The Standard Model of Particles and Interactions

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The Standard Modelof Particles and Interactions

• Ian Hinchliffe
• 26 June 2002

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What is the World Made of?

• Ancient times 4 elements
• 19th century atoms
• Early 20th century electrons, protons, neutrons
• Today quarks and leptons

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The Atom in 1900...

• Atoms get rearranged in chemical reactions
• More than 100 atoms (H, He, Fe )
• Internal structure was not understood known to
  have electric charge inside

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Periodic Table

• Elements are grouped into families with similar
  properties (e.g. Inert gasses He, Ne etc.) led to
  the Periodic Table
• This suggested an new structure with simpler
  building blocks

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Models of the Atom

• Experiments broke atoms apart
• Very light negative charged particles (electrons)
  surrounding a heavy positive nucleus
• Atom, is mostly empty

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The Nucleus

• Nucleus is small and dense it was thought for a
  while to be fundamental
• But still as many nucleii as atoms
• Simplification all nucleii are made up of
  charged protons and neutral neutrons

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Quarks

• We now know that even protons and neutrons are
  not fundamental
• They are made up of smaller particles called
  quarks
• So far, quarks appear to be fundamental
  (point-like)

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The Modern Atom

• A cloud of electrons in constant motion around
  the nucleus
• And protons and neutrons in motion in the nucleus
• And quarks in motion within the protons and
  neutrons

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Size inside atoms

• The nucleus is 10,000 times smaller than the atom
• Proton and neutron are 10 times smaller than
  nucleus
• No evidence that quarks have any size at all !

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New Particles

• Collisions of electrons and nucleii in cosmic
  rays and particle accelerators beginning in the
  1930s led to the discovery of many new particles
• Some were predicted but many others came as
  surprises
• Muon like a heavy electron Who ordered that?
• At first, all of them were thought to be
  fundamental

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Only a few at first
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These can be explained as made of a few quarks
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What is Fundamental?

• Physicists have discovered hundreds of new
  particles
• Most, we now know are not fundamental
• We have developed a theory, called The Standard
  Model, which appears to explain what we observe
• This model includes 6 quarks, 6 leptons and 13
  force-related particles

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What is the World made of?

• The real world is not made of individual quarks
  (more on that later)
• Quarks exist only in groups making up what we
  call hadrons (proton and neutron are hadrons)
• E.g. a proton is 2 up quarks and 1 down quark
• We are all made from up and down quarks and
  electrons

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Matter and Antimatter

• For every particle ever found, there is a
  corresponding antimatter particle or antiparticle
• They look just like matter but have the opposite
  charge
• Particles are created or destroyed in pairs

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Particles can decay

• Particles may decay, i.e. transform from one to
  another
• Most are unstable
• Proton and electron are stable

• Neutron can decay to electron and a proton
• Energy appears to be missing. It is carried off
  by a neutrino

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Generations

• The six quarks and the six leptons are each
  organized into three generations
• The generations are heavier Xerox copies
• Who ordered the 2nd and 3rd generations?
• The quarks have fractional charges (2/3 and
  -1/3) The leptons have charge -1 or 0

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What about Leptons?

• There are six leptons, three charged and three
  neutral
• They appear to be point-like particles with no
  internal structure
• Electrons are the most common and are the only
  ones found in ordinary matter
• Muons (m) and taus (t) are heavier and charged
  like the electron
• Neutrinos have no charge and very little mass

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Matter Summary

• So all the universe is made of First Generation
  quarks and leptons
• We now turn to how the quarks and leptons
  interact with each other, stick together and decay

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Four Forces

• There are four fundamental interactions in nature
• All forces can be attributed to these
  interactions
• Gravity is attractive others can be repulsive
• Interactions are also responsible for decay

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How do Particles Interact?

• Objects can interact without touching
• How do magnets feel each other to attract or
  repel?
• How does the sun attract the earth?
• A force is something communicated between objects

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Electromagnetism

• The electromagnetic force causes opposite charges
  to attract and like charges to repel
• The carrier is called the photon (g)
• The photon is massless and travels at the speed
  of light

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Residual E-M

• Normally atoms are neutral having the same number
  of protons and electrons
• The charged parts of one atom can attract the
  charged parts of another atom
• Can bind atoms into molecules

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Why Doesnt a Nucleus Explode?

