Download CHAPTER 14: Elementary Particles

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CHAPTER 14
Elementary Particles
14.1
14.2
14.8
14.3
14.4
14.5
14.6
14.7
Early Discoveries
The Fundamental Interactions
Accelerators
Classification of Elementary Particles
Conservation Laws and Symmetries
Quarks
The Families of Matter
Beyond the Standard Model
Steven Weinberg (1933 - )
“I have done a terrible thing: I have postulated a particle that cannot be detected.”
Wolfgang Pauli (after postulating the existence of the neutrino)
“If I could remember the names of all these particles, I’d be a botanist.”
Enrico Fermi
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Elementary
Particles
Finding answers to some of the
basic questions about nature is
a foremost goal of science:
What are the basic building
blocks of matter?
What is inside the nucleus?
What are the forces that hold
matter together?
How did the universe begin?
Will the universe end, and if so,
how and when?
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14.1: Early Discoveries
In 1930 the known elementary
particles were the proton, the
electron, and the photon.
Thomson identified the electron in
1897, and Einstein defined the
photon in 1905. The proton is the
nucleus of the hydrogen atom.
Despite the rapid progress of
physics in the first couple of
decades of the twentieth century,
no more elementary particles
were discovered until 1932, when
Chadwick proved the existence of
the neutron.
That would have seemed
sufficient…
James Chadwick (1891-1974)
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But particle physics measurements were
particles collide with stationary
happening… Energetic
particles in a “bubble chamber,” vaporizing
the nearby matter and leaving a visible track.
A magnetic field
(pointing into the
screen) causes
charged particles
to take curved
paths.
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The Positron
In 1928 Dirac introduced the
relativistic theory of the electron when
he combined quantum mechanics
with special relativity.
He found that his wave equation had
negative, as well as positive, energy
solutions.
His theory can be interpreted as a
vacuum being filled with an infinite
sea of electrons with negative
energies.
If enough energy is transferred to the
“sea,” an electron can be ejected with
positive energy leaving behind a hole
that is the positron, denoted by e+.
Paul Dirac (1902-1984)
Vacuum
Electron &
positron
Positron!
E
0
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Anti-particles
Dirac’s theory yields anti-particles, which:
Have the same mass and lifetime as their associated particles.
Have the same magnitude but are opposite in sign for such physical
quantities as electric charge and various quantum numbers.
All particles, even
neutral ones (with
some exceptions like
the neutral pion), have
antiparticles.
Magnetic field into screen
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Cosmic rays are highly
energetic particles, mostly
protons, that cross interstellar
space and enter the Earth’s
atmosphere, where their
interaction with particles
creates cosmic “showers” of
many distinct particles.
Carl Anderson (1905-1991)
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The Positron
Carl Anderson identified the
positron in cosmic rays. It was
easy: it had positive charge
and was light.
Anderson’s cloud chamber photo of
positron track
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Positron-Electron Interaction
The ultimate fate of positrons (anti-electrons) is annihilation with
electrons.
After a positron slows down by passing through matter, it is attracted
by the Coulomb force to an electron, where it annihilates through the
reaction:
All anti-matter eventually meets the
same fate. A lot of energy is released
in this process: all of the matter is
converted to energy.
Star Trek’s “dilithium
crystals” supposedly
contain anti-matter, which
powers the Enterprise.
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Feynman Diagrams
Feynman presented a particularly simple graphical technique to
describe interactions.
It predicts that, when two electrons approach each other, according to
the quantum theory of fields, they exchange a series of photons called
virtual photons, because they cannot be directly observed.
The action of the electromagnetic field (for example, the Coulomb
force) can be interpreted as the exchange of photons. In this case we
say that the photons are the
carriers or mediators of the
electromagnetic force.
Example of a Feynman spacetime diagram. Electrons interact
through mediation of a photon.
The axes are normally omitted.
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Yukawa’s Meson
The Japanese physicist Hideki
Yukawa had the idea of developing a
quantum field theory that would
describe the force between
nucleons—analogously to the
electromagnetic force.
To do this, he had to determine the
carrier or mediator of the nuclear
strong force analogous to the photon
in the electromagnetic force which he
called a meson (derived from the
Greek word meso meaning “middle”
due to its mass being between the
electron and proton masses).
