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Introduction to Particle
Physics
Particle Physics
• This is an introduction to the
• Phenomena (particles & forces)
• Theoretical Background (symmetry)
• Experimental Methods (accelerators &
detectors)
of modern particle physics
• That is, it is not a “real” introduction to
particle theory (there are other modules!)
• Rather, it will attempt to give you the
information and tools needed to understand
and appreciate the history and new results
in the field
Particle Physics
• Elementary particle physics is
concerned with the basic forces of
nature
• Combines the insights of our deepest
physical theories
• Special Relativity
• Quantum Mechanics
• Matter, at its deepest level, interacts
by the exchange of particles
Hierarchies of Nature
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Animal Life
Biology
Chemistry
Atomic Physics
Nuclear Physics
Subatomic physics
• Particle physics does not and will not
explain everything in nature.
• It does provide strong constraints on
what nature can do
What is a particle?
• Not an easy question!
•
•
•
•
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Is
Is
Is
Is
Is
a speck of dust a particle?
an atom a particle?
a nucleus a particle?
a proton a particle?
an electron a particle?
• At different times, each of these were
considered to be particles
• No substructure seen – need to break it
• No excited states seen – watch it decay
• How does one probe smaller and smaller
sizes?
Probing structure
• We see with our eyes by
• Light scattered from objects
• Light emitted from objects
• The size of the objects we can see are
limited by the wavelength of visible
light
• How do we see smaller structure?
Accelerators and Detectors
• Accelerators provide a consistent source of
charged particles traveling at speeds near
that of light
• The energy of the accelerated particles
dictates the kind of physics you are probing
• Atomic scale – 10’s of eV (Hydrogen)
• Nuclear physics – 10’s of MeV (Binding energy)
• Particle physics – 100’s of MeV (exciting proton
structure)  100’s of GeV (Electroweak
unification)
• At the lower scales, particles are really
particles since you do not perceive their
substructure or excited states
Conserved Quantities: Mechanics
• Noether’s theorem
• For every continuous symmetry of the laws
of physics, there must exist a conservation
law.
For every conservation law, there must
exist a continuous symmetry.
• Invariance under
• Time translation – Energy
• Space Translation – Momentum
• Rotation – Angular momentum
• These quantities are obeyed in any system
– on any level
• Easiest assumption is that they are obeyed
locally!
Waves and Particles
• Electromagnetic forces are
propagated by fields between charges
• Classically characterized by waves that
carry energy & momentum & spin
• Quantum mechanics describes
particles as a wave packet.
• The wave packet carries energy,
momentum, and spin
• The quantum theory of fields
(Quantum Field Theory) describes the
fields which couple to particles  as
particles!
Fundamental Matter Particles
LEPTONS
QUARKS
What is a Force?
• Every law of physics you have learned
boils down to involving two classes of
phenomena:
• Conserved quantities:
• Mechanical
• Energy, momentum, angular momentum
• Related to time, translation, and rotation
invariance
• Number
• Charge conservation, law mass action in
chemistry
Forces of Nature
• Now we know what there “is”
• How do they talk to each other?
We have managed to find four forces:
How did we get here?
• This picture of the world didn’t just
emerge naturally
• It is the synthesis of a wide variety of
experimental data
• It is worthwhile to consider how
certain things were discovered
Radioactivity
• End of the 19th century
• Discovery of three “particles” emitted
by nuclei
• Alpha  Turned out to be 4He
• Beta  Turned out to be an electron
• Gamma  Turned out to be a photon
• Amazing – already the strong, weak,
and electromagnetic interactions
were visible
• But they were not distinguishable at this
point
Proton & Neutron
• Rutherford identified the proton as the
nucleus of the hydrogen atom
• Neutron was discovered by James
Chadwick by bombarding beryllium
with alpha particles
4
2
He  Be  C  n
9
4
12
6
1
0
Nucleus
• Before Rutherford, people thought the
atom was a diffuse cloud of protons
and neutrons
• Rutherford found that there was
scattering off of a point source in the
atom
• Short distances allowed large momentum
transfers – even back-scattering
• Like firing a cannonball at tissue paper,
and having it bounce back!
The Electron
• Thomson identified
the cathode rays as a
new type of matter
• Same charge as a
proton
• Much lighter!
Mesons & The Strong Force
• But what held the nucleus together
• Coulomb forces should repel the
protons
• Something stronger must be present
• Yukawa postulated a force similar to
the photon, but massive
• Strong, but limited in range
• Nuclear size suggested / R ~ m  100 MeV
Particles from the Sky!
• Up in the mountains of
Europe, scientists
detected high-energy
particles in emulsion
and cloud chambers
• Discovered new
particles which were
lighter than nucleons
but much heavier than
electrons
• New particles
• Pion
• Muon
• Similar in mass, but
interacted very
differently
The Muon
• Did not suffer nuclear interactions
• Rather, was quite penetrating
• Like an electron, but slower (more
massive) at the same momentum
m  105.7 MeV
dE
Z z 2 ( c) 2  2me v 2  2 v 2 
2
 4 N A
ln
 2 
2
dx
A mev 
I
c 

