14'1Early Discoveries

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CHAPTER 14Elementary Particles

• 14.1 Early Discoveries
• 14.2 The Fundamental Interactions
• 14.8 Accelerators
• 14.3 Classification of Elementary Particles
• 14.4 Conservation Laws and Symmetries
• 14.5 Quarks
• 14.6 The Families of Matter
• 14.7 Beyond the Standard Model

Steven Weinberg (1933 - )
I have done a terrible thing I have postulated
a particle that cannot be detected. Wolfgang
Pauli (after postulating the existence of the
neutrino)
If I could remember the names of all these
particles, Id be a botanist. Enrico Fermi
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Elementary Particles

• Finding answers to some of the basic questions
  about nature is a foremost goal of science
• What are the basic building blocks of matter?
• What is inside the nucleus?
• What are the forces that hold matter together?
• How did the universe begin?
• Will the universe end, and if so, how and when?

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14.1 Early Discoveries

• In 1930 the known elementary particles were the
  proton, the electron, and the photon.
• Thomson identified the electron in 1897, and
  Einstein defined the photon in 1905. The proton
  is the nucleus of the hydrogen atom.
• Despite the rapid progress of physics in the
  first couple of decades of the twentieth century,
  no more elementary particles were discovered
  until 1932, when Chadwick proved the existence of
  the neutron.
• That would have seemed sufficient

James Chadwick (1891-1974)
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But particle physics measurements were happening
Energetic particles collide with stationary
particles in a bubble chamber, vaporizing the
nearby matter and leaving a visible track.
A magnetic field (pointing into the screen)
causes charged particles to take curved paths.
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The Positron

• In 1928 Dirac introduced the relativistic theory
  of the electron when he combined quantum
  mechanics with special relativity.
• He found that his wave equation had negative, as
  well as positive, energy solutions.
• His theory can be interpreted as a vacuum being
  filled with an infinite sea of electrons with
  negative energies.
• If enough energy is transferred to the sea, an
  electron can be ejected with positive energy
  leaving behind a hole that is the positron,
  denoted by e.

Paul Dirac (1902-1984)
Electron positron
Vacuum
0
E
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Anti-particles

• Diracs theory yields anti-particles, which
• Have the same mass and lifetime as their
  associated particles.
• Have the same magnitude but are opposite in sign
  for such physical quantities as electric charge
  and various quantum numbers.

All particles, even neutral ones (with some
exceptions like the neutral pion), have
antiparticles.
Magnetic field into screen
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The Positron
Cosmic rays are highly energetic particles,
mostly protons, that cross interstellar space and
enter the Earths atmosphere, where their
interaction with particles creates cosmic
showers of many distinct particles.

• Carl Anderson identified the positron in cosmic
  rays. It was easy it had positive charge and was
  light.

Andersons cloud chamber photo of positron track
Carl Anderson (1905-1991)
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Positron-Electron Interaction

• The ultimate fate of positrons (anti-electrons)
  is annihilation with electrons.
• After a positron slows down by passing through
  matter, it is attracted by the Coulomb force to
  an electron, where it annihilates through the
  reaction

All anti-matter eventually meets the same fate.
A lot of energy is released in this process all
of the matter is converted to energy.
Star Treks dilithium crystals supposedly
contain anti-matter, which powers the Enterprise.
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Feynman Diagrams

• Feynman presented a particularly simple graphical
  technique to describe interactions.
• It predicts that, when two electrons approach
  each other, according to the quantum theory of
  fields, they exchange a series of photons called
  virtual photons, because they cannot be directly
  observed.
• The action of the electromagnetic field (for
  example, the Coulomb force) can be interpreted as
  the exchange of photons. In this case we say that
  the photons are the carriers or mediators of the
  electromagnetic force.

Example of a Feynman space-time diagram.
Electrons interact through mediation of a photon.
The axes are normally omitted.
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Yukawas Meson

• The Japanese physicist Hideki Yukawa had the idea
  of developing a quantum field theory that would
  describe the force between nucleonsanalogously
  to the electromagnetic force.
• To do this, he had to determine the carrier or
  mediator of the nuclear strong force analogous to
  the photon in the electromagnetic force which he
  called a meson (derived from the Greek word meso
  meaning middle due to its mass being between
  the electron and proton masses).

