Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Chapter 29 Lecture
Particle Physics
Prepared by
Dedra Demaree,
Georgetown University
© 2014 Pearson Education, Inc.
Particle Physics
• What is antimatter?
• What are the fundamental particles and
interactions in nature?
• What was the Big Bang, and how has the
universe evolved since? © 2014 Pearson Education, Inc.
Be sure you know how to:
• Use the right-hand rule to determine the
direction of the magnetic force exerted by a
magnetic field on a moving charged particle
(Section 17.4).
• Explain beta decay (Section 28.6).
• Write an expression for the rest energy of a
particle (Section 25.8).
© 2014 Pearson Education, Inc.
What's new in this chapter
• Beta decay can produce antineutrinos, a form of
antimatter.
– Every known particle has a corresponding
antiparticle.
– In this chapter we investigate elementary
particles such as the positron and their
fundamental interactions.
– This area of physics is called particle physics.
© 2014 Pearson Education, Inc.
Antiparticles
• By 1930, physicists had identified four particles:
the electron, the proton, the neutron, and the
photon.
• At that time, these were the only known truly
elementary particles—a description used to
indicate the simplest and most basic particles.
• This view changed with the proposal and
discovery of so-called antiparticles.
© 2014 Pearson Education, Inc.
Antielectrons predicted
• Dirac predicted that free electrons would have
an infinite number of possible quantum states
with negative total energy.
– A free electron in a positive energy state
should be able to transition to one of these
negative energy states.
– How could a free electron have negative total
energy?
– These negative energy states are occupied
by an infinite number of positrons (then called
antielectrons), a new type of particle that had
not yet been observed. © 2014 Pearson Education, Inc.
Antielectrons detected
© 2014 Pearson Education, Inc.
Pair production
• Under the right conditions, a photon can
produce an electron and a positron:
© 2014 Pearson Education, Inc.
Pair annihilation
• If an electron and a positron meet, it is possible
for them to annihilate and produce a photon.
© 2014 Pearson Education, Inc.
Conceptual Exercise 29.1
• Imagine that an electron and a positron meet
and annihilate each other. Assume that they are
moving directly toward each other at constant
speed.
A. Will one or two photons be produced? Write
a reaction equation for this process.
B. In which directions do these photons move
relative to each other after the process?
© 2014 Pearson Education, Inc.
Beta-plus decay: Transforming a proton into
a neutron
• If a proton captures a gamma-ray photon, the
energy of the excited-state proton may be great
enough to produce a neutron and the other
particles.
• The proton absorbs the photon and then decays
into a neutron, a positron, and a neutrino.
– This process is called beta-plus decay to
indicate that a positron (not an electron) is
produced.
© 2014 Pearson Education, Inc.
Positron emission tomography
• Positron emission tomography (PET) is a process for
imaging the brain.
– Fluorine-18 isotopes undergo beta-plus decay
continually, producing positrons.
– The positrons meet electrons and annihilate each
other, producing a pair of gamma-ray photons that
move in opposite directions.
© 2014 Pearson Education, Inc.
Other antiparticles
• The positively charged proton that is part of all
nuclei has a negatively charged antiproton of the
same mass but opposite electric charge.
• Even though the neutron has zero electric
charge, it also has an antimatter counterpart;
other properties besides charge differentiate the
neutron from the antineutron.
• Occasionally, a particle is its own antiparticle;
the photon is an example.
© 2014 Pearson Education, Inc.
Fundamental interactions
• Fundamental interactions are the most basic
interactions known, such as the electromagnetic
interaction between charged particles.
• Nonfundamental interactions, such as friction,
can be understood in terms of fundamental
interactions.
– Friction is a macroscopic manifestation of the
electromagnetic interaction between the
electrons of two surfaces that are in contact.
© 2014 Pearson Education, Inc.
Fundamental interactions: Gravitational
interaction
• All objects in the universe participate in
gravitational interactions due to their mass.
– This interaction is important for very massive
(mega-) objects.
– It is much less important for objects in our
daily lives and extremely insignificant for
microscopic objects.
© 2014 Pearson Education, Inc.
Fundamental interactions: Electromagnetic
interaction
• Electrically charged objects participate in
electromagnetic interactions.
– The interaction is electric if the objects are at
rest or in motion with respect to each other.
– The interaction is magnetic only if the objects
are moving with respect to each other.
– The electromagnetic interactions between
nuclei and electrons are important in
understanding the structure of atoms.
© 2014 Pearson Education, Inc.
