Transcript Chapter 46

Chapter 46
Particle Physics and Cosmology
Atoms as Elementary Particles
Atoms
 From the Greek for “indivisible”
 Were once thought to be the elementary particles
Atom constituents
 Proton, neutron, and electron
 After 1932 these were viewed as elementary
 All matter was made up of these particles
Introduction
Discovery of New Particles
New particles
 Beginning in the 1940s, many “new” particles were discovered in
experiments involving high-energy collisions.
 Characteristically unstable with short lifetimes
 Over 300 have been catalogued
A pattern was needed to understand all these new particles.
Introduction
Elementary Particles – 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.
Section 45.7
Fundamental Forces
All particles in nature are subject to four fundamental forces:
 Nuclear force
 Electromagnetic force
 Weak force
 Gravitational force
 This list is in order of decreasing strength.
Section 46.1
Nuclear Force
Attractive force between nucleons
Strongest of all the fundamental forces
Very short-ranged
 Less than 10-15 m
 Negligible for separations greater than this
Section 46.1
Electromagnetic Force
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.
Section 46.1
Weak Force
Is responsible for instability in certain nuclei
 Is responsible for decay processes
Its strength is about 10-5 times that of the strong force.
Section 46.1
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 10-39 times the strength of the strong force.
 Weakest of the four fundamental forces
Section 46.1
Explanation of Forces
Forces between particles are often described in terms of the actions of field
particles or exchange particles.
 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.
Section 46.1
Forces and Mediating Particles
Section 46.1
Paul Adrien Maurice Dirac
1902 – 1984
British physicist
Understanding of antimatter
Unification of quantum mechanics and
relativity
Contributions of quantum physics and
cosmology
Nobel Prize in 1933
Section 46.2
Dirac’s Description of the Electron
Dirac developed a relativistic quantum mechanical description of the electron .
 It successfully explained the origin of the electron’s spin and its magnetic
moment.
The solutions to the wave equation required negative energy states.
Dirac postulated that all negative energy states were filled.
 The electrons occupying these states 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.
Section 46.2
Dirac’s Explanation
An interaction may cause the electron
to be excited to a positive energy.
 The minimum energy required is
2 me c2.
This would leave behind a hole in the
Dirac sea.
The hole can react to external forces
and is observable.
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.
 The electron’s antiparticle is now
called a positron.
Section 46.2
Antiparticles
For practically every known particle, there is an antiparticle.
 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.
 Since then, it has been observed in numerous experiments.
Antiprotons and antineutrons have also been discovered.
Section 46.2
Pair Production
A common source of positrons is pair production.
A gamma-ray photon with sufficient energy interacts with a nucleus and an
electron-positron pair is created from the photon.
The photon must have a minimum energy equal to 2mec2 to create the pair.
Section 46.2
Pair Production, cont.
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.
Section 46.2
Annihilation
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+ 
Section 46.2
Hideki Yukawa
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
Section 46.3
Mesons
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.
 It was called a meson.
Section 46.3
Mesons, cont.
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
 pi meson (pion)
 muon
 Found first, but determined to not be a meson
Section 46.3
Pion
There are three varieties of pions.
 Correspond to three charge states
 p+ and p Each has mass of 139.6 MeV/c2
 Antiparticles
 po
 Mass of 135.0 MeV/c2
 Very unstable particles
 For example, the p- decays into a muon and an antineutrino with a mean lifetime of
2.6 x 10-8 s
Section 46.3
Muons
Two muons exist.
 µ- and its antiparticle µ+
The muon is unstable.
 It has a mean lifetime of 2.2 µs.
 It decays into an electron, a neutrino, and an antineutrino.
Section 46.3
Richard Feynman
1918 – 1988
American physicist
Developed quantum electrodynamics
 The theory of interaction of light
and matter on a relativistic and
quantum basis.
Shared the Nobel Prize in 1965
Worked on the Manhattan Project
Worked on Challenger investigation
and demonstrated the effects of cold
temperatures on the rubber O-rings
used
Section 46.3
Feynman Diagrams
A graphical representation of the interaction between two particles.
 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.
 Actual values of time and space are not important.
 The overall appearance of the graph provides a pictorial representation of
the process.
Section 46.3
Feynman Diagram – Two Electrons
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.
