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Chapter 44
Particles Physics
& Cosmology
Elementary Particles
Atoms
From the Greek for “indivisible”
Were once thought to be the
elementary particles
Atom constituents
Proton, neutron, and electron
Were viewed as elementary because
they are very stable
Quarks
Physicists recognize that most particles
are made up of quarks
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
All particles in nature are subject
to four fundamental forces
Strong force
Electromagnetic force
Weak force
Gravitational force
Strong Force
Is responsible for the tight binding of
the quarks to form neutrons and
protons
Also responsible for the nuclear force
binding the neutrons and the protons
together in the nucleus
Strongest of all the fundamental forces
Very short-ranged
Less than 10-15 m
Electromagnetic Force
Is responsible for the binding of
atoms and molecules
About 10-2 times the strength of
the strong force
A long-range force that decreases
in strength as the inverse square
of the separation between
interacting particles
Weak Force
Is responsible for instability in certain
nuclei
Is responsible for beta decay
A short-ranged force
Its strength is about 10-6 times that of
the strong force
Scientists now believe the weak and
electromagnetic forces are two
manifestations of a single force, the
electroweak force
Gravitational Force
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 10-43 times the strength of
the strong force
Weakest of the four fundamental forces
Explanation of Forces
Forces between particles are often
described in terms of the actions of
field particles or quanta
For electromagnetic force, the photon
is the field particle
The electromagnetic force is
mediated, or carried, by photons
Forces and Mediating
Particles (also see table 30.1)
Interaction (force)
Mediating Field
Particle
Strong
Gluon
Electromagnetic
Photon
Weak
W± and Z0
Gravitational
Gravitons
Richard Feynmann
1918 – 1988
Contributions include
Work on the Manhattan
Project
Invention of diagrams to
represent particle
interactions
Theory of weak interactions
Reformation of quantum
mechanics
Superfluid helium
Challenger investigation
Shared Nobel Prize in
1965
Feynman Diagrams
A graphical representation of the
interaction between two particles
Feynman diagrams are named for
Richard Feynman who developed
them
Feynman Diagram – Two
Electrons
The photon is the field
particle that mediates
the interaction
The photon transfers
energy and momentum
from one electron to the
other
The photon is called a
virtual photon
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
The existence of the virtual photon
would be expected to violate the
law of conservation of energy
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 ∆t ≈ ħ
Paul Adrien Maurice Dirac
1902 – 1984
Instrumental in
understanding
antimatter
Aided in the
unification of
quantum mechanics
and relativity
Contributions to
quantum physics and
cosmology
Nobel Prize in 1933
Antiparticles
For every particle, there is an antiparticle
An antiparticle has the same mass as the
particle, but the opposite charge
The positron (electron’s antiparticle) was
discovered by Anderson in 1932
From Dirac’s version of quantum mechanics that
incorporated special relativity
Since then, it has been observed in numerous
experiments
Practically every known elementary particle
has a distinct antiparticle
Exceptions – the photon and the neutral pi particles
are their own antiparticles
Classification of Particles
Two broad categories
Classified by interactions
Hadrons
Interact through strong force
Composed of quarks
Leptons
Interact through weak force
Thought to be truly elementary
Some suggestions they may have some internal
structure
Hadrons
Interact through the strong force
Two subclasses
Mesons
Baryons
Decay finally into electrons, positrons, neutrinos
and photons
Integer spins
Masses equal to or greater than a proton
Noninteger spin values
Decay into end products that include a proton
(except for the proton)
Composed of quarks
Leptons
Interact through weak force
All have spin of ½
Leptons appear truly elementary
No substructure
Point- like particles
Scientists currently believe only six
leptons exist, along with their
antiparticles
Electron and electron neutrino
Muon and its neutrino
Tau and its neutrino
Conservation Laws
A number of conservation laws are
important in the study of
elementary particles
Two new ones are
Conservation of Baryon Number
Conservation of Lepton Number
Conservation of Baryon
Number
Whenever a baryon is created in a
reaction or a decay, an antibaryon is
also created
B is the Baryon Number
B = +1 for baryons
B =- 1for 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
Proton Stability
Absolute conservation of baryon
number indicates the proton must
be absolutely stable
Otherwise, it could decay into a
positron and a neutral pion
Never been observed
Currently can say the proton has a halflife of at least 1031 years
Some theories indicate the proton can
decay
Conservation of Lepton
Number
There are three conservation laws,
one for each variety of lepton
Law of Conservation of ElectronLepton Number states that the
sum of electron-lepton numbers
before a reaction or a decay must
equal the sum of the electronlepton number after the process
Conservation of Lepton
Number, cont
Assigning electron-lepton numbers
Le = 1 for the electron and the electron neutrino
Le =- 1for 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
Muon
- and tau
- lepton numbers are assigned
similarly to electron- lepton numbers
Strange Particles
Some particles discovered in the 1950’s
were found to exhibit unusual properties in
their production and decay and were given
the name strange particles
Peculiar features include
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
To explain these unusual properties, a new
law, conservation of strangeness, was
introduced
Also needed a new quantum number, S
The Law of Conservation of Strangeness 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 interactions do
not
Bubble Chamber
Example
The dashed lines
represent neutral
particles
At the bottom,
π- + p → Λ0 + K0
Then Λ0 → π- + p
and
K0 → π + µ- + νµ
Murray Gell-Mann
1929 –
Worked on
theoretical studies
of subatomic
particles
Nobel Prize in
1969
The Eightfold Way
Many classification schemes have been
proposed to group particles into families
These schemes are based on spin, baryon
number, strangeness, etc.