Comparing the electromagnetic interaction
to the gravitational interaction
• The electrostatic force that an electron and a
proton exert on each other in an atom is about
1039 times greater than the gravitational force
that they exert on each other.
© 2014 Pearson Education, Inc.
Fundamental interactions: Strong
interaction
• The binding of protons and neutrons together
into a nucleus is a residual interaction of the
strong interaction.
– The strong interaction is a very short-range
interaction, exerted by protons and neutrons
only on their nearest neighbors within the
nucleus.
© 2014 Pearson Education, Inc.
Fundamental interactions: Weak interaction
• The weak interaction is responsible for beta
decay.
– Protons, neutrons, electrons, and neutrinos all
participate in it.
– The weak interaction is significantly weaker
than the strong interaction and has a
significantly shorter range.
© 2014 Pearson Education, Inc.
Mechanisms of fundamental interactions
• This particle exchange mechanism has been
successful in describing the weak and strong
interactions.
• It has also had some success in describing the
gravitational interaction.
– The emitted and absorbed particle is called a
mediator.
– For the electromagnetic interaction, the
mediator is the photon.
© 2014 Pearson Education, Inc.
Interaction mediators
• Photons: electromagnetic interaction
• Gluons: strong interaction
• W and Z bosons: weak interaction
• Gravitons: gravitational interaction
– The mediators of the electromagnetic, strong,
and weak interactions have all been
discovered.
– The hypothetical mediator for the gravitational
interaction, the so-called graviton, has not.
© 2014 Pearson Education, Inc.
Fundamental interactions
© 2014 Pearson Education, Inc.
Quantitative Exercise 29.2
• Convert the masses of the W ± and Z
0 particles
into electron volts.
© 2014 Pearson Education, Inc.
Elementary particles and the Standard
Model
• Particle accelerators facilitate collisions between
particles with total energy significantly greater
than the rest energies, allowing for additional
particles to be produced.
– The properties of these additional particles
can be determined using elaborate detectors.
– Most of the particles produced are not stable.
© 2014 Pearson Education, Inc.
Leptons
• Leptons interact only through weak,
electromagnetic, and gravitational interactions, but
not through strong interactions.
– The electron and the electron neutrino are
examples of leptons.
– These two particles form a generation (or
family) of leptons.
© 2014 Pearson Education, Inc.
Lepton generations (families)
© 2014 Pearson Education, Inc.
Hadrons
• Two different types of hadrons can be
distinguished: baryons and mesons.
– The proton and the neutron are baryons.
– In 1935, Hideki Yukawa suggested the
existence of new particles that mediated the
strong interaction—the first example of a
meson.
– In 1947, physicists discovered a meson in
cosmic rays that participated in strong
interactions and had the correct properties to
be Yukawa's meson.
© 2014 Pearson Education, Inc.
Properties of hadrons
© 2014 Pearson Education, Inc.
Particle
© 2014 Pearson Education, Inc.
Quarks
• Hadrons are made up of smaller, more
fundamental particles that have fractional
electric charge, known as quarks.
– Six different quarks have been discovered
experimentally.
– These different quark types are known in the
physics community as flavors. © 2014 Pearson Education, Inc.
Quarks
© 2014 Pearson Education, Inc.
The proton and quark charge
• The total electric charge adds to e, and the total
color is neutral. © 2014 Pearson Education, Inc.
Conceptual Exercise 29.3
• Which combination of quarks will combine to
have the correct properties to be a neutron?
© 2014 Pearson Education, Inc.
Conceptual Exercise 29.3
© 2014 Pearson Education, Inc.
Particles (matter) and their interactions
© 2014 Pearson Education, Inc.
Confinement
• No experiment has ever produced a quark in
isolation.
– Every quark and antiquark ever produced have
always been part of a hadron.
– This phenomenon, called confinement, is an
indication of a feature of the strong interaction.
– The strong interaction between quarks is
weakest when they are close together and
gets stronger the farther apart the quarks are.
© 2014 Pearson Education, Inc.
Development of the Standard Model
• The Standard Model is the combined theory of
the building blocks of matter and their
interactions.
– In the late 1940s, physicists Feynman,
Schwinger, and Tomonaga independently
combined the ideas of special relativity and
quantum mechanics into a single model that
explains all electromagnetic phenomena.
– Their model, known as quantum
electrodynamics (QED), is a cornerstone of the
Standard Model.
© 2014 Pearson Education, Inc.
