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Introduction to ParticlePhysics
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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 theinformation and tools needed to understand
and appreciate the history and new results
in the field
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Particle Physics
Elementary particle physics is
concerned with the basic forces ofnature
Combines the insights of our deepest
physical theories Special Relativity
Quantum Mechanics
Matter, at its deepest level, interactsby the exchange of particles
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Hierarchies of Nature
Animal Life
Biology Chemistry
Atomic Physics
Nuclear Physics Subatomic physics
Particle physics does not and will notexplain everything in nature.
It does provide strong constraints onwhat nature can do
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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 visiblelight
How do we see smaller structure?
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Accelerators and Detectors
Accelerators provide a consistent source of
charged particles traveling at speeds nearthat of light
The energy of the accelerated particlesdictates the kind of physics you are probing
Atomic scale10s of eV (Hydrogen) Nuclear physics10s of MeV (Binding energy)
Particle physics100s of MeV (exciting protonstructure) 100s of GeV (Electroweakunification)
At the lower scales, particles are reallyparticles since you do not perceive theirsubstructure or excited states
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Conserved Quantities: Mechanics
Noethers theorem
For every continuous symmetry of the lawsof physics, there must exist a conservationlaw.For every conservation law, there mustexist a continuous symmetry.
Invariance under Time translationEnergy
Space TranslationMomentum
RotationAngular momentum These quantities are obeyed in any systemon any level
Easiest assumption is that they are obeyedlocally!
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Fundamental Matter Particles
QUARKSEPTONS
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What is a Force?
Every law of physics you have learned
boils down to involving two classes ofphenomena:
Conserved quantities:
Mechanical Energy, momentum, angular momentum
Related to time, translation, and rotation
invariance
Number
Charge conservation, law mass action in
chemistry
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Forces of Nature
Now we know what there is
How do they talk to each other?
We have managed to find four forces:
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How did we get here?
This picture of the world didnt just
emerge naturally It is the synthesis of a wide variety of
experimental data
It is worthwhile to consider howcertain things were discovered
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Radioactivity
End of the 19thcentury
Discovery of three particles emittedby nuclei
Alpha Turned out to be 4He
Beta Turned out to be an electron
Gamma Turned out to be a photon
Amazingalready the strong, weak,and electromagnetic interactions
were visible But they were not distinguishable at this
point
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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
2
4He 4
9Be6
12C 0
1n
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Nucleus
Before Rutherford, people thought the
atom was a diffuse cloud of protonsand neutrons
Rutherford found that there wasscattering off of a point source in theatom
Short distances allowed large momentumtransferseven back-scattering
Like firing a cannonball at tissue paper,and having it bounce back!
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The Electron
Thomson identifiedthe cathode rays as a
new type of matter
Same charge as a
proton
Much lighter!
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Mesons & The Strong Force
But what held the nucleus together
Coulomb forces should repel theprotons
Something stronger must be present
Yukawa postulated a force similar tothe photon, but massive
Strong, but limited in range
Nuclear size suggested / ~ 100R m MeV
f S
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Particles from the Sky! Up in the mountains of
Europe, scientists
detected high-energyparticles in emulsionand cloud chambers
Discovered newparticles which werelighter than nucleonsbut much heavier thanelectrons
New particles
Pion Muon
Similar in mass, butinteracted verydifferently
Th M
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The Muon
Did not suffer nuclear interactions
Rather, was quite penetrating Like an electron, but slower (more
massive) at the same momentum
105.7m MeV
dE
dx 42
NA
Z
A
z2( c)2
mev
2 ln
2mev
2
2
I
v2
c2
Ionization energy lossof charged particles
Th Pi
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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
Recognized as strongly-interacting particles, more
like Yukawas predictions!
