Transcript Chapter 46

Chapter 46
Particle Physics
and Cosmology
Atoms as Elementary Particles
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Atoms
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From the Greek for “indivisible”
Were once thought to be the elementary particles
Atom constituents
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Proton, neutron, and electron
After 1932 these were viewed as elementary
All matter was made up of these particles
Discovery of New Particles
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New particles
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Beginning in the 1940s, many “new” particles
were discovered in experiments involving highenergy collisions
Characteristically unstable with short lifetimes
Over 300 have been catalogued
A pattern was needed to understand all these
new particles
Elementary Particles – Quarks
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Physicists recognize that most particles are
made up of quarks
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Exceptions include photons, electrons and a few
others
The quark model has reduced the array of
particles to a manageable few
The quark model has successfully predicted
new quark combinations that were
subsequently found in many experiments
Fundamental Forces
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All particles in nature are subject to four
fundamental forces:
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Nuclear force
Electromagnetic force
Weak force
Gravitational force
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This list is in order of decreasing strength
Nuclear Force
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Attractive force between nucleons
Strongest of all the fundamental forces
Very short-ranged
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Less than 10-15 m
Negligible for separations greater than this
Electromagnetic Force
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Is responsible for the binding of atoms and
molecules
About 10-2 times the strength of the nuclear
force
A long-range force that decreases in strength
as the inverse square of the separation
between interacting particles
Weak Force
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Is responsible for instability in certain nuclei
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Is responsible for decay processes
Its strength is about 10-5 times that of the
strong force
Gravitational Force
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A familiar force that holds the planets, stars
and galaxies together
Its effect on elementary particles is negligible
A long-range force
It is about 10-39 times the strength of the
strong force
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Weakest of the four fundamental forces
Explanation of Forces
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Forces between particles are often described in
terms of the actions of field particles or exchange
particles
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Field particles are also called gauge bosons
The interacting particles continually emit and absorb field
particles
The emission of a field particle by one particle and its
absorption by another manifests itself as a force between
the two interacting particles
The force is mediated, or carried, by the field particles
Forces and Mediating Particles
Paul Adrien Maurice Dirac
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1902 – 1984
British physicist
Understanding of antimatter
Unification of quantum
mechanics and relativity
Contributions of quantum
physics and cosmology
Nobel Prize in 1933
Dirac’s Description of the
Electron
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Dirac developed a relativistic quantum mechanical
description of the electron
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The solutions to the wave equation required
negative energy states
Dirac postulated that all negative energy states were
filled
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It successfully explained the origin of the electron’s spin
and its magnetic moment
These electrons are collectively called the Dirac sea
Electrons in the Dirac sea are not directly
observable because the exclusion principle does not
let them react to external forces
Dirac’s Explanation
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An interaction may
cause the electron to
be excited to a positive
energy state
This would leave
behind a hole in the
Dirac sea
The hole can react to
external forces and is
observable
Dirac’s Explanation, cont.
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The hole reacts in a way similar to the
electron, except that it has a positive charge
The hole is the antiparticle of the electron
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The electron’s antiparticle is now called a positron
Antiparticles
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For every particle, there is an antiparticle
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This has been verified for all particles known today
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From Dirac’s version of quantum mechanics that
incorporated special relativity
Some particles are their own antiparticles
 Photon and po
An antiparticle of a charged particle has the same
mass as the particle, but the opposite charge
The positron (electron’s antiparticle) was discovered
by Anderson in 1932
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Since then, it has been observed in numerous experiments
Pair Production
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A common source of positrons is pair
production
A gamma-ray photon with sufficient energy
interacts with a nucleus and an electronpositron pair is created from the photon
The photon must have a minimum energy
equal to 2mec2 to create the pair
Pair Production, cont.
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A photograph of pair production produced by 300
MeV gamma rays striking a lead sheet
The minimum energy to create the pair is 1.02 MeV
The excess energy appears as kinetic energy of the
two particles
Annihilation
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The reverse of pair production can also occur
Under the proper conditions, an electron and
a positron can annihilate each other to
produce two gamma ray photons
e- + e+ 
Hideki Yukawa
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1907 – 1981
Japanese physicist
Nobel Prize in 1949 for
predicting the existence
of mesons
Developed the first
theory to explain the
nature of the nuclear
force
Mesons
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Developed from a theory to explain the
nuclear force
Yukawa used the idea of forces being
mediated by particles to explain the nuclear
force
A new particle was introduced whose
exchange between nucleons causes the
nuclear force
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It was called a meson
Mesons, cont.
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The proposed particle would have a mass
about 200 times that of the electron
Efforts to establish the existence of the
particle were done by studying cosmic rays in
the 1930s
Actually discovered multiple particles
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pi meson (pion)
muon
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Found first, but determined to not be a meson
Pion
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There are three varieties of pions
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Correspond to three charge states
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p+ and p
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po
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Each has mass of 139.6 MeV/c2
Antiparticles
Mass of 135.0 MeV/c2
Very unstable particles
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For example, the po decays into a muon and an
antineutrino with a mean lifetime of 2.6 x 10-8 s
Muons
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Two muons exist
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µ- and its antiparticle µ+
The muon is unstable
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It has a mean lifetime of 2.2 µs
It decays into an electron, a neutrino, and an
antineutrino
Richard Feynman
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1918 – 1988
American physicist
Developed quantum
electrodynamics
Shared the Nobel Prize in
1965
Worked on Challenger
investigation and
demonstrated the effects of
cold temperatures on the
rubber O-rings used
Feynman Diagrams
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A graphical representation of the interaction
between two particles
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Feynman diagrams are named for Richard Feynman who
developed them
A Feynman diagram is a qualitative graph of time on
the vertical axis and space on the horizontal axis
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Actual values of time and space are not important
The overall appearance of the graph provides a pictorial
representation of the process
Feynman Diagram – Two
Electrons
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The photon is the field
particle that mediates the
electromagnetic force
between the electrons
The photon transfers
energy and momentum
from one electron to the
other
The photon is called a
virtual photon
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It can never be detected
directly because it is absorbed
by the second electron very
shortly after being emitted by
the first electron
The Virtual Photon
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The existence of the virtual photon seems to
violate the law of conservation of energy
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But, due to the uncertainty principle and its very
short lifetime, the photon’s excess energy is less
than the uncertainty in its energy
The virtual photon can exist for short time
intervals, such that ∆E  h / 2 ∆t
Within the constraints of the uncertainty
principle, the energy of the system is
conserved
Feynman Diagram – Proton and
Neutron (Yukawa’s Model)
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The exchange is via the
nuclear force
The existence of the pion is
allowed in spite of
conservation of energy if this
energy is surrendered in a
short enough time
Analysis predicts the rest
energy of the pion to be 130
MeV / c2
 This is in close agreement
with experimental results
Nucleon Interaction – More
About Yukawa’s Model
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The time interval required for the pion to
transfer from one nucleon to the other is