Standard Model
© 2014 Pearson Education, Inc.
Higgs particle
• In 1967, Glashow, Salam, and Weinberg
independently put forth a model that unified the
electromagnetic and weak interactions into a
single interaction, which they called the
electroweak model.
• This model predicted the existence of a particle,
which became known as the Higgs particle after
physicist Peter Higgs.
© 2014 Pearson Education, Inc.
Predictions of the electroweak model
• In the very distant past when the universe was
much smaller and very much hotter, all particles
were massless.
• This situation led to the existence of the Higgs
particle.
• As the universe cooled, the Higgs particle began
interacting significantly with other elementary
particles, reconfiguring them into the familiar
forms they have today.
– This is known as the Higgs mechanism.
© 2014 Pearson Education, Inc.
Quantum chromodynamics
• In 1973, using Yang's and Mills' mathematical
framework, Fritzsch and Gell-Mann formulated
quantum chromodynamics (QCD), a
mathematical model of the strong interaction that
plays a role in the exchange of gluons between
quarks.
– Between 1976 and 1979, scientists
discovered the tau lepton and bottom quark
and found direct evidence for gluons.
© 2014 Pearson Education, Inc.
Additional particle discovery timeline
• The 1980s brought the discoveries of the
predicted weak interaction mediators W and Z.
• The 1990s gave us the top quark.
• In 2000, the tau neutrino was discovered.
• In July 2012, CERN announced the discovery of
a particle that may be the long-sought-after
Higgs particle.
© 2014 Pearson Education, Inc.
Unanswered questions of the Standard
Model
1. Can the strong interaction be unified with
the electroweak interaction?
2. Why are there only three families of
quarks/leptons?
3. Are the Standard Model particles truly
fundamental?
4. Are there additional particles beyond those
predicted by the Standard Model?
© 2014 Pearson Education, Inc.
Summary of the Standard Model
• Quarks and leptons, which make up the matter of
the universe
• The theory of strong interactions (QCD) mediated
by gluons
• The theory of electromagnetic (QED) and weak
interactions mediated by photons and the W and
Z particles
• The Higgs particle, which explains, through the
Higgs mechanism, why some of the fundamental
particles have nonzero mass
© 2014 Pearson Education, Inc.
Cosmology
• Why is our universe not filled with equal
numbers of particles and antiparticles? Why is
there an imbalance?
– These questions are answered in part by
particle physics and by cosmology—a branch
of physics that studies the composition and
evolution of the universe as a whole.
© 2014 Pearson Education, Inc.
Big Bang
© 2014 Pearson Education, Inc.
Standard Model
© 2014 Pearson Education, Inc.
Inflation
• When the universe first became "cold" enough
that quarks and leptons emerged as
distinguishable particles, a fundamental change
in the structure of the universe occurred,
resulting in an extremely rapid exponential
expansion known as cosmic inflation.
– During inflation, small fluctuations in the
density of the universe decreased.
– Areas where the density was slightly above
average would later act as the seeds of
galaxy formation.
© 2014 Pearson Education, Inc.
Nucleosynthesis
• A few minutes after the Big Bang, the
temperature had dropped to about 1 billion K, and
the average density of the universe was close to
the density of air at sea level on Earth today.
– For the first time, protons and neutrons were
able to combine to form the simplest nuclei:
deuterium, helium, and trace amounts of
lithium.
– This process is known as Big Bang
nucleosynthesis.
© 2014 Pearson Education, Inc.
Atoms, stars, and galaxies
• When the universe had cooled enough,
gravitational interactions became the dominant
driver of its further evolution.
– Density fluctuations led to the formation of the
first galaxies and stars just 500,000 years
after the Big Bang.
– These early stars went through their life
cycles, with some ending in a violent collapse
and explosion known as a supernova, which
created heavier elements such as carbon,
oxygen, iron, and gold.
© 2014 Pearson Education, Inc.
Dark matter and dark energy
• When astronomers measure the mass of all the
stars and gas that they can see, they find that
the total mass is only about one-tenth of the
mass needed to account for the speed of the
solar system around the center of the galaxy.
– The universe is "missing" about 90% of the
mass needed to account for the observed
motion of stars and galaxies.
– How can this contradiction be resolved?
© 2014 Pearson Education, Inc.
Dark matter
• In 1933, astrophysicist Fritz Zwicky speculated
that there must be some unseen dark matter
present in the Coma cluster; for about 40 years,
his observation was the only evidence for its
existence.