135m MeV
A ti tt
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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 energysolutions for fermions
Predicted anti-particles in
nature, with opposite charge but
same mass
Anti-electron positron wasdiscovered in cosmic rays
Andersons cloud chamber
Curvature gives momentum
Length gives rate of energy loss
Only consistent withlight positive particle
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A l t
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Accelerators
Cyclotron Linear Accelerator Synchrotron
D t t
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Detectors
Making subatomic particles visible to human
senses
Most commonly-used principles
Scintillationcharged particle produces light
Ionizationcharged particle produces charged ions
Magnetic spectrometerstracking a particle through a
magnetic field: p (MeV) = .3 qB(kG)R(cm)
B bbl Ch b
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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 andcomplexity of the collisions
have made the bubble chamber
obsolete
But excellent for pedagogy!
St P ti l
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Strange Particles
In cloud chamber, bubble
chamber and emulsion
experiments new particles
were being discovered at a
fast rate in the 40s and 50s
Some particles appeared to be
Produced immediately (strong
interactions)
Decaying only after a
considerable time (weak
interaction)
Produced in pairslooks like a
quantum number
Given name strangeness
C d titi
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Conserved quantities
Without detailed understanding of theinteractions, particles were classified bytheir quantum numbers, in the hope thatsome scheme would emerge
Multiplicative
Paritybehavior of wave function under spatialinversion
Charge conjugationsymmetry if charges wereflipped
Additive Isospinused to group particles into doublets
and triplets, like an internal spin
Strangenesscharacteristic of long livedparticles
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Th P ti l Z
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The Particle Zoo
Pre-standard model particle physics was
characterized by an increasing particle zoo
Quark Model
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Quark Model Gell-Mann and Neeman
explained the spectrum ofhadronic states with similar
quantum number by means ofquarks
Baryons (p, n, L) have 3 quarks
Mesons have one quark, and oneanti-quark
Transform states into each
other using rotations UpDown
DownStrange
StrangeUp
Particles with similar spin andparity fell into multiplets
SU(3) symmetry increasinglybroken with increasingstrangeness
Predicted unobserved states,like W
D
S
I3
DDD
S S S
Wq qq
~ 1230m MeV D
~ 1385m MeV S
~ 1530m MeV
~ 1672m MeV W
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The Later Years
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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
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Weak and Strong Interactions
While weak and strong interactions
were now extensively studied, andtheoretical 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
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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 possibleas if the proton has
pointlike consituents
Measured structure functions that characterize the momentum
distributions of the pieces of the proton
Electroweak Unification
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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
Weinbergwho 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
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Charmed Particles A case where theory led
experiment
Weak interactions seemed torequire a change of strangeness
Neutral currents not seen in
decays of kaons to pions Always
a change in charge
This was explained naturally bythe existence of a fourth quark
The J/Yparticle (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)
llK
llK 0
p p
Tau & Bottom
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Tau & Bottom
As energies increased in both e+e- colliders and
fixed target proton beams, new particles started
appearing in the mid-70s
Mark II observed strange events with one electron
and one muon
Suggested new lepton that decayed into e or m
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!
ee t tt t
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W&Z
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W&Z Electroweak unification
required W and Z
Found by Carlo Rubbia andcollaborators at the CERN
SppS exactly whereexpected!
MW~ 80 GeV
MZ ~ 90 GeV
Another case of theoryleading experiment.
But experimentalists got theNobel in 1984 (3 years later!)
The collider era had reallybegun!
eW e
0Z e e
Colliders in Use
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Colliders in Use
Tevatron, p p 2 TeV
HERA e p 30 900 GeV LEP, e e- 91-209 GeV
RHIC, Au Au 200 GeV/N
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Neutrino Oscillations
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Neutrino Oscillations Super-Kamiokande is
originally designed to
search for proton decay 50k tonnes of water
11k phototubes to detectlight
98 Detected a
significant deficit ofmuon neutrinos,especially when comingthrough the earth
Fit hypothesis ofneutrinos oscillatingchanging flavor Not part of the standard
modelyet!
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