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
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:
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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?
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.
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’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.
• 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.
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+.
Antiparticles
• Dirac’s 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.
Cosmic Rays
• 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.
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
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.
Yukawa’s 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).
Yukawa’s Meson
• 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.
14.2: The Fundamental Interactions
• The fundamental forces in nature responsible for all
interactions:
1) Gravitation
2) Electroweak (electromagnetic and weak)
3) Strong
• The electroweak is sometimes treated separately as
the electromagnetic and the weak force thus
creating four fundamental forces.
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.
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.
Other Mesons
• 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.
• Particles that interact by the strong interaction are
called hadrons; examples include the neutron, proton,
and mesons.
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.
The Fundamental Interactions
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.
14.3: Classification of Elementary
Particles
• We discussed in Chapter 9 that articles with halfintegral 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.
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.
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 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.
Boson Properties
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.
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:
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.
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.
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 (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.
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.
The Hadrons
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.
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.
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.
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.
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.
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 Lτ for the tau and its neutrino
similarly.
• Thus three additional conservation laws.
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
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,
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.
Further…
• 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.
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.
Symmetries
• Symmetries lead directly to conservation
laws.
• Three symmetry operators called parity,
charge conjugation, and time reversal are
considered.
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).
Charge conjugation C
• 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.
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.
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 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
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 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.
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
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.
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.

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 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).
Quark Description of Particles
• 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.
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 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).
Color
• There are three colors for quarks we call red (R), green
(G), and blue (B) with antiquark color antired
( ); G
R
antigreen ( )Band 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.
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
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
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,
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.
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.
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.
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.
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 (< 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.
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
14.7: Beyond the Standard Model
• 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?
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.
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
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.
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.
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.
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.
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.
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.
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.
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.