Download CHAPTER 14: Elementary Particles

CHAPTER 14
Elementary Particles
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14.1
14.2
14.3
14.4
14.5
14.6
14.7
14.8
Early Discoveries
The Fundamental Interactions
Classification of Elementary Particles
Conservation Laws and Symmetries
Quarks
The Families of Matter
Beyond the Standard Model
Accelerators
If I could remember the names of all these particles, I’d be a botanist.
- Enrico Fermi
1
Elementary Particles
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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:
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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
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We have thought of electrons, neutrons, and protons as
elementary particles, because we believe they are basic
building blocks of matter.
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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
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In 1930 the known elementary particles were the proton, the
electron, and the photon.
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Thomson identified the electron in 1897, and Einstein’s 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.
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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.
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The Positron
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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+.
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Antiparticles
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Dirac’s theory, along with refinements made by
others opened the possibility of antiparticles which:
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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
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Cosmic rays are highly energetic particles, mostly
protons, that cross interstellar space and enter the
Earth’s atmosphere, where their interaction with
particles creates cosmic “showers” of many
distinct particles.
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Positron-Electron Interaction
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The ultimate fate of positrons (antielectrons) is
annihilation with electrons.
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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
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Feynman Diagram
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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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Yukawa’s Meson
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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.
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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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Yukawa’s Meson
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Yukawa’s meson, called a pion (or pi-meson or πmeson), was identified in 1947 by C. F. Powell (1903–
1969) and G. P. Occhialini (1907–1993)
Charged pions have masses of 140 MeV/c2, and a
neutral pion π0 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 (Yukawa’s meson)
between a neutron and a proton.
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14.2: The Fundamental Interactions
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The fundamental forces in nature responsible for all
interactions:
1)
Gravitation
2)
Electroweak (electromagnetic and weak)
3)
Strong
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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
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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
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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.
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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
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We previously saw that Yukawa’s 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.
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Particles that interact by the strong interaction are called
hadrons; examples include the neutron, proton, and mesons.
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The Graviton
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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 massenergy.
The graviton has never been observed because of its
extremely weak interaction with objects.
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The Fundamental Interactions
17
The Standard Model
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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
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We discussed in Chapter 9 that articles with halfintegral spin are called fermions and those with
integral spin are called bosons.
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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
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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
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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.
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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
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The Standard Model proposes that there is a field called the
Higgs field that permeates space.
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By interacting with this field, particles acquire mass.
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Particles that interact strongly with the Higgs field have heavy
mass; particles that interact weakly have small mass.
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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.
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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
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The leptons are perhaps the simplest of the
elementary particles.
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They appear to be pointlike, that is, with no apparent
internal structure, and seem to be truly elementary.
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Thus far there has been no plausible suggestion
they are formed from some more fundamental
particles.
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There are only six leptons plus their six antiparticles.
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The Electron and the Muon
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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
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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
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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
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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 (π-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
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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
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The lifetimes of particles are also indications of their
force interactions.
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Particles that decay through the strong interaction are
usually the shortest-lived, normally decaying in less than
10−20 s.
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The decays caused by the electromagnetic interaction
generally have lifetimes on the order of 10−16 s, and
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The weak interaction decays are even slower, longer
than 10−10 s.
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Fundamental and Composite Particles
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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
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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
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These are helpful in understanding the many
possibilities of elementary particle interactions.
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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
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In low-energy nuclear reactions, the number of nucleons is
always conserved.
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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.
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The conservation of baryon number requires the same total
baryon number before and after the reaction.
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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.
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Lepton Conservation
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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 Lτ for the tau and its neutrino similarly.
Thus three additional conservation laws.
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Strangeness
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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 Λ and Σ
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 π0 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
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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.
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Further…
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π0 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 ΔS = ±1 violations are allowed by the weak
interaction.
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Hypercharge
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One more quantity, called hypercharge, has also become
widely used as a quantum number.
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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.
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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
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Symmetries lead directly to conservation
laws.
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Three symmetry operators called parity,
charge conjugation, and time reversal are
considered.
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The Conservation of Parity P
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The conservation of parity P describes the
inversion symmetry of space,
Inversion, if valid, does not change the laws of
physics.
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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
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Charge conjugation C reverses the sign of the particle’s
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
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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
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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 Ne’eman 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.
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Quarks
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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 Ω− 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
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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
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With these three quarks, all the known hadrons could be specified
by some combination of quarks and antiquarks.
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A fourth quark called the charmed quark (c) was proposed to
explain some additional discrepancies in the lifetimes of some of
the known particles.
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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.
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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.
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49
Quark Description of Particles
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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 π −
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).
50
Quark Description of Particles
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What about the quark composition of the Ω−,
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.
51
Quantum Chromodynamics (QCD)
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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 Ω−.
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).
52
Color
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There are three colors for quarks we call red (R), green (G),
and blue (B) with antiquark color antired ( R); antigreen (G)
and antiblue (B).
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(A “bar” above the symbol is usually used to describe the “anticolor”).
Color is the “charge” of the strong nuclear force, analogous
to electric charge for electromagnetism.
53
Color
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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
54
Color
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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
55
Color
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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
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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.

56
Confinement
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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.
57
14.6: The Families of Matter
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We now have a brief review of the particle
classifications and have learned how the hadrons
are made from the quarks.
In summary:
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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.
58
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.
59
The Families of Matter
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Most of the mass in the universe is made from the components in
the first generation (electrons and u and d quarks).
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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.
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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.
60
The Families of Matter
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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 (< 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.
61
The Families of Matter
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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.
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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.
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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.
62
14.7: Beyond the Standard Model
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Although the Standard Model has been
successful in particle physics, it doesn’t 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?
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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
2–3 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
Earth’s 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

1)
2)
3)
4)
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 ħ / 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.
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