• A heavy nucleus contains many protons, all with
  positive charge
• These repel each other
• Why does it not blow apart?

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Strong Force

• In addition to their electric charge, quarks also
  carry a new kind of charge called color charge
• The force between color charged particles is the
  strong force

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The Gluon

• The strong force holds quarks together to form
  hadrons
• Its carrier particles are called gluons there
  are 8 of these
• The strong force only acts on very short distances

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Color and Anti-color

• There are three color charges and three
  anti-color charges
• But note, these colors have nothing to do with
  color and visible light, they are only a way
  describing the physics

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Colored Quarks and Gluons

• Each quark has one of the three color charges and
  each antiquark has one of the three anticolor
  charges
• Baryons and mesons are color-neutral just as
  red-green-blue makes white light

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Quark Confinement

• Color force (QCD) gets stronger at long
  distances!!
• Color-charged particles cannot be isolated
• Color-charged quarks are confined in hadrons with
  other quarks
• The composites are color neutral

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Color Field

• Quarks in a hadron exchange gluons
• If one of the quarks is pulled away from its
  neighbors, the color field stretches between that
  quark and its neighbors
• New quark-antiquark pairs are created in the field

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Quarks Emit Gluons

• When a quark emits or absorbs a gluon, the quarks
  color charge must change to conserve color charge
• A red quark emits a red/antiblue gluon and
  changes into a blue quark

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Residual Strong Force

• The strong force between the quarks in one proton
  and the quarks in another proton is strong enough
  to overwhelm the repulsive electromagnetic force

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Weak Force

• Weak interactions are responsible for the decay
  of massive quarks and leptons into lighter quarks
  and leptons
• Example neutron to decay into proton electron
  neutrino
• This is why all matter consists of the lightest
  quarks and leptons (plus neutrinos)

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Electroweak Force

• In the Standard Model, the weak and the
  electromagnetic forces have been combined into a
  unified electroweak theory
• At very short distances (10-18 meters), the weak
  and electromagnetic interactions have comparable
  strengths
• Force particles are photon, W and Z

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What about Gravity?

• Gravity is very weak
• Relevant at macroscopic distances
• The gravity force carrier, the graviton, is
  predicted but has never been seen

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Interaction Summary
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Quantum Mechanics

• Behavior of atoms and particles is described by
  Quantum Mechanics
• Certain properties such as energy can have only
  discrete values, not continuous values
• Particle properties are described by these values
  (quantum numbers) such as
• Electric Charge
• Color Charge
• Flavor
• Spin

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The Pauli Exclusion Principle

• We can use quantum particle properties to
  categorize the particles we find
• Some particles, called Fermions, obey the
  Exclusion Principle while others, called Bosons,
  do not

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Fermions and Bosons
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What Holds the World Together?

• We have learned that the world is made up of six
  quarks and six leptons
• Everything we see is a conglomeration of quarks
  and leptons (and their antiparticles)
• There are four fundamental forces and there are
  force carrier particles associated with each force

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How does a particle decay?

• The Standard Model explains why some particles
  decay into other particles
• In nuclear decay, a nucleus can split into
  smaller nuclei
• When a fundamental particle decays, it has no
  constituents (by definition) so it must change
  into totally new particles

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The Unstable Nucleus

• We have seen that the strong force holds the
  nucleus together despite the electromagnetic
  repulsion of the protons
• However, not all nuclei live forever
• Some decay

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Nuclear Decay

• The nucleus can split into smaller nuclei
• This is as if the nucleus boiled off some of
  its pieces
• This happens in a nuclear reactor

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Muon Decay

• Muon decay is an example of particle decay
• Here the end products are not pieces of the
  starting particle but rather are totally new
  particles

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Missing Mass

• In most decays, the particles or nuclei that
  remain have a total mass that is less than the
  mass of the original particle or nucleus
• The missing mass gives kinetic energy to the
  decay products

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Particle Decay Mediators

• When a fundamental particle decays, it turns
  itself into a less massive particle and a
  force-carrier particle (the W boson)
• The force-carrier then emerges as other particles
• A particle can decay if it is heavier than the
  total mass of its decay products and if there is
  a force to mediate the decay