Ionization energy loss
of charged particles
The Pion
• Other meson events appeared to show a negative
particle which stopped in the emulsion, was
absorbed by a nucleus, and then “exploded” into
“stars” (D.H. Perkins was one who observed these!)
• The positive particles seemed to stop and then
decay into the previously-seen muons
• These had a similar mass to the mesons, but
clearly had different interactions m  135 MeV

• Recognized as strongly-interacting particles, more
like Yukawa’s predictions!
Antimatter
• As soon as Dirac combined
• Special Relativity
• Quantum Mechanics
in a way that was symmetric in
space & time, he found that his
equation described spin-1/2
particles
• It also predicted negative energy
solutions for fermions
• Predicted “anti-particles” in
nature, with opposite charge but
same mass
• Anti-electron  positron was
discovered in cosmic rays
• Anderson’s cloud chamber
• Curvature gives momentum
• Length gives rate of energy loss
Only consistent with
light positive particle
Accelerators and Detectors
• In order to probe down to smaller distances, you
need large energies
• Development of accelerator technology was rapid in the
first half of 20th century
• Three major types
• Linear accelerators
• Cyclotrons
• Synchrotrons
• With increasing energy,
require increasing
sophistication of tools used
to detect particles
• Detector technology
Accelerators
Cyclotron
Linear Accelerator
Synchrotron
Detectors
• Making subatomic particles visible to human
senses
• Most commonly-used principles
• Scintillation – charged particle produces light
• Ionization – charged particle produces charged ions
• Magnetic spectrometers – tracking a particle through a
magnetic field: p (MeV) = .3 qB(kG)R(cm)
Bubble Chamber
•
The bubble chamber was the
most instructive detector of
the early years
• Liquid kept under overpressure,
but below the boiling point
• When particles passed through,
stopper pulled out, reducing
boiling point and bubbles
formed around tracks
• Photograph of tank created a
full image of the event
• However, slow and difficult to
extract only the events you
wanted (e.g. for rare particles)
•
These days, the granularity and
complexity of the collisions
have made the bubble chamber
obsolete
• But excellent for pedagogy!
Strange Particles
• In cloud chamber, bubble
chamber and emulsion
experiments new particles
were being discovered at a
fast rate in the 40’s and 50’s
• Some particles appeared to be
• Produced immediately (strong
interactions)
• Decaying only after a
considerable time (weak
interaction)
• Produced in pairs – looks like a
quantum number
• Given name “strangeness”
Conserved quantities
• Without detailed understanding of the
interactions, particles were classified by
their quantum numbers, in the hope that
some scheme would emerge
• Multiplicative
• Parity – behavior of wave function under spatial
inversion
• Charge conjugation – symmetry if charges were
flipped
• Additive
• Isospin – used to group particles into doublets
and triplets, like an internal spin
• Strangeness – characteristic of long lived
particles
The Particle Zoo
• Pre-standard model particle physics was
characterized by an increasing particle zoo
Quark Model
•
Gell-Mann and Ne’eman
explained the spectrum of
hadronic states with similar
quantum number by means of
“quarks”
• Baryons (p, n, L) have 3 quarks
• Mesons have one quark, and one
anti-quark
•
•
Transform states into each other
using “rotations”
• UpDown
• DownStrange
• StrangeUp
Particles with similar spin and
parity fell into multiplets
• SU(3) symmetry increasingly
broken with increasing
strangeness
•
Predicted unobserved states,
like W
S
mD ~ 1230 MeV
D
Do
D
mS ~ 1385 MeV
S
So
m ~ 1530 MeV

D
S

mW ~ 1672 MeV
q
q
W
q
I3
Neutrinos
• Neutrino proposed by Pauli to
account for energy released in
b-decay
• Reines and Cowan showed
that neutrinos were actual
particles
• Steinberger, Schwartz and
Lederman showed that muons
had their own neutrino
New law of nature:
Lepton number is
conserved separately
n  p   e