 It can never be detected directly
because it is absorbed by the
second electron very shortly after
being emitted by the first electron.
Section 46.3
The Virtual Photon
The existence of the virtual photon seems 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  h / 2 ∆t.
Within the constraints of the uncertainty principle, the energy of the system is
conserved.
Section 46.3
Feynman Diagram – Proton and Neutron (Yukawa’s Model)
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 100 MeV / c2.
 This is in close agreement with
experimental results.
Section 46.3
Nucleon Interaction – More About Yukawa’s Model
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.
Section 46.3
Nucleon Interaction, final
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.
 These fluctuations are a consequence of quantum mechanics and special
relativity.
Section 46.3
Feynman Diagram – Weak Interaction
An electron and a neutrino are
interacting via the weak force.
The Z0 is the mediating particle.
 The weak force can also be
mediated by the W± .
 The W± and Z0 were discovered in
1983 at CERN.
Section 46.3
Nuclear Force and Strong Force
Historically, the nuclear force was called the strong force.
Now the strong force is reserved for the force between quarks.
 Or between particles made from quarks
The nuclear force is the force between nucleons.
 It is a secondary result of the strong force.
 Sometimes called residual strong force
Section 46.3
Classification of Particles
Two broad categories for particles other than field particles
Classified by interactions
 Hadrons – interact through strong force
 Leptons – interact through weak force
Section 46.4
Hadrons
Interact through the strong force
Two subclasses distinguished by masses and spins
 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
Section 46.4
Leptons
Do not interact through strong 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
Section 46.4
Conservation Laws
A number of conservation laws are important in the study of elementary particles.
Already have seen conservation of
 Energy
 Linear momentum
 Angular momentum
 Electric charge
Two additional laws are
 Conservation of Baryon Number
 Conservation of Lepton Number
Section 46.5
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 = -1 for antibaryons
 B = 0 for all other particles
Conservation of Baryon Number states whenever a nuclear reaction or decay
occurs, the sum of the baryon numbers before the process must equal the sum of
baryon numbers after the process.
Section 46.5
Conservation of Baryon Number and Proton Stability
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.
 For now, we can say that the proton has a half-life of at least 1033 years.
Section 46.5
Conservation of Baryon Number, Example
Is baryon number conserved in the following reaction?

 Baryon numbers:
 Before: 1 + 1 = 2
 After: 1 + 1 + 1 + (-1) = 2
 Baryon number is conserved
 The reaction can occur as long as energy is conserved.
Section 46.5
Conservation of Lepton Number
There are three conservation laws, one for each variety of lepton.
The law of conservation of electron lepton number states whenever a nuclear
reaction or decay occurs, the sum of electron lepton numbers before the process
must equal the sum of the electron lepton number after the process.
Assigning electron lepton numbers:
 Le = 1 for the electron and the electron neutrino
 Le = -1 for the positron and the electron antineutrino
 Le = 0 for all other particles
Section 46.5
Conservation of Lepton Number, cont.
When a process involves muons, muon lepton number must be conserved.
When a process involves tau particles, tau lepton numbers must be conserved.
 Muon and tau lepton numbers are assigned similarly to electron lepton
numbers.
Section 46.5
Conservation of Lepton Number, Example
Is lepton number conserved in the following reaction?

 Check electron lepton numbers:
 Before: Le = 0
After: Le = 1 + (-1) + 0 = 0
 Electron lepton number is conserved
 Check muon lepton numbers:
 Before: Lµ = 1
After: Lµ = 0 + 0 + 1 = 1
 Muon lepton number is conserved
 Both lepton numbers are conserved and on this basis the decay is possible.
Section 46.5
Strange Particles
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:
 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.
Section 46.6
Strangeness
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.
 It states in a nuclear reaction or decay that occurs via the strong force,
strangeness is conserved.
 That is, the sum of strangeness numbers before a reaction or a decay must
equal the sum of the strangeness numbers after the process.
 In processes that occur via the weak interactions, strangeness may not be
conserved.
 Strong and electromagnetic interactions obey the law of conservation of
strangeness.
Section 46.6
Bubble Chamber Example of Strange Particles
The dashed lines represent
neutral particles
At the bottom,
π - + p  K0 + Λ0
Then Λ0  π - + p and
K o  π  +µ - + ν μ
Section 46.6
Murray Gell-Mann
1929 –
American physicist
Studies dealing with subatomic
particles
 Named quarks
 Developed pattern known as
eightfold way
Nobel Prize in 1969
Section 46.7
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
Section 46.7
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.
