Transcript PSE4_Lecture_Ch43 - Elementary Particles

Chapter 43
Elementary Particles
Units of Chapter 43
• High-Energy Particles and Accelerators
• Beginnings of Elementary Particle Physics –
Particle Exchange
• Particles and Antiparticles
• Particle Interactions and Conservation Laws
• Neutrinos – Recent Results
• Particle Classification
Units of Chapter 43
• Particle Stability and Resonances
• Strange Particles? Charm? Toward a New
Model
• Quarks
• The “Standard Model”: Quantum
Chromodynamics (QCD) and the Electroweak
Theory
• Grand Unified Theories
• Strings and Supersymmetry
43.1 High-Energy Particles and
Accelerators
If an incoming particle in a nuclear reaction has
enough energy, new particles can be produced.
This effect was first observed in cosmic rays;
later particle accelerators were built to provide
the necessary energy.
43.1 High Energy Particles and
Accelerators
As the momentum of a particle increases, its
wavelength decreases, providing details of
smaller and smaller structures:
In addition, with additional kinetic energy
more massive particles can be produced.
43.1 High-Energy Particles and
Accelerators
One early particle
accelerator was the
cyclotron. Charged
particles are maintained
in near-circular paths by
magnets, while an electric
field accelerates them
repeatedly. The voltage is
alternated so that the
particles are accelerated
each time they traverse
the gap.
43.1 High-Energy Particles and
Accelerators
Larger accelerators are a type called
synchrotrons. Here, the magnetic field is
increased as the particles accelerate, so
that the radius of the path stays
constant. This allows the construction of
a narrow circular tunnel to house a ring
of magnets.
43.1 High-Energy Particles and
Accelerators
Synchrotrons can be very large, up to several
miles in diameter. These pictures are of
Fermilab, a synchrotron outside Chicago,
Illinois.
43.1 High-Energy Particles and
Accelerators
Accelerating particles radiate; this causes them to lose
energy. This is called synchrotron radiation for particles
in a circular path. For protons this is usually not a
problem, but the much lighter electrons can lose
substantial amounts. One solution is to construct a linear
accelerator for electrons; the largest is about 3 km long.
43.1 High-Energy Particles and
Accelerators
The maximum possible energy is obtained from an
accelerator when two counter-rotating beams of
particles collide head-on. Fermilab is able to obtain
1.8 TeV in proton–antiproton collisions; a new
accelerator called the Large Hadron Collider (LHC)
will reach energies of 14 TeV.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
The electromagnetic force acts over a
distance – direct contact is not necessary.
How does that work?
Because of wave–particle duality, we can
regard the electromagnetic force between
charged particles as due to:
1. an electromagnetic field, or
2. an exchange of photons.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
This is a crude analogy
for how particle
exchange would work to
transfer energy and
momentum. The force
can be either attractive
or repulsive.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
Physicists visualize interactions using
Feynman diagrams, which are a kind of x-t
graph.
Here is a Feynman diagram for photon
exchange by electrons:
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
The photon is emitted by one electron and
absorbed by the other; it is never visible and is
called a virtual photon. The photon carries the
electromagnetic force.
Originally, the strong force was thought to be
carried by mesons. The mesons have nonzero
mass, which is what limits the range of the
force, as conservation of energy can only be
violated for a short time.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
The mass of the meson
can be calculated,
assuming the range, d,
is limited by the
uncertainty principle:
For d = 1.5 x 10-15 m,
this gives 130 MeV.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
This meson was soon discovered, and is called
the pi meson, or pion, with the symbol π.
Pions are created in interactions in particle
accelerators. Here are two examples:
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
The weak nuclear force is also carried by
particles; they are called the W+, W-, and Z0.
They have been directly observed in
interactions.
A carrier for the gravitational force, called the
graviton, has been proposed, but there is as yet
no theory that will accommodate it.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
This picture shows the reconstruction of the
creation of a Z particle, and the detector that
discovered it.
43.2 Beginnings of Elementary Particle
Physics – Particle Exchange
This table details the four known forces, their
relative strengths for two protons in a nucleus,
and their field particles.
43.3 Particles and Antiparticles
The positron is the same as the electron, except
for having the opposite charge (and lepton
number). We call the positron the antiparticle of
the electron.
Every type of particle has its own antiparticle,
with the same mass and most with the opposite
quantum number.
A few particles, such as the photon and the π0,
are their own antiparticles, as all the relevant
quantum numbers are zero for them.