The distance the pion could travel is c∆t
Using these pieces of information, the rest
energy of the pion is about 100 MeV
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Nucleon Interaction, final
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This concept says that a system of two
nucleons can change into two nucleons plus
a pion as long as it returns to its original state
in a very short time interval
It is often said that the nucleon undergoes
fluctuations as it emits and absorbs field
particles
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These fluctuations are a consequence of quantum
mechanics and special relativity
Feynman Diagram – Weak
Interaction
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An electron and a
neutrino are interacting
via the weak force
The Z0 is the mediating
particle
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The weak force can also be
mediated by the W
The W and Z0 were
discovered in 1983 at
CERN
Nuclear Force and Strong
Force
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Historically, the nuclear force was called the
strong force
Now the strong force is reserved for the force
between quarks
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Or between particles made from quarks
The nuclear force is the force between
nucleons
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It is a secondary result of the strong force
Sometimes called residual strong force
Classification of Particles
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Two broad categories
Classified by interactions
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Hadrons – interact through strong force
Leptons – interact through weak force
Note on terminology
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The strong force is reserved for the force between
quarks
The nuclear force is reserved for the force
between nucleons
Hadrons
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Interact through the strong force
Two subclasses distinguished by masses and spins
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Mesons
 Integer spins (0 or 1)
 Decay finally into electrons, positrons, neutrinos and
photons
Baryons
 Masses equal to or greater than a proton
 Half integer spin values (1/2 or 3/2)
 Decay into end products that include a proton (except for
the proton)
Not elementary, but composed of quarks
Leptons
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Do not interact through strong force
All have spin of 1/2
Leptons appear truly elementary
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No substructure
Point-like particles
Scientists currently believe only six leptons exist,
along with their antiparticles
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Electron and electron neutrino
Muon and its neutrino
Tau and its neutrino
Conservation Laws
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A number of conservation laws are important in the
study of elementary particles
Already have seen conservation of
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Energy
Linear momentum
Angular momentum
Electric charge
Two additional laws are
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Conservation of Baryon Number
Conservation of Lepton Number
Conservation of Baryon
Number
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Whenever a baryon is created in a reaction or
a decay, an antibaryon is also created
B is the baryon number
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B = +1 for baryons
B = -1 for antibaryons
B = 0 for all other particles
The sum of the baryon numbers before a
reaction or a decay must equal the sum of
baryon numbers after the process
Conservation of Baryon
Number and Proton Stability
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There is a debate over whether the proton
decays or not
If baryon number is absolutely conserved, the
proton cannot decay
Some recent theories predict the proton is
unstable and so baryon number would not be
absolutely conserved
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For now, we can say that the proton has a half-life
of at least 1033 years
Conservation of Baryon
Number, Example
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Is baryon number conserved in the following
reaction?
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Baryon numbers:
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Before: 1 + 1 = 2
After: 1 + 1 + 1 + (-1) = 2
Baryon number is conserved
The reaction can occur as long as energy is
conserved
Conservation of Lepton
Number
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There are three conservation laws, one for
each variety of lepton
The law of conservation of electron lepton
number states that the sum of electron
lepton numbers before the process must
equal the sum of the electron lepton number
after the process
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The process can be a reaction or a decay
Conservation of Lepton
Number, cont.
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Assigning electron lepton numbers
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Le = 1 for the electron and the electron neutrino
Le = -1 for the positron and the electron antineutrino
Le = 0 for all other particles
Similarly, when a process involves muons, muon
lepton number must be conserved and when a
process involves tau particles, tau lepton numbers
must be conserved
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Muon and tau lepton numbers are assigned similarly to
electron lepton numbers
Conservation of Lepton
Number, Example
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Is lepton number conserved in the following
reaction?
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Check electron lepton numbers:
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Before: Le = 0
After: Le = 1 + (-1) + 0 = 0
Electron lepton number is conserved
Check muon lepton numbers:
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Before: Lµ = 1
After: Lµ = 0 + 0 + 1 = 1
Muon lepton number is conserved
Strange Particles
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Some particles discovered in the 1950s were found
to exhibit unusual properties in their production and
decay and were given the name strange particles
Peculiar features include:
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Always produced in pairs
Although produced by the strong interaction, they do not
decay into particles that interact via the strong interaction,
but instead into particles that interact via weak interactions
 They decay much more slowly than particles decaying via
strong interactions
Strangeness
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To explain these unusual properties, a new quantum
number S, called strangeness, was introduced
A new law, the law of conservation of
strangeness was also needed
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It states that the sum of strangeness numbers before a
reaction or a decay must equal the sum of the strangeness
numbers after the process
Strong and electromagnetic interactions obey the
law of conservation of strangeness, but the weak
interaction does not
Bubble Chamber
Example of Strange Particles
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The dashed lines
represent neutral
particles
At the bottom,
π - + p  K0 + Λ0
Then Λ0  π - + p
and