Hideki Yukawa (1907-1981)
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Yukawas Meson

• Yukawas meson, called a pion (or pi-meson or
  p-meson), was identified in 1947 by C. F. Powell
  (19031969) and G. P. Occhialini (19071993)
• Charged pions have masses of 140 MeV/c2, and a
  neutral pion p0 was later discovered that has a
  mass of 135 MeV/c2, a neutron and a proton.

Feynman diagram indicating the exchange of a pion
(Yukawas meson) between a neutron and a proton.
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Other Mesons, Quarks, and Gluons

• Yukawas pion is responsible for the nuclear
  force.
• Later well see that the nucleons and mesons are
  part of a general group of particles formed from
  even more fundamental particles quarks. The
  particle that mediates the strong interaction
  between quarks is called a gluon (for the glue
  that holds the quarks together) its massless
  and has spin 1, just like the photon.

Computed image of quarks and gluons in a nucleon
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The Weak Interaction

• In the 1960s Sheldon Glashow, Steven Weinberg,
  and Abdus Salam predicted that particles that
  they called W (for weak) and Z should exist that
  are responsible for the weak interaction.
• They have been observed.

Abdus Salam (1926-1996)
Sheldon Glashow (1932- )
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The Graviton

• It has been suggested that the particle
  responsible for the gravitational interaction
  be called a graviton.
• The graviton is the mediator of gravity in
  quantum field theory and has been postulated
  because of the success of the photon in quantum
  electrodynamics theory.
• It must be massless, travel at the speed of
  light, have spin 2, and interact with all
  particles that have mass-energy.
• The graviton has never been observed because of
  its extremely weak interaction with objects.

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The Fundamental Interactions
One of the main goals of particle physics is to
unify these forces (to show that theyre all just
different aspects of the same force), just as
Maxwell did for the electric and magnetic forces
many years earlier.
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The Fundamental Interactions
A finite range effectively confines the particle,
which, by the uncertainty principle, gives it a
minimal momentum and hence a minimum kinetic
energy and mass. Photons and gravitons are
massless. W and Z bosons are heavy.
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14.8 Accelerators
Particle accelerators generate high enough
energies to create particles 1 GeV/c2 or greater.
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Accelerators

• There are three main types of accelerators used
  presently in particle physics experiments
  synchrotrons, linear accelerators, and colliders.

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Synchrotron Radiation

• One difficulty with cyclic accelerators is that
  when charged particles are accelerated, they
  radiate electromagnetic energy called
  synchrotron radiation. This problem is
  particularly severe when electrons, moving very
  close to the speed of light, move in curved
  paths. If the radius of curvature is small,
  electrons can radiate as much energy as they
  gain.
• Physicists have learned to take advantage of
  these synchrotron radiation losses and now build
  special electron accelerators (called light
  sources) that produce copious amounts of photon
  radiation used for both basic and applied
  research.

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Linear Accelerators

• Linear accelerators or linacs typically have
  straight electric-field-free regions between gaps
  of RF voltage boosts. The particles gain speed
  with each boost, and the voltage boost is on for
  a fixed period of time, and thus the distance
  between gaps becomes increasingly larger as the
  particles accelerate.
• Linacs are sometimes used as pre-acceleration
  device for large circular accelerators.

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Colliders

• Because of the limited energy available for
  reactions like that found for the Tevatron,
  physicists decided they had to resort to
  colliding beam experiments, in which the
  particles meet head-on.
• If the colliding particles have equal masses and
  kinetic energies, the total momentum is zero and
  all the energy is available for the reaction and
  the creation of new particles.

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Large Hadron Collider
Counter-propagating protons will each have an
energy of 7 TeV, giving a total collision energy
of 14 TeV. The LHC can also be used to collide
heavy ions such as lead (Pb) with a collision
energy of 1,150 TeV.
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14.3 Classification of Elementary Particles

• Particles with half-integral spin are called
  fermions and those with integral spin are called
  bosons.
• This is a particularly useful way to classify
  elementary particles because all stable matter in
  the universe appears to be composed, at some
  level, of constituent fermions.
• Fermions obey the Pauli Exclusion Principle.
  Bosons dont.