43.3 Particles and Antiparticles
This drawing, from a bubble chamber photograph, is of
an interaction between an incoming antiproton and a
proton (not seen) that results in the creation of several
different particles and antiparticles.
43.4 Particle Interactions and
Conservation Laws
In the study of particle interactions, it was found
that certain interactions did not occur, even
though they conserve energy and charge, such
as:
A new conservation law was proposed: the
conservation of baryon number. Baryon
number is a generalization of nucleon
number to include more exotic particles.
43.4 Particle Interactions and
Conservation Laws
Particles such as the proton and neutron have
baryon number B = +1; antiprotons,
antineutrons, and the like have B = -1; all other
particles (electrons, photons, etc.) have B = 0.
There are three types of leptons – the electron,
the muon (about 200 times more massive), and
the tau (about 3000 electron masses). Each
type of lepton is conserved separately.
43.4 Particle Interactions and
Conservation Laws
This accounts for the following decays:
Decays that have an unequal mix of e-type
and μ-type leptons are not allowed.
43.4 Particle Interactions and
Conservation Laws
Conceptual Example 43-5: Lepton
number in muon decay.
Which of the following decay schemes
is possible for muon decay?
(a)
(b)
(c)
All of these particles have Lτ = 0.
43.4 Particle Interactions and
Conservation Laws
Example 43-6: Energy and momentum are
conserved.
In addition to the “number” conservation laws which
help explain the decay schemes of particles, we can
also apply the laws of conservation of energy and
momentum. The decay of a Σ+ particle at rest with a
mass of 1189 MeV/c2 commonly yields a proton (mass
= 938 MeV/c2) and a neutral pion, (mass = 135 MeV/c2):
What are the kinetic energies of the decay products,
assuming the Σ+ parent particle was at rest?
43.5 Neutrinos – Recent Results
Neutrinos are currently a subject of active
research. Evidence has shown that a neutrino
of one type may change into a neutrino of
another type; this is called flavor oscillation.
This suggests that the individual lepton
numbers are sometimes not strictly conserved,
although there is no evidence that the total
lepton number is not.
In addition, these oscillations cannot take place
unless at least one neutrino type has a nonzero
mass.
43.6 Particle Classification
As work continued, more and more particles of all
kinds were discovered. They have now been
classified into different categories.
• Gauge bosons are the particles that mediate the
forces.
• Leptons interact weakly and (if charged)
electromagnetically, but not strongly.
• Hadrons interact strongly; there are two types of
hadrons, baryons (B = 1) and mesons (B = 0).
The table of particle properties on the next slide
gives some indication of the complexity of the
known particles.
43.6 Particle Classification
43.6 Particle Classification
Example 43-7: Baryon decay.
Show that the decay modes of the
Σ+ baryon given in Table 43–2 do
not violate the conservation laws
we have studied up to now: energy,
charge, baryon number, lepton
numbers.
43.7 Particle Stability and Resonances
Almost all of the particles that have been
discovered are unstable. If they decay weakly,
their lifetimes are around 10-13 s; if
electromagnetically, around 10-16 s; and if
strongly, around 10-23 s.
Strongly decaying particles do not travel far
enough to be observed; their existence is
inferred from their decay products.
43.7 Particle Stability and Resonances
The lifetime of strongly decaying particles is
calculated from the variation in their effective
mass using the uncertainty principle. These
particles are often called resonances.
43.8 Strange Particles? Charm? Toward
a New Model
When the K, Λ, and Σ particles were first
discovered in the early 1950s, there were
mysteries associated with them:
• They are always produced in pairs.
• They are created in a strong interaction, decay
to strongly interacting particles, but have
lifetimes characteristic of the weak interaction.
To explain this, a new quantum number, called
strangeness, S, was introduced.
43.8 Strange Particles? Charm? Toward
a New Model
Particles such as the K, Λ, and Σ have S = 1
(and their antiparticles have S = -1); other
particles have S = 0.
The strangeness number is conserved in
strong interactions but not in weak ones;
therefore, these particles are produced in
particle–antiparticle pairs, and decay weakly.
More recently, another new quantum number
called charm was discovered to behave in the
same way.
43.8 Strange Particles? Charm? Toward
a New Model
Conceptual Example 43-8: Guess the
missing particle.
Using the conservation laws for particle
interactions, determine the possibilities for
the missing particle in the reaction
in addition to K0 + Λ0 mentioned above.