Section 46.7
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.
 These three particles form their
own antiparticles.
Section 46.7
Eightfold Way for Spin 3/2 Baryons
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.
Section 46.7
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 Gell-Mann and Zweig
 Named by Gell-Mann
Section 46.8
Original Quark Model
Three types or flavors
 u – up
 d – down
 s – strange
Quarks have fractional electrical charges
 -a e and b e
Quarks have spin ½
 All quarks are fermions
Associated with each quark is an antiquark
 The antiquark has opposite charge, baryon number and strangeness
Section 46.8
Original 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.
Section 46.8
Quark Composition of Particles – Examples
Mesons are quark-antiquark pairs.
Baryons are quark triplets.
Section 46.8
Additions to the Original Quark Model – Charm
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.
Section 46.8
More Additions – Top and Bottom
Discovery led to the need for a more elaborate quark model
This need led to the proposal of two new quarks:
 t – top (or truth)
 b – bottom (or beauty)
Added quantum numbers of topness and bottomness
Verification
 b quark was found in a Y meson in 1977.
 t quark was found in 1995 at Fermilab.
Section 46.8
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
 Four field particles
Section 46.8
Quark Composition of Some Baryons
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.
Section 46.8
Particle Properties
Section 46.8
More About Quarks
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.
 Both RHIC and CERN have announced evidence for a quark-gluon plasma,
but neither laboratory has provided definitive data to verify the existence of
the plasma.
Section 46.8
Color
It was noted that certain particles had quark compositions that violated the
exclusion principle.
 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.
Section 46.9
Colored Quarks
Color charge occurs in red, blue, or green.
 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.
Section 46.9
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 mediated by gluons.
 Gluons are massless particles.
When a quark emits or absorbs a gluon, its color may change.
Section 46.9
More About Color Charge
Particles with like colors repel and those with opposite colors attract.
 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.
 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.
Section 46.9
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.
Section 46.9
Quark Structure of a Baryon
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.
Section 46.9
QCD Explanation of a Neutron-Proton Interaction
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.
This quark model of interactions
between nucleons is consistent with the
pion-exchange model.
Section 46.9
Elementary Particles – A Current View
Scientists now believe there are three classifications of truly elementary particles:
 Leptons
 Quarks
 Field particles
These three particles are further classified as fermions or bosons.
 Quarks and leptons are fermions (spin ½).
 Field particles are bosons (integral spin 1 and up).
Section 46.10
Weak Force
The weak force is believed to be mediated by the W+, W-, and Z0 bosons.
 These particles are said to have weak charge.
Therefore, each elementary particle can have
 Mass
 Electric charge
 Color charge
 Weak charge
 One or more of these could be zero.
Section 46.10
Electroweak Theory
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
Section 46.10
The Standard Model
A combination of the electroweak theory and QCD for the strong interaction 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.
 Also in gravitational 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.
Section 46.10
The Standard Model – Chart
Section 46.10
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.
 In a collider, particles with equal masses and equal kinetic energies,
traveling in opposite directions, collide head-on to produce the required
reaction.
Section 46.10
Particle Paths After a Collision – Fermi Lab Example
Section 46.10
The Big Bang
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.
 All matter was contained in a quark-gluon plasma.
As time increased and temperature decreased, the forces broke apart.
Section 46.11
A Brief History of the Universe
Section 46.11
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 black body at 2.7K.
There are small irregularities that
allowed for the formation of galaxies
and other objects.
The COBE satellite found that the
background radiation had irregularities
that corresponded to temperature
variations of 0.000 3 K.
Section 46.11
Hubble’s Law
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 can be written as v = HR.
 H is called the Hubble constant.
 H 22 x 10-3 m/s•ly
Section 46.11
Remaining Questions About the Universe
Will the universe expand forever?
 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.
Section 46.11
Another Remaining Question About the Universe
Is there mysterious energy in the universe?
 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.
Section 46.11
Some Questions in Particle Physics
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?
Do leptons and quarks have an underlying structure?
Section 46.12
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.
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.
Section 46.12
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.
Section 46.12
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.
Section 46.12
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.