Photons, gluons, W, and the Z are called gauge
bosons and are responsible for the strong and
electroweak interactions. Gravitons are also
bosons, having spin 2. Fermions exert attractive
or repulsive forces on each other by exchanging
gauge bosons, which are the force carriers.
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The Higgs Boson

• One other boson that has been predicted, but not
  yet detected, is necessary in quantum field
  theory to explain why the W and Z have such
  large masses, yet the photon has no mass.
• This missing boson is called the Higgs particle
  (or Higgs boson) after Peter Higgs, who first
  proposed it.
• The Standard Model proposes that there is a field
  called the Higgs field that permeates all of
  space.
• By interacting with this field, particles acquire
  mass. Particles that interact strongly with the
  Higgs field have heavy mass particles that
  interact weakly have small mass.
• The Higgs boson is very heavy, and it hasnt
  been observed yet.
• The search for the Higgs boson is of the highest
  priority in elementary particle physics.

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Boson Properties
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Leptons electrons, muons, taus neutrinos

• The leptons are perhaps the simplest of the
  elementary particles.
• They appear to be point-like, that is, with no
  apparent internal structure, and seem to be truly
  elementary.

Thus far there has been no plausible suggestion
they are formed from some more fundamental
particles. Each of the leptons has an associated
neutrino, named after its charged partner (for
example, muon neutrino). There are only six
leptons plus their six antiparticles.
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Muon and tau decay

• The muon decays into an electron, and the tau can
  decay into an electron, a muon, or even hadrons.
• The muon decay (by the weak interaction) is

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Neutrinos
Picture of the sun, taken not with light, but
with neutrinos, made at the Japanese neutrino
observatory Super-Kamiokande.

• Neutrinos have zero charge.
• The electron neutrino occurs in the beta decay
  of the neutron.
• Their masses are known to be very small. The
  precise mass of neutrinos may have a bearing on
  current cosmological theories of the universe
  because of the gravitational attraction of mass.
• Like all other leptons, they have spin 1/2, and
  all three neutrinos have been identified
  experimentally.
• Neutrinos are particularly difficult to detect
  because they have no charge and little mass, and
  they interact very weakly (they easily pass
  through the earth!).

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Neutrino Oscillations

• One of the most perplexing problems over the
  last three decades has been the solar neutrino
  problem the number of neutrinos reaching Earth
  from the sun is a factor of 2 to 3 too small if
  our understanding of the energy-producing
  (nuclear fusion) is correct.
• Neutrinos come in three varieties or flavors
  electron, muon, and tau. The solution was found
  when researchers saw neutrinos generated in the
  Earths atmosphere (from cosmic rays) changing or
  oscillating into another flavor (the sun only
  emits electron neutrinos).
• Also, this could only happen if neutrinos have
  mass.

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Hadrons

• Hadrons are particles that act through the
  strong force.
• Two classes of hadrons mesons and baryons.
• Mesons are particles with integral spin having
  masses greater than that of the muon (106
  MeV/c2). (Mesons are made up of pairs of quarksa
  quark and an anti-quark.) Theyre unstable and
  rare.
• Baryons have masses at least as large as the
  proton and have half-integral spins. Baryons
  include the proton and neutron, which make up the
  atomic nucleus, but many other unstable baryons
  exist as well. The term "baryon" is derived from
  the Greek ßa??? (barys), meaning "heavy," because
  at the time of their naming it was believed that
  baryons were characterized by having greater mass
  than other particles. (Theyre made up of three
  quarks.) All baryons decay into protons.

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The Hadrons
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Particles and Lifetimes

• The lifetimes of particles are also indications
  of their force interactions.
• Particles that decay through the strong
  interaction are usually the shortest-lived,
  normally decaying in less than 10-20 s.
• The decays caused by the electromagnetic
  interaction generally have lifetimes on the order
  of 10-16 s, and
• The weak interaction decays are even slower,
  longer than 10-10 s.

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Fundamental and Composite Particles

• We call certain particles fundamental this means
  that they are not composed of other, smaller
  particles. We believe leptons, quarks, and gauge
  bosons are fundamental particles.
• Although the Z and W bosons have very short
  lifetimes, they are regarded as particles, so a
  definition of particles dependent only on
  lifetimes is too restrictive.
• Other particles are composites, made from the
  fundamental particles.

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14.4 Conservation Laws

• Physicists like to have clear rules or laws that
  determine whether a certain process can occur or
  not.
• It seems that everything occurs in nature that is
  not forbidden.
• Certain conservation laws are already familiar
  from our study of classical physics. These
  include mass-energy, charge, linear momentum, and
  angular momentum.
• These are absolute conservation laws they are
  always obeyed.

