M. Velasco -- Lecture 1

1
Introduction Historical Overview of Particle
Physics, Accelerators and Detectors

• M. Velasco -- Lecture 1 2
• Problems 1.1, 1.2, 1.7, 1.12, 1.13

2
Particle Physics

• What are the fundamental building blocks of the
  universe ? Visible matter, dark matter, dark
  energy
• What are the interactions between them?
  Gravitational, electro-weak, strong
• How can we explain the universe?
• its history
• its present form
• its future
• Is there a theory of everything? bring us back
  at the beginning of the universe

3
Clear relationship between energy of particle
and time
4
What is a particle and how we learn from them?

• a small piece of matter...
• characterized by
• charge
• mass
• lifetime
• spin
• particles can scatter off each other like
  billiard balls
• unlike billiard balls, most particles are
  unstable and decay
• particles can be produced by colliding other
  particles and form bound states

5
Models used to described general principles
Small
Classical Mechanics Quantum Mechanics
Relativistic Mechanics Quantum Field Theory
Fast
Quantum Gravity
What is missing?
6
Quantum Field Theories included in Standard Model
QEDQuantum Electro Dynamics
QCDQuantum Chromo Dynamics
Electro-Weak
7
Production of Particles
Primary cosmic rays
90 protons, 9 He nuclei
Air nuclei (Nitrogen Oxygen)
?
?
e
??
Nuclear Reactors ? alpha, neutrons, etc.
8
Chronology of Early DiscoveriesInterplay with
introduction of new detectors/particle sources

• Electron (1897) J.J. Thompson
• Cloud Chamber(1912) C.T.R.Wilson
• Cosmic Rays(1913) V.F.Hess C.Anderson
• Discovery of Proton(1919) E. Rutherford
• Compton Scattering ge?ge (1923) C.T.R.Wilson
• Waves nature of es(1927) C. Davisson

1900 1910 1920 1930 1940 1950
1960 1970 1980 1990 2000
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Cloud Chamber

• Supersaturated Gas
• Cloud formation
• Used until 1950s
• Condensation started around the ions generated by
  passing charged particles (ionization), and the
  resulting droplets were photographed.

9
10
Scattering
GeigerMarsden
?, b
source
Zinc Sulphide Screen
E. Rutherford 1927, Rutherford, as President of
the Royal Society, expressed a wish for a supply
of "atoms and electrons which have an individual
energy far transcending that of the alpha and
beta particles from radioactive bodies..."
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Penetrating Power
?
?
?
Neutron
Paper sheet
Lead
Paraffin
Aluminum
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Cross-Section
1 barn 10-24 cm2 Approximately the area of a
proton Radii of nuclei fm

• Distribution of scattering angles tell us about
  the force/particles
• Precision required

13
Accelerator technology
The first successful cyclotron, built by Lawrence
and his graduate student M. Stanley Livingston,
accelerated a few hydrogen-molecule ions to an
energy of 80,000 electron volts. (80KeV)
1932- 1MeV
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Particle guidance

• In circular machines use magnetic field to guide
  particles along orbit (Lorentz force)
• in early machines e.g. cyclotrons B field
  occupied entire accelerating plane
• What about machines with larger energies like the
  one we need today?
• Can you guess based on your basic knowledge of
  EM ?

16
Kinematics of circular accelerators

• Use relativistic equations of motion (v c)
• Centripetal force Lorentz force (magnetic)
• ?mv2/r ?mv? qvB/c v ? B ?
  1/?(1 - v²/c²)
• ? rev. freq. ? ?/2? qB/2??mc
• at relativistic speeds v c and momentum P ?mc
• ? ? c/2?? qB/2?P ? radius of orbit
• ? P qB?/c
• or, P (GeV/c) 0.3 B (Tesla) ? (m) ( q
  e )
• In another words, ? (q/2?mc) ?(1 - v²/c²) B
• as particle accelerates, v increases, ? B and/or
  ? must increase to compensate
• in electron synchrotrons (LEP) ? fixed , B
  increases

17
Summary of what the world was made of by 1932

• electrons (1897)
• orbit atomic nucleus
• photon (1905)
• quantum of the electromagnetic field
• proton (1911)
• nucleus of lightest atom
• neutron (1932)
• neutral constituent of the nucleus
• ? Required new experimental techniques...not
  stable more questions

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Postulates ? to explain what was observed known
at that time

• 1927 Diracs relativistic quantum mechanics
• antiparticles for every particle there exists an
  antiparticle with same mass, lifetime, spin, but
  opposite charge
• 1931 the positive electron (positron) found
• 1930 Paulis neutrino
• energy conservation in beta (b) decay requires
  the existence of a light, neutral particle
• n ? p e- ? (e- b)
• observed in 1956 ? Why it took so long?
• To come 1937 Yukawas pion to explain
  inter-nuclear forces

