties, decay
modes, ..
 confirmed by
experiment
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Summary
 we’ve come a long way ……
Standard Model (theory of particle interactions)
works embarrassingly well!
Has been tested by many hundreds of precision
measurements over last three decades – very
few measurements differ by more than 1 or 2
standard deviations
Even some amount of frustration – always hope
to see experimental result in disagreement with
theory
 But there are some open questions ………………… 31