Introduction to Particle Physics

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Introduction to Particle Physics
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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 the
  information 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 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

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Hierarchies of Nature

• 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

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What is a particle?

• Not an easy question!
• Is a speck of dust a particle?
• Is an atom a particle?
• Is a nucleus a particle?
• Is a proton a particle?
• Is 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?

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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 visible light
• How do we see smaller structure?

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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 10s of eV (Hydrogen)
• Nuclear physics 10s of MeV (Binding energy)
• Particle physics 100s of MeV (exciting proton
  structure) ? 100s of GeV (Electroweak
  unification)
• At the lower scales, particles are really
  particles since you do not perceive their
  substructure or excited states

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Conserved Quantities Mechanics

• Noethers 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!

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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!

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Fundamental Matter Particles
QUARKS
LEPTONS
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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

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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 how certain things
  were discovered

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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

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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

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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!

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The Electron

• Thomson identified the 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 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

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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

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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

Ionization energy lossof charged particles
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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!

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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 energy solutions for
  fermions
• Predicted anti-particles in nature, with
  opposite charge but same mass
• Anti-electron ? positron was discovered in cosmic
  rays
• Andersons cloud chamber
• Curvature gives momentum
• Length gives rate of energy loss

Only consistent withlight positive particle
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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
  increasingsophistication of tools usedto detect
  particles
• Detector technology

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Accelerators
Cyclotron
Linear Accelerator
Synchrotron
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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)

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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 and complexity of the
  collisions have made the bubble chamber obsolete
• But excellent for pedagogy!

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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 pairs looks like a quantum number
• Given name strangeness

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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

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The Particle Zoo

• Pre-standard model particle physics was
  characterized by an increasing particle zoo

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Quark Model

• Gell-Mann and Neeman 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
D
D
Do
D-
S-
So
S
I3
?
?-
?-
q
q
q
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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
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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

nt
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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

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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 possible
  as if the proton has pointlike consituents
• Measured structure functions that characterize
  the momentum distributions of the pieces of the
  proton

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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
  Weinberg who received the Nobel in 1979
• The weak force is carried by W and Z bosons of
  M90 GeV
• The massless photon is induced by the presence of
  a condensate of Higgs bosons, that
  spontaneously breaks the symmetry of the
  interaction

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Charmed Particles

• A case where theory led experiment
• Weak interactions seemed to require a change of
  strangeness
• 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 (M3.1 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)

m-
p
p
y
m
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Tau Bottom

• As energies increased in both ee- 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 m5 GeV
• A new family was found
• Required another neutrino and another quark
• Took around 20 years to find both!

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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 1970s
• 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

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WZ

• Electroweak unification required W and Z
• 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!

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Colliders in Use
HERA ep 30900 GeV
LEP, ee- 91-209 GeV
Tevatron, pp 2 TeV
RHIC, AuAu 200 GeV/N
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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 pp
  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 Ws
• QCD backgrounds (soft hard)
• Essentially completed the standard model
• OK, the tau neutrino was only established in 2000

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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!

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The Higgs

• The Higgs particle, couples to all massive
  particles (quarks and leptons)
• However, direct searches for the Higgs have been
  without success
• The data may suggest MH114 GeV
• The LHC is the ultimate hope for understanding
  the origin of mass

Higgs Condensate
M0
Mm
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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!

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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 1980s
• Particles are vibrating strings, embedded in a
  many-dimensional space where only 4 are allowed
  to be macroscopic!