K  π +µ + ν μ
o
-
Murray Gell-Mann
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1929 –
American physicist
Studies dealing with
subatomic particles
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Named quarks
Developed pattern
known as eightfold way
Nobel Prize in 1969
The Eightfold Way
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Many classification schemes have been proposed to
group particles into families
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The eightfold way is a symmetric pattern proposed
by Gell-Mann and Ne’eman
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These schemes are based on spin, baryon number,
strangeness, etc.
There are many symmetrical patterns that can be
developed
The patterns of the eightfold way have much in
common with the periodic table
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Including predicting missing particles
An Eightfold Way for Baryons
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A hexagonal pattern for the
eight spin 1/2 baryons
Strangeness vs. charge is
plotted on a sloping
coordinate system
Six of the baryons form a
hexagon with the other two
particles at its center
An Eightfold Way for Mesons
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The mesons with spins of 0
can be plotted
Strangeness vs. charge on a
sloping coordinate system is
plotted
A hexagonal pattern emerges
The particles and their
antiparticles are on opposite
sides on the perimeter of the
hexagon
The remaining three mesons
are at the center
Eightfold Way for Spin 3/2
Baryons
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The nine particles known at
the time were arranged as
shown
An empty spot occurred
Gell-Mann predicted the
missing particle and its
properties
About three years later, the
particle was found and all
its predicted properties
were confirmed
Quarks
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Hadrons are complex particles with size and
structure
Hadrons decay into other hadrons
There are many different hadrons
Quarks are proposed as the elementary
particles that constitute the hadrons
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Originally proposed independently by Gell-Mann
and Zweig
Original Quark Model
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Three types or flavors
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Quarks have fractional electrical charges
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u – up
d – down
s – strange
-a e and b e
Associated with each quark is an antiquark
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The antiquark has opposite charge, baryon
number and strangeness
Original Quark Model – Rules
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All the hadrons at the time of the original
proposal were explained by three rules
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Mesons consist of one quark and one antiquark
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This gives them a baryon number of 0
Baryons consist of three quarks
Antibaryons consist of three antiquarks
Quark Composition of
Particles – Examples
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Mesons are quarkantiquark pairs
Baryons are quark
triplets
Active Figure 46.11
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Use the active figure to
observe the quark
composition for various
mesons and baryons
PLAY
ACTIVE FIGURE
Additions to the Original
Quark Model – Charm
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Another quark was needed to account for some
discrepancies between predictions of the model and
experimental results
A new quantum number, C, was assigned to the
property of charm
Charm would be conserved in strong and
electromagnetic interactions, but not in weak
interactions
In 1974, a new meson, the J/, was discovered that
was shown to be a charm quark and charm
antiquark pair
More Additions –
Top and Bottom
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Discovery led to the need for a more elaborate
quark model
This need led to the proposal of two new quarks
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t – top (or truth)
b – bottom (or beauty)
Added quantum numbers of topness and
bottomness
Verification
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b quark was found in a Y meson in 1977
t quark was found in 1995 at Fermilab
Numbers of Particles
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At the present, physicists believe the
“building blocks” of matter are complete
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Six quarks with their antiparticles
Six leptons with their antiparticles
Quark Composition of Some
Baryons
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The table shows the
quark composition of
various baryons
Baryons are made from
three quarks
Only u and d quarks
are contained in
hadrons encountered in
ordinary matter
Particle Properties
More About Quarks
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No isolated quark has ever been observed
It is believed that at ordinary temperatures,
quarks are permanently confined inside
ordinary particles due to the strong force
Current efforts are underway to form a
quark-gluon plasma where quarks would be
freed from neutrons and protons
Color
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It was noted that certain particles had quark
compositions that violated the exclusion
principle
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Quarks are fermions, with half-integer spins and
so should obey the exclusion principle
The explanation is an additional property
called color charge