Hideki Yukawa (1907-1981)
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Yukawa’s Meson
Yukawa’s meson, called a pion (or pi-meson or p-meson), was
identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini
(1907–1993)
Charged pions have masses of 140 MeV/c2, and a neutral pion p0
was later discovered that has a mass of 135 MeV/c2, a neutron and
a proton.
Feynman diagram indicating the
exchange of a pion (Yukawa’s
meson) between a neutron and a
proton.
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Other Mesons, Quarks, and Gluons
Yukawa’s pion is responsible for the nuclear force.
Later we’ll see that the nucleons and mesons are part of a general
group of particles formed from even more fundamental particles:
quarks. The particle
that mediates the strong
interaction between quarks
is called a gluon (for the “glue”
that holds the quarks together);
it’s massless and has
spin 1, just like the photon.
Computed image of quarks
and gluons in a nucleon
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The Weak Interaction
Abdus Salam
(1926-1996)
Sheldon
Glashow
(1932- )
In the 1960s Sheldon
Glashow, Steven
Weinberg, and Abdus
Salam predicted that
particles that they
called W (for weak)
and Z should exist that
are responsible for the
weak interaction.
They have been
observed.
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The Graviton
It has been suggested that
the particle responsible for
the gravitational interaction
be called a graviton.
The graviton is the mediator of gravity in quantum field theory and
has been postulated because of the success of the photon in
quantum electrodynamics theory.
It must be massless, travel at the speed of light, have spin 2, and
interact with all particles that have mass-energy.
The graviton has never been observed because of its extremely
weak interaction with objects.
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The Fundamental Interactions
One of the main goals of particle physics is to unify these forces (to
show that they’re all just different aspects of the same force), just as
Maxwell did for the electric and magnetic forces many years earlier.
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The Fundamental Interactions
A finite range effectively confines the particle, which, by the
uncertainty principle, determines energy and mass. Photons and
gravitons are massless. W and Z bosons are heavy.
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14.8 Accelerators
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Particle accelerators generate
high enough energies to create
particles 1 GeV/c2 or greater.
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Accelerators
There are three main types of accelerators used presently in
particle physics experiments: synchrotrons, linear
accelerators, and colliders.
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Synchrotron
Radiation
One difficulty with cyclic
accelerators is that when
charged particles are
accelerated, they radiate
electromagnetic energy called synchrotron radiation. This problem is
particularly severe when electrons, moving very close to the speed of
light, move in curved paths. If the radius of curvature is small,
electrons can radiate as much energy as they gain.
Physicists have learned to take advantage of these synchrotron
radiation losses and now build special electron accelerators (called
light sources) that produce copious amounts of photon radiation
used for both basic and applied research.
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Linear
Accelerators
Linear accelerators or linacs
typically have straight
electric-field-free regions
between gaps of RF voltage
boosts. The particles gain
speed with each boost, and
the voltage boost is on for a
fixed period of time, and thus
the distance between gaps
becomes increasingly larger
as the particles accelerate.
Linacs are sometimes used
as pre-acceleration device
for large circular accelerators.
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Colliders
Because of the
limited energy
available for
reactions like that
found for the Tevatron, physicists decided they had to resort to
colliding beam experiments, in which the particles meet head-on.
If the colliding particles have equal masses and kinetic energies, the
total momentum is zero and all the energy is available for the reaction
and the creation of new particles.
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Large
Hadron
Collider
Counterpropagating
protons will each
have an energy of
7 TeV, giving a
total collision
energy of 14 TeV.
The LHC can also
be used to collide
heavy ions such
as lead (Pb) with a
collision energy of
1,150 TeV.
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14.3: Classification of Elementary
Particles
Particles with half-integral spin are called fermions and those with
integral spin are called bosons.
This is a particularly useful way to classify elementary particles
because all stable matter in the universe appears to be composed, at
some level, of constituent fermions.
Fermions obey the Pauli Exclusion Principle. Bosons don’t.
Photons, gluons, W±, and the Z are called gauge bosons and are
responsible for the strong and electroweak interactions.
Gravitons are also bosons, having spin 2.
Fermions exert attractive or repulsive forces on each other by
exchanging gauge bosons, which are the force carriers.
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The Higgs Boson
One other boson that has been predicted, but not yet detected, is
necessary in quantum field theory to explain why the W± and Z have
such large masses, yet the photon has no mass.
This missing boson is called the Higgs particle (or Higgs boson)
after Peter Higgs, who first proposed it.