• In the 1970s, Vera Rubin presented further
evidence.
• It was at this point that the dark matter
explanation started to become more widely
accepted.
© 2014 Pearson Education, Inc.
Dark matter
• Dark matter does not emit photons or otherwise
participate in the electromagnetic interaction
(this is why it is called "dark").
• Dark matter cannot be some sort of dark cloud
of protons or gaseous atoms, because these
could be detected by the scattering of radiation
passing through them.
© 2014 Pearson Education, Inc.
MACHOs: Massive compact halo objects
• These objects could be black holes, neutron
stars, or brown dwarfs.
• Astronomers have detected MACHOs through
their gravitational effects on the light from distant
objects.
• The small number of detected events translates
into MACHOs accounting for at most 20% of the
dark matter in our galaxy.
– There must be another (or an additional)
explanation.
© 2014 Pearson Education, Inc.
WIMPs: Weakly interacting massive
particles
• WIMPs are "weakly interacting": they can pass
through ordinary matter with almost no interaction,
and they neither absorb nor emit light.
• WIMPs are "massive": their mass is not zero.
– Prime candidates for WIMPs include neutrinos,
axions, and neutralinos.
– Axions and neutralinos are not Standard Model
particles and require the Standard Model to be
extended to accommodate their existence.
© 2014 Pearson Education, Inc.
Grand unified theories
• Grand unified theories combine the strong,
weak, and electromagnetic interactions into a
single interaction.
• These theories predict the existence of "sterile
neutrinos," which could have even fewer
interactions and be far more massive than
Standard Model neutrinos.
– Physicists do not know how to detect such a
particle.
– If it exists in sufficient abundance, it could
account for dark matter.
© 2014 Pearson Education, Inc.
Supersymmetry
• Supersymmetry is an extension of the Standard
Model:
– It effectively doubles the number of
elementary particles.
– It gives insight into the cosmological constant
problem.
– It allows for a more precise understanding of
the unification of interactions in grand unified
theories.
– It gives a potential candidate for dark matter.
© 2014 Pearson Education, Inc.
Explaining the accelerating expansion of the
universe
• Invoke a discarded feature of Einstein's general
theory of relativity (our current best model of the
gravitational interaction) known as the
cosmological constant.
• Suggest the existence of a strange kind of
energy-fluid that fills space and has a repulsive
gravitational effect.
• Propose a modified version of general relativity
that includes a new kind of field that creates this
cosmic acceleration.
© 2014 Pearson Education, Inc.
The cosmological constant model
• The dominant model of the universe was the
steady-state model, which asserted that the
universe essentially did not change in any major
way as time passed.
– General relativity predicted that a static
universe was unstable.
– Einstein introduced the cosmological constant
into general relativity in an attempt to allow
the theory to accommodate a steady-state
universe.
© 2014 Pearson Education, Inc.
The dark energy model
• The cosmological constant seems to represent a
type of dark energy that is present at every point
in space with equal density.
– Even as the universe expands, the density
does not decrease because it is a property of
space itself.
– Dark energy has a negative pressure.
– In general relativity, this produces a
gravitationally repulsive effect on space.
© 2014 Pearson Education, Inc.
Modified general relativity
• Some better theory of the gravitational interaction
would make even better predictions than general
relativity.
– The challenge has been to construct the new
theory in such a way that it does not make
predictions that contradict experiments that
have already been done.
– Thus far physicists have been unsuccessful in
achieving this goal.
© 2014 Pearson Education, Inc.
The proportion of matter, dark matter, and
dark energy in the universe
© 2014 Pearson Education, Inc.
Cosmological constant problem
• Dark energy is the sum of the zero-point
energies of all quantum fields in the universe.
– When physicists predict values for the
cosmological constant, they get a result that
is 10120 times the observed value.
– This is the largest disagreement between
prediction and experiment in all of science.
– This so-called cosmological constant problem
is a major unsolved problem in physics.
© 2014 Pearson Education, Inc.
Tip
© 2014 Pearson Education, Inc.
Is our pursuit of knowledge worthwhile?
• Our models describe the behavior of only 4% of
the content of our universe.
– The nature of the remaining 96% of our
universe currently remains an unsolved
problem.
• Will our eventual knowledge of the other 96% of
the universe someday make people's lives better?
– It is impossible to say for sure, but history
suggests that it very likely will.
© 2014 Pearson Education, Inc.
Summary
© 2014 Pearson Education, Inc.
Summary
© 2014 Pearson Education, Inc.