The color has nothing to do with the visual
sensation from light, it is simply a name
Colored Quarks
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Color “charge” occurs in red, blue, or green
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Antiquarks have colors of antired, antiblue, or
antigreen
These are the quantum “numbers” of color charge
Color obeys the exclusion principle
A combination of quarks of each color
produces white (or colorless)
Baryons and mesons are always colorless
Quantum Chromodynamics
(QCD)
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QCD gave a new theory of how quarks interact with
each other by means of color charge
The strong force between quarks is often called the
color force
The strong force between quarks is mediated by
gluons
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Gluons are massless particles
When a quark emits or absorbs a gluon, its color
may change
More About Color Charge
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Particles with like colors repel and those with
opposite colors attract
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Different colors attract, but not as strongly as a color and
its anticolor
The color force between color-neutral hadrons is
negligible at large separations
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The strong color force between the constituent quarks
does not exactly cancel at small separations
This residual strong force is the nuclear force that binds the
protons and neutrons to form nuclei
Quark Structure of a Meson
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A green quark is
attracted to an
antigreen quark
The quark – antiquark
pair forms a meson
The resulting meson is
colorless
Quark Structure of a Baryon
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Quarks of different
colors attract each
other
The quark triplet
forms a baryon
Each baryon contains
three quarks with
three different colors
The baryon is
colorless
QCD Explanation of a NeutronProton Interaction
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Each quark within the
proton and neutron is
continually emitting and
absorbing gluons
The energy of the gluon can
result in the creation of
quark-antiquark pairs
When close enough, these
gluons and quarks can be
exchanged, producing the
strong force
Elementary Particles –
A Current View

Scientists now believe there are three
classifications of truly elementary particles
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Leptons
Quarks
Field particles
These three particles are further classified as
fermions or bosons
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Quarks and leptons are fermions (spin ½)
Field particles are bosons (integral spin 1 and up)
Weak Force

The weak force is believed to be mediated by
the W+, W-, and Z0 bosons
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These particles are said to have weak charge
Therefore, each elementary particle can have
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Mass
Electric charge
Color charge
Weak charge