Additional conservation laws will be helpful in
understanding the many possibilities of
elementary particle interactions. Some of these
laws are absolute, but others may be valid for
only one or two of the fundamental interactions.
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Baryon Conservation

• In low-energy nuclear reactions, the number of
  nucleons is always conserved.
• Empirically this is part of a more general
  conservation law for what is assigned a new
  quantum number called baryon number that has the
  value B 1 for baryons and -1 for anti-baryons,
  and 0 for all other particles.
• The conservation of baryon number requires the
  same total baryon number before and after the
  reaction.
• Although there are no known violations of baryon
  conservation, there are theoretical indications
  that it was violated sometime in the beginning of
  the universe when temperatures were quite high.
  This is thought to account for the preponderance
  of matter over anti-matter in the universe today.

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Lepton Conservation

• The leptons are all fundamental particles, and
  there is a conservation of leptons for each of
  the three kinds (families) of leptons.
• The number of leptons from each family is the
  same both before and after a reaction.
• We let Le 1 for the electron and the electron
  neutrino Le -1 for their antiparticles and
  Le 0 for all other particles.
• We assign the quantum numbers Lµ for the muon and
  its neutrino and Lt for the tau and its neutrino
  similarly.
• Thus three additional conservation laws.

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Strangeness

• The behavior of the K mesons seemed very odd.
• There is no conservation law for the production
  of mesons, but it appeared that K mesons, as well
  as the ? and S baryons, were always produced in
  pairs in the p p reaction. One would expect the
  K0 meson to also decay into two photons very
  quickly, but it does not.
• A new quantum number was defined Strangeness, S,
  which is conserved in the strong and
  electromagnetic interactions, but not in the weak
  interaction.
• The kaons have S 1, lambda and sigmas have S
  -1, the xi has S -2, and the omega has S -3.
• When the strange particles are produced by the p
  p strong interaction, they must be produced in
  pairs to conserve strangeness.

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Strangeness is strange

• p0 can decay into two photons by the strong
  interaction, it is not possible for K0 to decay
  at all by the strong interaction. The K0 is the
  lightest S 0 particle, and there is no other
  strange particle to which it can decay. It can
  decay only by the weak interaction, which
  violates strangeness conservation.
• Because the typical decay times of the weak
  interaction are on the order of 10-10 s, this
  explains the longer decay time for K0.
• Only ?S 1 violations are allowed by the weak
  interaction.

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Hypercharge

• One more quantity, called hypercharge, has also
  become widely used as a quantum number.
• The hypercharge quantum number Y is defined by Y
  S B.
• Hypercharge, the sum of the strangeness and
  baryon quantum numbers, is conserved in strong
  interactions.
• The hypercharge and strangeness conservation laws
  hold for the strong and electromagnetic
  interactions, but are violated for the weak
  interaction.

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Symmetries

• Symmetries lead directly to conservation laws.
• Three symmetry operators called parity, charge
  conjugation, and time reversal are considered.

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The Conservation of Parity P

• The conservation of parity P describes the
  inversion symmetry of space,
• Inversion, if valid, does not change the laws of
  physics.
• The conservation of parity is valid for the
  strong and electromagnetic interactions, but not
  for the weak interaction (experimentally).

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Charge conjugation C

• Charge conjugation C reverses the sign of the
  particles charge and magnetic moment.
• It has the effect of interchanging every particle
  with its antiparticle.
• Charge conjugation is valid for the strong and
  electromagnetic interactions, but it is also not
  conserved in the weak interactions.
• Even though both C and P are violated for the
  weak interaction it was believed that when both
  charge conjugation and parity operations are
  performed (called CP), conservation was still
  valid.

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Time Reversal T

• Here time t is replaced with t.
• When all three operations are performed (CPT),
  where T is the time reversal symmetry,
  conservation holds.
• We speak of the invariance of the symmetry
  operators, such as T, CP, and CPT.

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Unifying all these interactions proved difficult.
In the 1950s, it was rumored that Heisenberg had
done it, and just the details remained to be
sketched in. But nothing ever emerged from
Heisenberg. So Wolfgang Pauli responded with the
following Below is the proof that I am as great
an artist as Rembrandt the details remain to be
sketched in.
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The Weak Interaction The Electroweak Theory

• In the 1960s Sheldon Glashow, Steven Weinberg,
  and Abdus Salam unified the electro-magnetic and
  weak interactions into what they called the
  electroweak theory, much as Maxwell had unified
  electricity and magnetism into the
  electromagnetic theory a hundred years earlier.