The Standard Model proposes that there is a field called the Higgs
field that permeates all of space.
By interacting with this field, particles acquire mass. Particles that
interact strongly with the Higgs field have heavy mass; particles that
interact weakly have small mass.
The Higgs boson is very heavy, and
it hasn’t been observed yet.
The search for the Higgs boson is of
the highest priority in elementary
particle physics.
Simulated
event
featuring the
appearance
of the Higgs
boson.
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Boson Properties
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Leptons: electrons, muons, taus & neutrinos
The leptons are perhaps the simplest of the elementary particles.
They appear to be point-like, that is, with no apparent internal
structure, and seem to be truly elementary.
Thus far there has been no
plausible suggestion they
are formed from some more
fundamental particles.
Each of the leptons has an
associated neutrino, named
after its charged partner (for
example, muon neutrino).
There are only six leptons
plus their six antiparticles.
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Muon and tau decay
The muon decays into an electron, and the tau can decay into an
electron, a muon, or even hadrons.
The muon decay (by the weak interaction) is:
e
m
nm
ne
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Hadrons
Hadrons are particles that
act through the strong force.
Two classes of hadrons:
mesons and baryons.
Mesons are particles with integral spin having masses greater than
that of the muon (106 MeV/c2). (Mesons are made up of pairs of
quarks—a quark and an anti-quark.) They’re unstable and rare.
Baryons have masses at least as large as the proton and have halfintegral spins. Baryons include the proton and neutron, which make
up the atomic nucleus, but many other unstable baryons exist as
well. The term "baryon" is derived from the Greek βαρύς (barys),
meaning "heavy," because at the time of their naming it was
believed that baryons were characterized by having greater mass
than other particles. (They’re made up of three quarks.) All baryons
decay into protons.
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The
Hadrons
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Fundamental and Composite Particles
We call certain particles
fundamental; this means that
they are not composed of other,
smaller particles. We believe
leptons, quarks, and gauge
bosons are fundamental
particles.
Although the Z and W bosons
have very short lifetimes, they
are regarded as particles, so a
definition of particles dependent
only on lifetimes is too restrictive.
Other particles are composites,
made from the fundamental
particles.
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14.4: Conservation Laws
Physicists like to have clear rules or laws that determine whether a
certain process can occur or not.
It seems that everything occurs in nature that is not forbidden.
Certain conservation laws are already familiar from our study of
classical physics. These include mass-energy, charge, linear
momentum, and angular momentum.
These are absolute conservation laws: they are always obeyed.
Additional conservation laws will be helpful in understanding the many
possibilities of elementary particle interactions.
Some of these laws are absolute, but others may be valid for only one
or two of the fundamental interactions.
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Baryon Conservation
In low-energy nuclear reactions, the number of nucleons is always
conserved.
Empirically this is part of a more general conservation law for what is
assigned a new quantum number called baryon number that has the
value B = +1 for baryons and −1 for anti-baryons, and 0 for all other
particles.
The conservation of baryon number requires the same total baryon
number before and after the reaction.
Although there are no known violations of baryon conservation, there
are theoretical indications that it was violated sometime in the
beginning of the universe when temperatures were quite high. This is
thought to account for the preponderance of matter over anti-matter in
the universe today.
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Lepton Conservation
The leptons are all fundamental
particles, and there is a conservation
of leptons for each of the three kinds
(families) of leptons.
The number of leptons from each family is the same both before and
after a reaction.
We let Le = +1 for the electron and the electron neutrino; Le = −1 for
their antiparticles; and Le = 0 for all other particles.
We assign the quantum numbers Lμ for the muon and its neutrino and
Lτ for the tau and its neutrino similarly.
Thus three additional conservation laws.
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Strangeness
The behavior of the K mesons seemed very odd.
There is no conservation law for the production of mesons, but it
appeared that K mesons, as well as the Λ and Σ baryons, were
always produced in pairs in the p + p reaction. One would expect
the K0 meson to also decay into two photons very quickly, but it
does not.
A new quantum number was defined: Strangeness, S, which is
conserved in the strong and electromagnetic interactions, but not in
the weak interaction.
The kaons have S = +1, lambda and sigmas have S = −1, the xi has
S = −2, and the omega has S = −3.
When the strange particles are produced by the p + p strong
interaction, they must be produced in pairs to conserve
strangeness.
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Unifying all these interactions proved
difficult.