One or more of these could be zero
Electroweak Theory
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The electroweak theory unifies
electromagnetic and weak interactions
The theory postulates that the weak and
electromagnetic interactions have the same
strength when the particles involved have
very high energies

Viewed as two different manifestations of a single
unifying electroweak interaction
The Standard Model

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A combination of the electroweak theory and QCD
for the strong interaction form the Standard Model
Essential ingredients of the Standard Model
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The strong force, mediated by gluons, holds the quarks together
to form composite particles
Leptons participate only in electromagnetic and weak interactions
The electromagnetic force is mediated by photons
The weak force is mediated by W and Z bosons
The Standard Model does not actually yet include
the gravitational force
The Standard Model – Chart
Mediator Masses

Why does the photon have no mass while the W
and Z bosons do have mass?
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Not answered by the Standard Model
The difference in behavior between low and high energies
is called symmetry breaking
The Higgs boson has been proposed to account for the
masses
 Large colliders are necessary to achieve the energy needed
to find the Higgs boson

In a collider, particles with equal masses and equal kinetic
energies, traveling in opposite directions, collide head-on to
produce the required reaction
Particle Paths After a Collision
The Big Bang

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This theory states that the universe had a
beginning, and that it was so cataclysmic that
it is impossible to look back beyond it
Also, during the first few minutes after the
creation of the universe, all four interactions
were unified

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All matter was contained in a quark-gluon plasma
As time increased and temperature
decreased, the forces broke apart
A Brief History of the Universe
Cosmic Background Radiation
(CBR)
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CBR represents the cosmic
“glow” left over from the Big
Bang
The radiation had equal
strengths in all directions
The curve fits a black body at
2.7K
There are small irregularities
that allowed for the formation
of galaxies and other objects
CBR, cont.

The COBE satellite found that the
background radiation had irregularities that
corresponded to temperature variations of
0.000 3 K
Hubble’s Law

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The Big Bang theory predicts that the
universe is expanding
Hubble claimed the whole universe is
expanding
Furthermore, the speeds at which galaxies
are receding from the earth is directly
proportional to their distance from us

This is called Hubble’s law
Hubble’s Law, cont.

Hubble’s law can be
written as
v = HR

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H is called the Hubble
constant
H 17 x 10-3 m/s•ly
Remaining Questions About
the Universe

Will the universe expand forever?
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Today, astronomers and physicists are trying to
determine the rate of expansion
It depends on the average mass density of the
universe compared to a critical density
Missing mass in the universe


The amount of non-luminous (dark) matter seems
to be much greater than what we can see
Various particles have been proposed to make up
this dark matter
Another Remaining Question
About the Universe

Is there mysterious energy in the universe?

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Observations have led to the idea that the
expansion of the universe is accelerating
To explain this acceleration, dark energy has
been proposed
The dark energy results in an effective repulsive
force that causes the expansion rate to increase
Some Questions in Particle
Physics
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Why so little antimatter in the Universe?
Is it possible to unify electroweak and strong forces?
Why do quarks and leptons form similar but distinct
families?
Are muons the same as electrons apart from their
difference in mass?
Why are some particles charged and others not?
Why do quarks carry fractional charge?
What determines the masses of fundamental
particles?
Can isolated quarks exist?
A New Perspective –
String Theory


String theory is one current effort at
answering some of the previous questions
It is an effort to unify the four fundamental
forces by modeling all particles as various
vibrational modes of an incredibly small string
String Theory, cont.

The typical length of a string is 10-35 m


This is called the Planck length
According to the string theory, each
quantized mode of vibration of the string
corresponds to a different elementary particle
in the Standard Model
Complications of the String
Theory

It requires space-time to have ten dimensions


Four of the ten dimensions are visible to us, the
other six are compactified (curled)
Another complication is that it is difficult for
theorists to guide experimentalists as to what
to look for in an experiment

Direct experimentation on strings is impossible
String Theory Prediction –
SUSY

One prediction of string theory is
supersymmetry (SUSY)


It suggests that every elementary particle has a
superpartner that has not yet been observed
Supersymmetry is a broken symmetry and the
masses of the superpartners are above our
current capabilities to detect
Another Perspective –
M-Theory


M-theory is an eleven-dimensional theory
based on membranes rather than strings
M-theory is claimed to reduce to string theory
if one compactifies from the eleven
dimensions to ten