e  p  n  e

  X Y  
  p  n  

The Later Years
• After the quark model, the zoo reduced to six microbes.
Then it became chase after heavier and heavier particles
t
Weak and Strong Interactions
• While weak and strong interactions
were now extensively studied, and
theoretical concepts existed for their
deeper structure, experiments were
still limited in energy
• Thus, difficult to probe
• Force carriers of weak interactions
• Substructure of hadrons
Partons
•
•
•
For a long time, quarks were seen as simply a convenient
mathematical tool to account for quantum numbers
No evidence for free quarks in nature
Scattering experiments at SLAC did the same thing as Rutherford
• Found that large momentum transfers were possible – as if the proton
has pointlike consituents
•
Measured “structure functions” that characterize the momentum
distributions of the “pieces” of the proton
Electroweak Unification
• Many features of the weak interactions
• Long lifetimes
• Parity violation
• Isotropic decays
• Explained by
• Heavy intermediate bosons (like the Yukawa force, but
much shorter range)
• Coupled to left-handed fermions
• The features were then unified with the
electromagnetic force by Glashow, Salam and
Weinberg – who received the Nobel in 1979
• The weak force is carried by W and Z bosons of M~90 GeV
• The massless photon is induced by the presence of a
condensate of “Higgs” bosons, that spontaneously breaks
the symmetry of the interaction
Charmed Particles
• A case where theory led
experiment
• Weak interactions seemed to
require a change of strangeness
K      l  l
K    0  l  l
• “Neutral currents” not seen in
decays of kaons to pions  Always
a change in charge
• This was explained naturally by
the existence of a fourth quark
• The J/Y particle (M~3.1 GeV!) was
found near-simultaneously at BNL
and SLAC in 1974!
• Not just a new quark:
• Completed the second family of
quarks and leptons
• Nobel prize awarded in 1976 (just
two years later…)

p
p
y

Tau & Bottom
• As energies increased in both e+e- colliders and
fixed target proton beams, new particles started
appearing in the mid-70’s
• Mark II observed strange events with one electron
and one muon
• Suggested new lepton that decayed into e or m
t       t t  e   e  t




• Leon Lederman et al observed new peaks around
10 GeV.
• Suggestive of yet another quark m~5 GeV
• A new family was found
• Required another neutrino and another quark
• Took around 20 years to find both!
Gluons
•
Still, there were some mysteries
• It seemed as if the quarks only carried ½ the momentum of a proton
•
Moreover, it was clear that quarks could not be the whole story
• No way for a particle to be in the uuu state unless each u quark carried a
distinct quantum number!
•
This led to the “colour hypothesis” of Nambu, which evolved into
Quantum Chromodynamics in the early 1970’s
• Quarks came in 3 colors – so each u quark was a different particle
•
Another gauge symmetry  “long range” force to maintain it
• QCD predicted that gluons could be radiated from quarks (and gluons)
just like photons from electrons
W&Z
• Electroweak unification
required W and Z


e
0


W  e 
Z e e
• Found by Carlo Rubbia and
collaborators at the CERN
SppS exactly where
expected!
• MW ~ 80 GeV
• MZ ~ 90 GeV
• Another case of theory
leading experiment.
• But experimentalists got the
Nobel in 1984 (3 years later!)
• The collider era had really
begun!
Colliders in Use
HERA e+p 30+900 GeV
LEP, e+e- 91-209 GeV
Tevatron, p+p 2 TeV
RHIC, Au+Au 200 GeV/N
The Top Quark
• The discovery of the charm
quark led us to believe that all
quarks come in doublets.
• Thus, the lonely bottom quark
(5 GeV) was a problem for many
years
• Only in 1995 was the top quark
identified in p+p collisions at
Fermilab
• Mass of 170 GeV – Almost like a
gold nucleus!
• Required deep understanding
of almost everything before it
• Single lepton production
• Jet production from W’s
• QCD backgrounds (soft & hard)
• Essentially completed the
standard model
• OK, the tau neutrino was only
established in 2000…
Neutrino Oscillations
• Super-Kamiokande is
originally designed to
search for proton decay
• 50k tonnes of water
• 11k phototubes to detect
light
• ’98 Detected a
significant deficit of
muon neutrinos,
especially when coming
through the earth
• Fit hypothesis of
neutrinos oscillating –
changing flavor
• Not part of the standard
model – yet!
The Higgs
• The Higgs particle,
couples to all massive
particles (quarks and
leptons)
Higgs Condensate
M=0
M=m
• However, direct searches
for the Higgs have been
without success
• The data may suggest
MH~114 GeV…
• The LHC is the ultimate
hope for understanding the
origin of mass
The Future??
• As we push towards a deeper
understanding of nature, our laboratories
are seeming less and less sufficient
• Much recent progress in particle physics
comes from the side of cosmology
• Kind of ironic
• Many subatomic particles seemed to come from
space (pion, muon, etc)
• We learned all about the world at hand through
the patterns these particles made
• Now we are heading back to space, to see what
more we can figure out!
What is left (i.e. What I may not cover!)
• Heavy Ion Physics
• Search for quark-gluon matter
• Supersymmetry
• Symmetry between Bosons & Fermions
• Dark Matter / Dark Energy
• Seems to require new particles, which are
clearly all around us!
• Superstrings / Extra Dimensions
• Physics of the 21st century that appeared
miraculously in the 1980’s
• Particles are vibrating strings, embedded in a
many-dimensional space where only 4 are
allowed to be macroscopic!