14.1Early Discoveries

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CHAPTER 14Elementary Particles

• 14.1 Early Discoveries
• 14.2 The Fundamental Interactions
• 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
• 14.8 Accelerators

If I could remember the names of all these
particles, Id be a botanist. - Enrico Fermi
2
Elementary Particles

• We began our study of subatomic physics in
  Chapter 12. We investigated the nucleus in
  Chapters 12 and 13. We now delve deeper, because
  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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The Building Blocks of Matter

• We have thought of electrons, neutrons, and
  protons as elementary particles, because we
  believe they are basic building blocks of matter.
• However, in this chapter the term elementary
  particle is used loosely to refer to hundreds of
  particles, most of which are unstable.

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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
  Einsteins work on the photoelectric effect can
  be said to have defined the photon (originally
  called a quantum) 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, and Carl Anderson identified the
  positron in cosmic rays.

5
The Positron

• Dirac in 1928 introduced the relativistic theory
  of the electron when he combined quantum
  mechanics with 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.

6
Antiparticles

• Diracs theory, along with refinements made by
  others opened the possibility of antiparticles
  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
  notable exceptions like the neutral pion), have
  antiparticles.

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Cosmic Rays

• 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.

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Positron-Electron Interaction

• The ultimate fate of positrons (antielectrons) 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

9
Feynman Diagram

• Feynman presented a particularly simple graphical
  technique to describe interactions.
• For example, 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.

Figure 14.2 Example of a Feynman spacetime
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 nucleons analogous 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).

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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.

Figure 14.3 A Feynman diagram indicating the
exchange of a pion (Yukawas meson) between a
neutron and a proton.
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14.2 The Fundamental Interactions

• The fundamental forces in nature responsible for
  all interactions
• Gravitation
• Electroweak (electromagnetic and weak)
• Strong
• The electroweak is sometimes treated separately
  as the electromagnetic and the weak force thus
  creating four fundamental forces.

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The Fundamental Interactions

• We have learned that the fundamental forces act
  through the exchange or mediation of particles
  according to the quantum theory of fields. The
  exchanged particle in the electromagnetic
  interaction is the photon. All particles having
  either electric charge or a magnetic moment (and
  also the photon) interact with the
  electromagnetic interaction. The electromagnetic
  interaction has very long range.

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The Fundamental Interactions

• In the 1960s Sheldon Glashow, Steven Weinberg,
  and Abdus Salam (Nobel Prize for Physics, 1979)
  predicted that particles, which they called W
  (for weak) and Z, should exist that are
  responsible for the weak interaction.
• This theory, called the electroweak theory,
  unified the electromagnetic and weak interactions
  much as Maxwell had unified electricity and
  magnetism into the electromagnetic theory a
  hundred years earlier.

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Other Mesons

• We previously saw that Yukawas pion is
  responsible for the nuclear force. Now we know
  there are other mesons that interact with the
  strong force. Later we will 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) it is massless and has spin 1, just
  like the photon.
• Particles that interact by the strong interaction
  are called hadrons examples include the neutron,
  proton, and mesons.

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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
18
The Standard Model

• The most widely accepted theory of elementary
  particle physics at present is the Standard
  Model.
• It is a 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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14.3 Classification of Elementary Particles

• We discussed in Chapter 9 that articles 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.

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Bosons and Fermions

• 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.

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The Higgs Boson

• The Standard Model proposes that there is a field
  called the Higgs field that permeates 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 field has at least one particle
  associated with it, and that is the Higgs
  particle (or Higgs boson). The properties of the
  gauge and Higgs bosons, as well as the graviton,
  are given in the next slide.
• 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

• The leptons are perhaps the simplest of the
  elementary particles.
• They appear to be pointlike, 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.
• There are only six leptons plus their six
  antiparticles.

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The Electron and the Muon

• Each of the charged particles has an associated
  neutrino, named after its charged partner (for
  example, muon neutrino).
• The muon decays into an electron, and the tau can
  decay into an electron, a muon, or even hadrons
  (which is most probable).
• The muon decay (by the weak interaction) is

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Neutrinos

• We are already familiar with the electron
  neutrino that occurs in the beta decay of the
  neutron (Chapter 12).
• Neutrinos have zero charge.
• 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.
• All leptons 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.

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Hadrons

• These 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
  note that the muon is a lepton and not a meson).
• All baryons have masses at least as large as the
  proton and have half-integral spins.

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Mesons

• Mesons are bosons because of their integral spin.
  