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Just as the equation x24 can have two possible
solutions (x2 OR x-2), so Dirac's equation
could have two solutions, one for an electron
with positive energy, and one for an electron
with negative energy. Dirac interpreted this
to mean that for every particle that exists there
is a corresponding antiparticle, exactly matching
the particle but with opposite charge. For the
electron, for instance, there should be an
"anti-electron" called the positron identical in
every way but with a positive electric charge.
E2 p2c2 (m0c2) 2 relativistic
invariant (same value in all reference frames)
20

• 1931 the positive electron (positron)

21
Neutrinos must be present to account for
conservation energy momentum
__
Wolfgang Pauli

• Large variations in the emission velocities of
  the ? particle seemed to indicate that both
  energy and momentum were not conserved.
• This led to the proposal by Wolfgang Pauli of
  another particle, the neutrino, being emitted in
  ? decay to carry away the missing mass and
  momentum.

22
1937 Theory of nuclear forces
Hideki Yukawa
Existence of a new light particle (meson) as
the carrier of nuclear forces (140GeV) Relation
between interaction radius meson mass m
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1932-1947

• Neutron(1932) J.
  Chadwick
• Triggered Cloud Chamber(1932) P.Blackett
• Muon(1937) S.H.
  Neddermeyer
• Muon Decay(1939) B.Rossi, Williams
• Kaon(1944) L.
  Leprince-Ringuet
• Pion(1947) .H.Perkins,G.P.S.Occialini
  

1900 1910 1920 1930 1940 1950
1960 1970 1980 1990 2000
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Emulsion heavily used in the early days of
Cosmic Ray experiments

• Dates back to Becquerel (1896)
• Three components
• silver halide (600mm thick)
• plate
• target
• Grain diameter 0.2mm
• Still the highest resolution device

25
Emulsion ? used in discovery of m, p, k, etc.
m
Scale 100mm
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The particle Zoo ? Cosmic rays 1st , followed
by accelerator

• 1947 strange particles
• K0?? ?-, K?? ? ?-
• ??p ?-
• ?, ?
• long lifetime ? 10-10 s
• more particles...
• ??p?,
• ????
• short lifetime ? 10-24 s

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1947-1953

• Efficient production of particles with higher
  masses is going to required high energy
• Before 50s Emc2 was still just a theory
• Next period will required the development of both
  accelerators in addition to detectors
• Cockcroft and Walton

28
Energy and momentum for relativistic particles
(velocity v comparable to c) Speed of light in
vacuum c 2.99792 x 108 m / s
Total energy
Expansion in powers of (v/c)
Momentum
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Cockcroft Walton Accelerator

• First artificial splitting of nucleus
• First transmutation using artificially
  accelerated particles
• First experimental verification of
• E mc2

30

• Relativity
• Mass not conserved
• ? Energy Momentum are conserved

Experimental verification of E mc2
17.3 MeV
1 MeV
Proton Lithium 2 a particles Energy
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Other discoveries between 1947-1953

• Scintillation Counters(1947) F.
  Marshall
• pion decay(1947) C.
  Lattes
• Unstable Vs(1947)
  G.D.Rochester
• Semi-Conductor Detectors(1949) K.G.McKay
• SparkChambers(1949) J.W.Keuffel
• K Meson decays(1951) R.Armenteros

1900 1910 1920 1930 1940 1950
1960 1970 1980 1990 2000
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Basic principles of particle detection
Passage of charged particles through
matter Interaction with atomic electrons


K
p
ionization (neutral atom ? ion free electron)
p
e
excitation of atomic energy levels (de-excitation
? photon emission)
m
Momentum
Mean energy loss rate dE /dx

• proportional to (electric charge)2
• of incident particle

• for a given material, function only
• of incident particle velocity

• typical value at minimum
• -dE /dx 1 2 MeV /(g cm-2)
• What causes this shape?