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Virtual Particles

• Particles decay via force-carrier particles
• In some cases, a particle may decay via a
  force-carrier that is more massive than the
  initial particle
• The force-carrier particle is immediately
  transformed into lower-mass particles
• The short-lived massive particle appears to
  violate the law of energy conservation

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The Uncertainty Principle

• A result of the Heisenberg Uncertainty Principle
  is that these high-mass particles may come into
  being if they are very short-lived.
• These particles are called virtual particles

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Different Interactions

• Strong, electromagnetic, and weak interactions
  all cause particle decays. However, only weak
  interactions can cause the decay of fundamental
  particles into other types of particles.
• Physicists call particle types "flavors." The
  weak interaction can change a charm quark into a
  strange quark while emitting a virtual W boson
  (charm and strange are flavors).
• Only the weak interaction (via the W boson) can
  change flavor and allow the decay of a truly
  fundamental particle.

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Other Interactions

• Electromagnetic Decays
• The p0 (neutral pion) is a meson. The quark and
  antiquark can annihilate from the annihilation
  come two photons. This is an example of an
  electromagnetic decay.
• Strong Decays
• The hc particle is a meson made up of a c and an
  anti-c. It can undergo a strong decay into two
  gluons (which emerge as hadrons).

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Annihilations

• These are not decays but they also take place
  through virtual particles
• Annihilations of light quarks at very high energy
  can produce very massive quarks in the laboratory

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Antiproton Annihilation

• This bubble chamber shows an antiproton colliding
  with a proton, annihilating and producing eight
  pions
• One pion decays into a muon and a neutrino (which
  leaves no track)

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Fundamental Processes

• With what you have now learned, you can make
  models of the fundamental processes that
  physicists study
• These models are the foundation for detailed
  calculations of what happens at a high energy
  accelerator

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Neutron Beta Decay
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Electron-Positron Annihilation
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Top Production
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Mysteries and Failures

• The Standard Model is a theory of the universe
• It provides a good description of phenomena
  observed by experiments
• It is still incomplete in many ways why 3
  generations? What is dark matter?

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Is the Standard Model Wrong?

• We need to go beyond the Standard Model in the
  same way that Einsteins Theory of Relativity
  extended Newtons laws of mechanics
• We will need to extend the Standard Model with
  something new to explain mass, gravity, etc.

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Three Generations

• There are three sets of quarks and three sets of
  leptons
• Why are there exactly three generations of
  matter?
• Why do we see only one in the real world?

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What About Masses?

• The Standard Model cannot explain why a particle
  has a certain mass
• Physicists have theorized the existence of a new
  field, called the Higgs field, which interacts
  with other particles to give them mass
• So far, the Higgs has not been seen by experiment

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Grand Unified Theory

• We believe that GUT will unify the strong, weak
  and electromagnetic forces
• All three forces would be different aspects of
  the same, unified interaction
• The three forces would merge into one at high
  enough energy

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Supersymmetry

• Some physicists attempting to unify gravity with
  the other fundamental forces have suggested that
  every fundamental particle should have a massive
  shadow particle

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• Modern physics has theories for quantum
  mechanics, relativity and gravity but they do not
  quite work with each other
• If we lived in a world of more than three spatial
  dimensions, these problems can be resolved
• String theory suggests that in a world with three
  ordinary dimensions and some additional very
  small dimensions, particles are strings and
  membranes

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Extra Dimensions

• String Theory requires more than three space
  dimensions
• These extra dimensions could be very small so
  that we do not see them
• Experiments are now searching for evidence of
  extra dimensions

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Dark Matter

• It appears that the universe is not made of the
  same kind of matter as our sun and the stars
• The dark matter does exert a gravitational
  attraction on ordinary matter but has not been
  detected directly

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The Accelerating Universe

• Recent experiments using Type Ia Supernovae have
  shown that the universe is still expanding and
  the rate of expansion is increasing
• This acceleration must be driven by a new
  mechanism which has been named dark energy

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The Expanding Universe

• Studies of the most distant supernova ever
  detected indicates that the universe did go
  through a phase where the expansion slowed down
• It is now speeding up

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Conclusion

• The Standard Model is a powerful synthesis that
  explains a huge number of observations in a
  simple framework. It is to physics what
  evolution is to biology.
• There are many important questions beyond the
  Standard Model