The eightfold way is a symmetric pattern
proposed by Gell-Mann and Ne’eman
There are many symmetrical patterns that
can be developed
The patterns of the eightfold way have
much in common with the periodic table
Including predicting missing particles
An Eightfold Way for
Baryons
A hexagonal pattern
for the eight spin ½
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
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
Quarks
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
Originally proposed independently by GellMann and Zweig
Quark Model
Three types
u – up
d – down
s – strange
c – charmed
t – top
b – bottom
Associated with each quark is an
antiquark
The antiquark has opposite charge, baryon
number and strangeness
Quark Model, cont
Quarks have fractional electrical
charges
+1/3 e and –2/3 e
All ordinary matter consists of just
u and d quarks
Quark Model – Rules
All the hadrons at the time of the
original proposal were explained
by three rules
Mesons consist of one quark and one
antiquark
This gives them a baryon number of 0
Baryons consist of three quarks
Antibaryons consist of three
antiquarks
Numbers of Particles
At the present, physicists believe
the “building blocks” of matter are
complete
Six quarks with their antiparticles
Six leptons with their antiparticles
See table 30.3 for quark summary
Color
Isolated quarks
Physicist now believe that quarks are
permanently confined inside ordinary
particles
No isolated quarks have been observed
experimentally
The explanation is a force called the color
force
Color force increases with increasing distance
This prevents the quarks from becoming isolated
particles
Colored Quarks
Color “charge” occurs in red, blue,
or green
Antiquarks have colors of antired,
antiblue, or antigreen
Color obeys the Exclusion Principle
A combination of quarks of each
color produces white (or colorless)
Baryons and mesons are always
colorless
Quark Structure of a
Meson
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
Quarks of
different colors
attract each other
The quark triplet
forms a baryon
The baryon is
colorless
Quantum
Chromodynamics (QCD)
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
carried by gluons
Gluons are massless particles
There are 8 gluons, all with color charge
When a quark emits or absorbs a gluon,
its color changes
More About Color Charge
Like colors repel and opposite colors
attract
Different colors also attract, but not as strongly
as a color and its anticolor
The color force between color-neutral
hadrons is negligible at large separations
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
Weak Interaction
The weak interaction is an extremely
short-ranged force
This short range implies the mediating
particles are very massive
The weak interaction is responsible for
the decay of c, s, b, and t quarks into u
and d quarks
Also responsible for the decay of µ and
τ leptons into electrons
Weak Interaction, cont
The weak interaction is very important
because it governs the stability of the
basic particles of matter
The weak interaction is not symmetrical
Not symmetrical under mirror reflection
Not symmetrical under charge exchange
Electroweak Theory
The electroweak theory unifies
electromagnetic and weak
interactions
The theory postulates that the
weak and electromagnetic
interactions have the strength at
very high particle energies
Viewed as two different
manifestations of a single interaction
The Standard Model
A combination of the electroweak
theory and QCD form the standard
model
Essential ingredients of the standard
model
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 –
Chart
Mediator Masses
Why does the photon have no mass
while the W and Z bosons do have
mass?
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
Grand Unification Theory
(GUT)
Builds on the success of the
electroweak theory
Attempted to combine electroweak
and strong interactions
One version considers leptons and
quarks as members of the same
family
They are able to change into each other
by exchanging an appropriate particle
The Big Bang
This theory of cosmology states that
during the first few minutes after the
creation of the universe all four
interactions were unified
All matter was contained in a quark soup
As time increased and temperature
decreased, the forces broke apart
Starting as a radiation dominated
universe, as the universe cooled it
changed to a matter dominated
universe
A Brief History of the
Universe
George Gamow
1904 – 1968
Among the first to
look at the first half
hour of the universe
Predicted:
Abundances of
hydrogen and helium
Radiation should still
be present and have
an apparent
temperature of about
5K
Cosmic Background
Radiation (CBR)
CBR represents the
cosmic “glow” left over
from the Big Bang
The radiation had
equal strengths in all
directions
The curve fits a
blackbody at 2.9 K
There are small
irregularities that
allowed for the
formation of galaxies
and other objects
Connection Between Particle
Physics and Cosmology
Observations of events that occur
when two particles collide in an
accelerator are essential to
understanding the early moments
of cosmic history
There are many common goals
between the two fields
Some Questions
Why so little antimatter in the Universe?
Do neutrinos have mass?
How do they contribute to the dark mass in the
universe?
Explanation of why the expansion of the
universe is accelerating?
Is there a kind of antigravity force acting
between widely separated galaxies?
Is it possible to unify electroweak and
strong forces?
Why do quark and leptons form similar but
distinct families?
More Questions
Are muons the same as electrons, except
for their mass?
Why are some particles charged and others
neutral?
Why do quarks carry fractional charge?
What determines the masses of
fundamental particles?
Do leptons and quarks have a
substructure?