33
Most detectors at that time based on Ionization

• Charged particles
• interaction with material

track of ionisation
34
Ionization
Density of electrons

• Important for all charged particles

• Bethe-Bloch Equation

velocity
Mean ionization potential (10ZeV)
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Ionization

• In low fields the ions eventually recombine with
  the electrons
• However under higher fields it is possible to
  separate the charges

Note e-s and ions generally move at a different
rate


E






36
1953-1968

• Neutrino (1953) F. Reines
• Bubble Chamber(1953) D.A. Glaser
• K Lifetime(1955)
  L.W.Alvarez
• Flash Tubes(1955) M.
  Conversi
• Spark Chamber(1959) S. Fukui
• Streamer Chambers(1964) B.A.Dolgoshein
• MWPC(1968) G.
  Charpak

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1960 1970 1980 1990 2000
37
CERN
LEP-1984-1999
SC 1957-1990
Synchrotron Radiation
SLAC
38
Before we move to accelerator based measurement
lets talk about neutrinos - n ? Puzzle in b
decay the continuous e- energy spectrum
First measurement by Chadwick (1914)
Radium E 210Bi83 (a radioactive isotope
produced in the decay chain of 238U)
If ? decay is (A, Z) ? (A, Z1) e, then the
emitted electron is mono-energetic ? e- total
energy E M(A, Z) M(A, Z1)c2 (neglecting
the kinetic energy of the recoil nucleus
½p2/M(A,Z1) ltlt E)
39
Theory of ?-decay
Fermis theory ? particles emitted in ? decay
need not exist before emission
they are created at the instant of
decay
Prediction of ? decay rates and electron energy
spectra as a function of only one parameter
Fermi coupling constant GF (determined from
experiments)
40
First neutrino detection
(Reines, Cowan 1953)
E? 0.5 MeV

• detect 0.5 MeV ?-rays from
• ee ? ?? (t 0)

• neutron thermalization
• Followed by capture in Cd nuclei
• Emission of delayed ?-rays
• (average delay 30 ?s)

Event rate at the Savannah River nuclear power
plant 3.0 ? 0.2 events / h in
agreement with expectations
41
Muon decay
Decay electron momentum distribution
Muon spin ½
Muon lifetime at rest ?? 2.197 x 10 - 6 s ?
2.197 ?s
Muon decay mean free path in flight
? muons can reach the Earth surface after a
path ? 10 km because the decay mean
free path is stretched by the relativistic time
expansion
42
Lepton Number Conservation
Electron, Muon and Tau Lepton Number
Lepton Conserved Quantity Lepton Number
e- Le 1
ne Le 1
m- Lm 1
nm Lm 1
t- Lt 1
nt Lt 1
Anti-Lepton Conserved Quantity Lepton Number
e Le -1
ne Le -1
m Lm -1
nm Lm -1
t Lt -1
nt Lt -1
We find that Le , Lm and Lt are each conserved
quantities
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Lepton Number Conservation

• .

• .

• .

• .

m ? e g
44
Other conserved quantities
Baryon Number Conservation When we collide
particles together, we find that the number
ofbaryons is conserved.
A B ? C D

• For each baryon, we simply assign B 1
  (protons, neutrons,for example)
• For each anti-baryon ,we assign B -1
  (antiprotons, antineutrons,for example)
• ? Compute the total baryon number on each side
  and they must be equal!

45
Baryon number conservation B 1 for baryon in a
decay or reaction, and B -1 for each
anti-baryon, then the total baryon number must
be the same before and after the process.
Eg p n ? p p n p-
1 1 ? 1 1
1 -1

• .

p n ? p p p- 1 1 ?
1 1 -1
X
46
Recall We had many new types of matter!
More and More Mystery particles
Fermilab Bubble Chamber Photo
47
Strange particles observedLong lifetimes Heavy
48
Invention of a new, additive quantum number
Strangeness (S) (Gell-Mann, Nakano, Nishijima,
1953)

• conserved in strong interaction
• processes

• not conserved in weak decays

S 1 K, K S 1 ?, ?, ? S 2
?, ? S 0 all other particles (and
opposite strangeness S for the corresponding
antiparticles)
49
Summary of strangeness puzzles their
contribution to the SM

• 1944-47 Strangeness ? quark model
• ? Basis for QCD
• 1956 Parity violation
• ?Spin-dependence of weak interactions
• 1964 Suppression of Flavour Changing NC
• ? Suggested charm quark
• ? Properties of the neutral currents
• 1964 CP violation
• ? Absolute matter-antimatter asymmetry

50
Puzzle 1 -- Strange particles observedLong
lifetimes Heavy

• Strangeness
• - produced by strong interaction
• conserved by strong interactions
• ? these strange particles produced in pairs

d
g
s
u
u
d
51
Invariance under Lorentz transformation implies ?
CPT invariance
Therefore big impact on the foundation of the
theory, if interactions behave in different ways
under Charge conjugation(C) reverses the
electric charge all the internal quantum
numbers. Parity (P) space inversion reversal
of the space coordinates. Time reversal (T)
replacing t by -t. This reverses time derivatives
like momentum and angular momentum. ? Particles
and antiparticles have identical masses and
lifetimes. This arises from CPT invariance of
physical theories and is used experimentally to
test CPT.
52
Puzzle 2 Parity violating DecaysV-A Theory
of Weak Interactions (WI)