43.9 Quarks
Due to the regularities seen in the particle
tables, as well as electron scattering results that
showed internal structure in the proton and
neutron, a theory of quarks was developed.
There are six different “flavors” of quarks; each
has baryon number B = ⅓.
Hadrons are made of three quarks; mesons are a
quark–antiquark pair.
43.9 Quarks
Here are the quark
compositions for some
baryons and mesons:
43.9 Quarks
This table gives the properties of the
six known quarks.
43.9 Quarks
This is a list of some of the hadrons that have
been discovered that contain c, t, or b quarks.
43.9 Quarks
The particles that we now consider to be truly
elementary – having no internal structure – are
the quarks, the gauge bosons, and the leptons.
The quarks and leptons are arranged in three
“generations”; each has the same pattern of
electric charge, but the masses increase from
generation to generation.
43.9 Quarks
Conceptual Example 43-9: Quark
combinations.
Find the baryon number, charge, and
strangeness for the following quark
combinations, and identify the
hadron particle that is made up of
these quark combinations: (a) udd,
(b) uū, (c) uss, (d) sdd, and (e) bū.
43.10 The “Standard Model”: Quantum
Chromodynamics (QCD) and the
Electroweak Theory
Soon after the quark theory was proposed, it
was suggested that quarks have another
property, called color, or color charge.
Unlike other quantum numbers, color takes on
three values. Real particles must be colorless;
this explains why only 3-quark and quark–
antiquark configurations are seen. Color also
ensures that the exclusion principle is still
valid.
43.10 The “Standard Model”: Quantum
Chromodynamics (QCD) and the
Electroweak Theory
Each quark carries a color charge, and the
force between them is called the color force –
hence the name quantum chromodynamics.
The particles that transmit the color force are
called gluons; there are eight different ones,
with all possible color–anticolor
combinations.
43.10 The “Standard Model”: Quantum
Chromodynamics (QCD) and the
Electroweak Theory
The color force becomes much larger as quarks
separate; quarks are therefore never seen as
individual particles, as the energy needed to
separate them is less than the energy needed to
create a new quark–antiquark pair.
Conversely, when the quarks are very close
together, the force is very small.
43.10 The “Standard Model”: Quantum
Chromodynamics (QCD) and the
Electroweak Theory
These Feynman diagrams show a quark–quark
interaction mediated by a gluon; a baryon–baryon
interaction mediated by a meson; and the baryon–
baryon interaction as mediated on a quark level by
gluons.
Figure 43-16 goes here.
43.11 Grand Unified Theories
A Grand Unified Theory (GUT) would unite the
strong, electromagnetic, and weak forces into
one. There would be (rare) transitions that
would transform quarks into leptons and vice
versa.
This unification would occur at extremely high
energies; at lower energies the forces would
“freeze out” into the ones we are familiar with.
This is called “symmetry breaking.”
43.11 Grand Unified Theories
Conceptual Example 43-12: Symmetry.
The table shown has four identical place
settings. Four people sit down to eat.
Describe the symmetry of this table and what
happens to it when someone starts the meal.
43.11 Grand Unified Theories
GUTs predict that the proton will
eventually decay; in fact, the simplest
GUT predicts a lifetime for the proton
that is shorter than the measured limit,
so a more complex GUT must be the
correct theory.
43.12 Strings and Supersymmetry
Finally, there are theories that attempt to include
the gravitational force as well.
String theory models the fundamental particles as
different resonances on tiny loops of “string”.
Supersymmetry postulates a fermion partner for
each boson, and vice versa.
Neither of these theories has any experimental
evidence either favoring or disfavoring it at the
moment.
Summary of Chapter 43
• Particle accelerators accelerate particles to a
very high energy, to probe the detailed structure
of matter and to produce new massive particles.
• Every particle has an antiparticle, with the same
mass and opposite charge (and some other
quantum numbers).
• Other quantum numbers: baryon number,
lepton number, strangeness, charm, topness,
bottomness
• Strong force is mediated by gluons.
Summary of Chapter 43
• Fundamental force carriers are called gauge
bosons.
• Leptons interact weakly and
electromagnetically.
• Hadrons are made of quarks, and interact
strongly.
• Most particles decay quickly, weakly,
electromagnetically, or strongly.
• There are six quarks and six leptons.
Summary of Chapter 43
• The quarks also carry color charge.
• Quantum chromodynamics is the theory of
the strong interaction.
• Electroweak theory unites the
electromagnetic and weak forces.
• Grand unified theories attempt to unite all
three forces.