• The meson family is rather large and consists of
  many variations, distinguished according to their
  composition of quarks.
• The pion (p-meson) is a meson that can either
  have charge or be neutral.
• In addition to the pion there is also a K meson,
  which exists in both charged (K) and neutral
  forms (K0). The K- meson is the antiparticle of
  the K, and their common decay mode is into muons
  or pions.
• All mesons are unstable and not abundant in
  nature.

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Baryons

• The neutron and proton are the best-known
  baryons.
• The proton is the only stable baryon, but some
  theories predict that it is also unstable with a
  lifetime greater than 1030 years.
• All baryons except the proton eventually 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 and Symmetries

• 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.

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Additional Conservation Laws

• These are 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 antibaryons,
  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 antimatter in the universe today.

36
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

• In the early 1950s physicists had considerable
  difficulty understanding the myriad of observed
  reactions and decays. For example, 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 proton reaction studied most often,
  namely the p p reaction.
• In addition, the very fast decay of the p0 meson
  into two photons (10-16 s) is the preferred mode
  of decay.
• One would expect the K0 meson to also decay into
  two photons very quickly, but it does not. The
  long and short decay lifetimes of the K0 are 10-8
  and 10-10 s, respectively.

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The New Quantum Number Strangeness

• Strangeness, S, 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.

39
Further

• 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.

42
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
  in 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 not conserved in the weak
  interactions, but it is valid for the strong and
  electromagnetic 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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14.5 Quarks

• We are now prepared to discuss quarks and how
  they form the many baryons and mesons that have
  been discovered experimentally.
• In 1961 Murray Gell-Mann and Yuval Neeman
  independently proposed a classification system
  called the eightfold way that separated the known
  particles into multiplets based on charge,
  hypercharge, and another quantum number called
  isospin, which we have not previously discussed.
  Isospin is a characteristic that can be used to
  classify different charged particles that have
  similar mass and interaction properties.
• The neutron and proton are members of an isospin
  multiplet we call the nucleon. In this case the
  isospin quantum number (I) has the value ½, with
  the proton having the substate value ½ (spin
  up) and the neutron having -½ (spin down).
  Isospin is conserved in strong interactions, but
  not in electromagnetic interactions.

46
Quarks

• After the eightfold way was developed, it was
  noticed that some members of the multiplets were
  missing. Because of physicists strong belief in
  symmetry, experimentalists set to work to find
  them, a task made easier because many of the
  particles properties were predicted by the
  theoretical model.
• The O- was detected in 1964 at Brookhaven
  National Laboratory (see Figure 14.10 and Example
  14.5) in this manner, a discovery that confirmed
  the usefulness of the eightfold way.

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Quarks

• However, as other particles were discovered it
  soon became clear that the eightfold way was not
  the final answer. In 1963 Gell-Mann and,
  independently, George Zweig proposed that hadrons
  were formed from fractionally charged particles
  called quarks. The quark theory was unusually
  successful in describing properties of the
  particles and in understanding particle 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 pointlike,
  just like leptons.

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Quarks, Antiquarks, and Charm

• With these three quarks, all the known hadrons
  could be specified by some combination of quarks
  and antiquarks.
• 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 antiquark 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.

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Quark Properties

• We can now present the given quark properties
  and see how they are used to make up the
  hadrons. In Table 14.5 we give the name, symbol,
  mass, charge, and the quantum numbers for
  strangeness, charm, bottomness, and topness. The
  spin of all quarks (and antiquarks) is 1/2.

50
Quark Description of Particles

• A meson consists of a quark-antiquark pair, which
  gives the required baryon number of 0. Baryons
  normally consist of three quarks.
• We present the quark content of several mesons
  and baryons in Table 14.6. The structure is quite
  simple. For example, a p - consists of ,
  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).

51
Quark Description of Particles

• What about the quark composition of the O-, which
  has a strangeness of S -3? We look in Table
  14.6 and find that its quark composition is sss.
  According to the properties in Table 14.5 its
  charge must be 3(-e/3) -e, and its spin is due
  to three quark spins aligned, 3(1/2) 3/2. Both
  of these values are correct. There is no other
  possibility for a stable omega (lifetime 10-10
  s) in agreement with Table 14.4.

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Quantum Chromodynamics (QCD)

• Because quarks have spin 1/2, they are all
  fermions and according to the Pauli exclusion
  principle, no two fermions can exist in the same
  state. Yet we have three 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).

53
Color

• There are three colors for quarks we call red
  (R), green (G), and blue (B) with antiquark 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.

54
Color

• The two theories, 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. We show a Feynman
  diagram of two quarks interacting in Figure
  14.11. 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.

Fig 14.11
55
Color

• A color and its anticolor cancel out. We call
  this colorless (or white). All hadrons are
  colorless. In Figure 14.11 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.