• Kaons are mesons (Spin 0 Parity -1)
• K ? pp0 P(-1)(-1) Even
• ? pp-p P(-1)(-1)(-1) Odd

• Strangeness not conserved WI

Extra confidence in the V-A theory (Spin-Flip) ?
BR 63
Helicity suppressed due to low mass of e ? BR
0.0015
53
Puzzle 3 Low rate of KL?mm- Predicts no
mixing with Z0 boson existence of Charm Quark
Consistent with observed rate 10-5
If possible should represent 60 of the decays
? Not Observed
Flavor Changing Neutral Current (FCNC) not
allowed
Extra u like quark needed to get proper rate?
Charm
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Puzzle 4 CP violating Decays (CP) K0
reveals a more intricate picture

• Flavor Eigenstate K0 - K0

oscillations
d
s
-
W-
K0
K0
u, c, t
u, c, t
_
_
W
d
s
s
d
u, c, t
-
_
W-
W
_
_
K0
K0
_
_
u, c, t
d
s
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K0 - K0 Oscillation quantified from leptonic
decay
Get positron
Kaon Interferometry G gtgt G- G Dm
Or electron
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Puzzle 4 CP violating Decays (CP) K0
reveals a more intricate picture

• Flavor Eigenstate K0 - K0

oscillations

• CP Eigenstate

d
s
-
W-
K1??o?o K1???- K2???-?o K2??o?o?o
K0
K0
u, c, t
u, c, t
_
CP1
_
W
d
s
s
d
u, c, t
-
_
W
W-
_
CP-1
_
K0
K0
_
_
u, c, t
d
s

• Mass Eigenstate ? Before observation of CP
  violation

? 0.9 x 10-10 s
? 5.2 x 10-8 s
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Why Puzzle 4 was so interesting? Potential
Solution to the Baryon Asymmetry in the Early
Universe
? 2 g
10,000,000,001
10,000,000,000
? They basically have all annihilated away except
a tiny difference between them
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Baryon Asymmetry in the Current Universe
us
1
This is us TODAY!!!
After 40 years of studying CP-violation in the
quark sector Now we know that the effect is too
small to be source of the Baryon Asymmetry
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THE STATIC QUARK MODEL
Late 1950s early 1960s discovery of many
strongly interacting particles at the high energy
proton accelerators (Berkeley Bevatron, BNL AGS,
CERN PS), all with very short mean life times
(1020 1023 s, typical of strong decays) ?
catalog of gt 100 strongly interacting particles
(collectively named hadrons)
ARE HADRONS ELEMENTARY PARTICLES?
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The quark model

• 1964 Gell-Mann, Zweig
• there are three quarks and their antiparticles
• each quark can carry one of three colors
• red blue green
• anti-quarks carry anticolor
• anti-red anti-blue anti-green

Quark Up Down Strange
Charge 2/3 -1/3 -1/3
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The quark model

• only colorless (white) combinations of quarks
  and antiquarks can form particles
• qqq
• qq
• ?no others observed

62
SU(3) Flavor Symmetry ? uds

• 1964 Symmetry operations on an octahedron
  illustrate the theory of quarks. Theorist Murray
  Gell-Mann (and, independently, Yuval Ne'eman)
  discovered a theory that organized all the
  particles into families with properties
  mathematically the same as those of a "group of
  eight" in abstract algebra. Gell-Mann called it
  "The Eightfold Way." When physicists recognized
  that underlying fundamental particles could
  explain the eightfold pattern, the idea of quarks
  was born.
• In the 1970s, experiments at the Department of
  Energy's SLAC showed that quarks were not just
  mathematical constructs but real building blocks
  of protons and neutrons.

63
The 8-fold way
baryons qqq
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Prediction and discovery of ? particle
A success of the static quark model
The decuplet of spin baryons
65
http//math.ucr.edu/home/baez/qg-spring2003/eightf
old/
ETC
66
Quark confinement

• What holds quarks/antiquarks together?
• strong force
• acts between all colored objects
• short range
• independent of distance

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1968-1999
First major discovery with Solid State Detectors

• J/? (charm) (1974) J.J, Aubert, J.E. Augustin
• t lepton(1975)
  M.Perl et al
• B-mesons(1981)
  CLEO
• W,Z(1983)
  UA1
• number of n (1991)
  LEP
• t-quark(1994)
  CDF

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1960 1970 1980 1990 2000
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Conclusion -- Standard Model Fundamental
Particles
Leptons q1 q0
Missing H Higgs q0
Quarks q2/3 q-1/3
Force Carriers
q0
q0
q0
q1