Abdus Salam (1926-1996)
Sheldon Glashow (1932- )
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Unification of the Strong and Electroweak
Interactions The Standard Model

• Over the latter half of the 20th century,
  numerous physicists combined efforts to generate
  The Standard Model.
• It is a widely accepted theory of elementary
  particle physics at present.
• It is a relatively simple, comprehensive theory
  that explains hundreds of particles and complex
  interactions with six quarks, six leptons, and
  three force-mediating particles.
• It is a combination of the electroweak theory and
  quantum chromodynamics (QCD), but does not
  include gravity.

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Quarks
Murray Gell-Mann (1929- )
In 1963 Murray Gell-Mann and, independently,
George Zweig proposed that hadrons were formed
from fractionally charged particles called
quarks. The quark theory successfully described
the properties of the particles and reactions and
decay.
Three quarks were proposed, named the up (u),
down (d), and strange (s), with the charges
2e/3, -e/3, and -e/3, respectively. The strange
quark has the strangeness value of -1, whereas
the other two quarks have S 0. Quarks are
believed to be essentially point-like, just like
leptons. With these three quarks, all the known
hadrons (at the time) could be specified by some
combination of quarks and anti-quarks.
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Charm, Truth, and Beauty

• A fourth quark called the charmed quark (c) was
  proposed to explain some additional discrepancies
  in the lifetimes of some of the known particles.
• A new quantum number called charm C was
  introduced so that the new quark would have C
  1 while its anti-quark would have C -1 and
  particles without the charmed quark have C 0.
• Charm is similar to strangeness in that it is
  conserved in the strong and electromagnetic
  interactions, but not in the weak interactions.
  This behavior was sufficient to explain the
  particle lifetime difficulties.
• Two additional quarks, top and bottom (or truth
  and beauty), were also required to construct some
  exotic particles (the Upsilon-meson).

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Quark Properties
The spin of all quarks (and anti-quarks) is 1/2.
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Quark Description of Particles

• Baryons normally consist of three quarks or
  anti-quarks.
• A meson consists of a quark-anti-quark pair,
  yielding the required baryon number of 0.

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Quark Description of Particles

• A meson consists of a quark-anti-quark pair,
  which gives the required baryon number of 0.
  Baryons normally consist of three quarks.
• The structure is quite simple. For example, a p -
  consists of ud, which gives a charge of (-2e/3)
  (-e/3) -e, and the two spins couple to give 0
  (-1/2 1/2 0).
• A proton is uud, which gives a charge of (2e/3)
  (2e/3) (-e/3) e its baryon number is 1/3
  1/3 1/3 1 and two of the quarks spins
  couple to zero, leaving a spin 1/2 for the proton
  (1/2 1/2 - 1/2 1/2).

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Other Particles

• What about the quark composition of the O-, which
  has a strangeness of S -3? Its quark
  composition is sss. And its charge is 3(-e/3)
  -e, and its spin is due to three quark spins
  aligned, 3(1/2) 3/2. There is no other
  possibility for a stable omega (lifetime 10-10
  s) in agreement with the table.

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Quantum Chromodynamics (QCD)

• Because quarks have spin 1/2, they are all
  fermions. According to the Pauli exclusion
  principle, no two fermions can exist in the same
  state. Yet we have three identical strange quarks
  in the O-!
• This is not possible unless some other quantum
  number distinguishes each of these quarks in one
  particle.
• A new quantum number called color circumvents
  this problem and its properties establish quantum
  chromodynamics (QCD).

Discovery of the W-
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Color

• There are three colors for quarks we call red
  (R), green (G), and blue (B) with anti-quark
  color antired ( ) antigreen ( ) and antiblue
  ( ). (A bar above the symbol is usually used
  to describe the anti-color).
• Color is the charge of the strong nuclear
  force, analogous to electric charge for
  electromagnetism.