Fig 14.11
56
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 Figure 14.11 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.

57
Confinement

• Physicists now believe that free quarks cannot be
  observed they can only exist within hadrons.
  This is called confinement.

Figure 14.12 When a high-energy gamma ray is
scattered from a neutron, a free quark cannot
escape because of confinement. For high enough
energies, an antiquark-quark pair is created (for
example, ), and a pion and proton are the
final particles.
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14.6 The Families of Matter

• We now have a brief review of the particle
  classifications and have learned how the hadrons
  are made from the quarks.
• In summary
• We presently believe that the two varieties of
  fermions, called leptons and quarks, are
  fundamental particles.
• These fundamental particles can be divided into
  three simple families or generations.
• Each generation consists of two leptons and two
  quarks. The two leptons are a charged lepton and
  its associated neutrino. The quarks are combined
  by twos or threes to make up the hadrons.

59
The Families of Matter
Figure 14.14 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.

61
The Families of Matter

• Leptons are essentially pointlike, because they
  have no internal structure.
• There are three leptons with mass and three
  others with little mass (the neutrinos).
• Quarks and antiquarks make up the hadrons (mesons
  and baryons). Quarks may also be pointlike (lt
  10-18 m) and are confined together, never being
  in a free state.
• There are six flavors of quarks (up, down,
  strange, charmed, bottom, and top) and there are
  three colors (green, red, and blue) for each
  flavor.
• Rules for combining the colored quarks allow us
  to represent all known hadrons.

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The Families of Matter

• Bosons mediate the four fundamental forces of
  nature gluons are responsible for the strong
  interaction, photons for the electromagnetic
  interaction, W and Z for the weak interaction,
  and the as yet unobserved graviton for the
  gravitational interaction.
• In our study of nuclear physics we discussed the
  pion as the mediator of the strong force. At a
  more fundamental level, we can now say that the
  gluon is responsible.
• The gluon is responsible for the attraction
  between the antiquark and quark that make up the
  pion, and the gluon is responsible for the
  attraction between the quarks that make up the
  nucleons.

63
14.7 Beyond the Standard Model

• Although the Standard Model has been successful
  in particle physics, it doesnt answer all the
  questions. For example, it is not by itself able
  to predict the particle masses.
• Why are there only three generations or families
  of fundamental particles?
• Do quarks and/or leptons actually consist of more
  fundamental particles?

64
Neutrino Oscillations

• One of the most perplexing problems over the last
  three decades has been the solar neutrino problem
  where the number of neutrinos reaching Earth from
  the sun is a factor of 23 too small if our
  understanding of the energy-producing (nuclear
  fusion) is correct.
• Suggestions were made that other processes were
  going on. Neutrinos come in three varieties or
  flavors electron, muon, and tau. Researchers had
  seen neutrinos generated in the Earths
  atmosphere (from cosmic rays) changing or
  oscillating into another flavor. This could
  only happen if neutrinos have mass.
• Physicists have seen various oscillations between
  the three flavors of neutrinos.

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Matter-Antimatter

• According to the Big Bang theory, matter and
  antimatter should have been created in exactly
  equal quantities. It appears that matter
  dominates over antimatter now in our universe,
  and the reason for this has concerned physicists
  and cosmologists for years.
• The tiny violation of CP symmetry in the kaon
  decay tilts the scales in terms of matter over
  antimatter however, the Standard Model indicates
  that this violation is too small to account for
  the predominance of matter.
• B meson decays may yield more about CP violations
  than with kaons and physicists are exploring
  theories going beyond the Standard Model.

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Grand Unifying Theories

• There have been several attempts toward a grand
  unified theory (GUT) to combine the weak,
  electromagnetic, and strong interactions.
• Predictions
• The proton is unstable with a lifetime of 1029 to
  1031 years. Current experimental measurements
  have shown the lifetime to be greater than 1032
  years.
• Neutrinos may have a small, but finite, mass.
  This has been confirmed.
• Massive magnetic monopoles may exist. 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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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 supersymmetry resulted in the
  name theory of superstrings.
• In superstring 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 superstring theory is a promising
  approach to unify the four fundamental forces,
  including gravity.

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Supersymmetry

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

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M-theory

• Recently theorists have proposed a successor to
  superstring theory called M-theory.
• M-theory has 11 dimensions 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 will scientists be able
  to wade through the vast number of unifying
  theories.

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14.8 Accelerators

• Particle physics was not able to develop fully
  until particle accelerators were constructed with
  high enough energies to create particles with a
  mass of about 1 GeV/c2 or greater.
• 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 preacceleration
  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.