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Color
Quantum electrodynamics and quantum
chromodynamics, are similar in structure color
is often called color charge and the force
between quarks is sometimes referred to as color
force. Earlier we saw that gluons are the
particles that hold the quarks together. Below is
a Feynman diagram of two quarks interacting. A
red quark comes in from the left and interacts
with a blue quark coming in from the right. They
exchange a gluon, changing the blue quark into a
red one and the red quark into a blue one.
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Color
A color and its anti-color cancel out. We call
this colorless (or white). All hadrons are
colorless. In the figure, the gluon itself must
have the color in order for the diagram
to work. Quarks change color when they emit or
absorb a gluon, and quarks of the same color
repel, whereas quarks of different color attract.
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Color
To finish the story we should mention that the
six different kinds of quarks are referred to as
flavors. There are six flavors of quarks (u, d,
s, c, b, t). Each flavor has three colors.
Finally, how many different gluons are possible?
Using the three colors red, blue, and green,
there are nine possible combinations for a gluon.
They are Note in the diagram that the gluon is
and not . The combination does not
have any net color change and cannot be
independent. Therefore, there are only eight
independent gluons, and that is what quantum
chromodynamics predicts. Gluons can interact
with each other because each gluon carries a
color charge. Note that in this case gluons, as
the mediator of the strong force, are much
different from photons, the mediator of the
electromagnetic force.
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Quark-anti-quark creation

• Physicists now believe that free quarks cannot be
  observed they can only exist within hadrons.
  This is called confinement.
• This occurs because the force between the quarks
  increases rapidly with distance, and the energy
  supplied to separate them creates new quarks.

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Confinement
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The Families of Matter
The three generations (or families) of matter.
Note that both quarks and leptons exist in three
distinct sets. One of each charge type of quark
and lepton make up a generation. All visible
matter in the universe is made from the first
generation second- and third-generation
particles are unstable and decay into
first-generation particles.
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The Families of Matter

• Most of the mass in the universe is made from the
  components in the first generation (electrons and
  u and d quarks).
• The second generation consists of the muon, its
  neutrino, and the charmed and strange quarks. The
  members of this generation are found in certain
  astrophysical objects of high energy and in
  cosmic rays, and are produced in high-energy
  accelerators.
• The third generation consists of the tau and its
  neutrino and two more quarks, the bottom (or
  beauty) and top (or truth). The members of this
  third generation existed in the early moments of
  the creation of the universe and can be created
  with very high energy accelerators.

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Grand Unifying Theories (GUTs)

• There have been several attempts toward a grand
  unified theory (GUT) to combine the weak,
  electromagnetic, and strong interactions and
  explain why
• Current experimental measurements have shown the
  proton lifetime to be greater than 1032 years.
  Current theory has it at 10-29 to 10-31 years.
• Neutrinos may have a small, but finite, mass.
  This has been confirmed.
• Massive magnetic monopoles may exist. If one
  exists anywhere in the universe, it explains why
  charge is quantized. There is presently no
  confirmed experimental evidence for magnetic
  monopoles.
• The proton and electron electric charges should
  have the same magnitude.

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Another challengeMatter-Antimatter
Could this galaxy be made entirely of anti-matter?

• According to the Big Bang theory, matter and
  antimatter should have been created in exactly
  equal quantities. But it appears that matter
  dominates over antimatter now in our universe,
  and the reason for this has puzzled physicists
  and cosmologists for years.
• Events in the early universe may be responsible
  for this asymmetry. But explanations go far
  beyond the standard model.

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Including Gravity String Theory

• For the last two decades there has been a
  tremendous amount of effort by theorists in
  string theory, which has had several variations.
  The addition of super-symmetry resulted in the
  name theory of super-strings.
• In super-string theory, elementary particles do
  not exist as points, but rather as tiny, wiggling
  loops that are only 10-35 m in length.
• Further work has revealed that they describe not
  just strings, but other objects including
  membranes and higher-dimensional objects. The
  addition of membranes has resulted in brane
  theories.
• Presently super-string theory is a promising
  approach to unify the four fundamental forces,
  including gravity.

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Super-symmetry

• Super-symmetry is a necessary ingredient in
  many of the theories trying to unify all four
  forces of nature.
• The symmetry relates fermions and bosons. All
  fermions will have a super-partner that is a
  boson of equal mass, and vice versa.
• The super-partner spins differ by h / 2.
• Presently, none of the known leptons, quarks, or
  gauge bosons can be identified with a
  super-partner of any other particle type.

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M-theory

• Recently theorists have proposed a successor to
  super-string theory called M-theory.
• M-theory has 11 dimensions (ten spatial and one
  for time) and predicts that strings coexist with
  membranes, called branes for short.
• The number of particles that have been predicted
  from a variety of different theories include the
  fancifully named sleptons, squarks, axions,
  winos, photinos, zinos, gluinos, and preons.
• Only through experiments (which no one currently
  knows how to do) will scientists be able to wade
  through the vast number of unifying theories.