In the 1950s, it was rumored that Heisenberg had done it, and just
the details remained to be sketched in. But nothing ever emerged
from Heisenberg. So Wolfgang Pauli responded with the following:
“Below is the proof that I am as great an artist as Rembrandt; the
details remain to be sketched in.”
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The Weak Interaction: The Electroweak
Theory
Abdus Salam
(1926-1996)
Sheldon
Glashow
(1932- )
In the 1960s Sheldon
Glashow, Steven
Weinberg, and Abdus
Salam unified the electromagnetic and weak
interactions into what they
called the electroweak
theory, much as Maxwell
had unified electricity and
magnetism into the
electromagnetic theory a
hundred years earlier.
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Unification of the Strong and Electroweak
Interactions: The Standard Model
Over the latter half of the 20th century, numerous physicists
combined efforts to generate The Standard Model.
It is a widely accepted theory of elementary particle physics at
present.
It is a relatively simple, comprehensive theory that explains
hundreds of particles and complex interactions with six quarks,
six leptons, and three force-mediating particles.
It is a combination of the electroweak theory and quantum
chromodynamics (QCD), but does not include gravity.
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Quarks
Murray Gell-Mann (1929- )
In 1963 Murray Gell-Mann and,
independently, George Zweig proposed
that hadrons were formed from fractionally
charged particles called quarks. The quark
theory successfully described the
properties of the particles and reactions
and decay.
Three quarks were proposed, named the up (u), down (d), and
strange (s), with the charges +2e/3, −e/3, and −e/3, respectively. The
strange quark has the strangeness value of −1, whereas the other
two quarks have S = 0.
Quarks are believed to be essentially point-like, just like leptons.
With these three quarks, all the known hadrons (at the time) could be
specified by some combination of quarks and anti-quarks.
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Charm, Truth,
and Beauty
A fourth quark called the charmed quark (c) was proposed to explain
some additional discrepancies in the lifetimes of some of the
known particles.
A new quantum number called charm C was introduced so that the
new quark would have C = +1 while its anti-quark would have C =
−1 and particles without the charmed quark have C = 0.
Charm is similar to strangeness in that it is conserved in the strong
and electromagnetic interactions, but not in the weak interactions.
This behavior was sufficient to explain the particle lifetime
difficulties.
Two additional quarks, top and bottom (or truth and beauty), were also
required to construct some exotic particles (the Upsilon-meson).
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Quark Properties
The spin of all quarks (and anti-quarks) is 1/2.
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Quark
Description
of Particles
Baryons
normally consist
of three quarks
or anti-quarks.
-2/3e
2/3e
2/3e
-1/3e
-2/3e
2/3e
-1/3e
2/3e
-1/3e
-1/3e
2/3e
1/3e
A meson
consists of a
quark-anti-quark
pair, yielding the
required baryon
number of 0.
1/3e
-2/3e
2/3e
-1/3e
-2/3e
-1/3e
1/3e
2/3e
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Other Particles
What about the quark
composition of the Ω−, which
has a strangeness of S = −3?
Its quark composition is sss.
And its charge is 3(−e/3) = −e,
and its spin is due to three
quark spins aligned, 3(1/2) =
3/2. There is no other
possibility for a stable omega
(lifetime ~10−10 s) in
agreement with the table.
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Quantum Chromodynamics (QCD)
Because quarks have spin 1/2, they
are all fermions. According to the
Pauli exclusion principle, no two
fermions can exist in the same state.
Yet we have three identical strange
quarks in the Ω−!
This is not possible unless some other
quantum number distinguishes each
of these quarks in one particle.
A new quantum number called color
circumvents this problem and its
properties establish quantum
chromodynamics (QCD).
Discovery of the W-
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Color
There are three colors for quarks we call red (R), green (G), and blue
(B) with anti-quark color antired ( R ); antigreen (G ) and antiblue (B ).
(A “bar” above the symbol is usually used to describe the “anti-color”).
Color is the “charge” of the strong nuclear force, analogous to electric
charge for electromagnetism.
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Quarkanti-quark
creation
Physicists now believe
that free quarks cannot
be observed; they can
only exist within hadrons.
This is called
confinement.
This occurs because the
force between the
quarks increases rapidly
with distance, and the
energy supplied to
separate them creates
new quarks.
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The Families of
Matter
The three generations (or families)
of matter. Note that both quarks and
leptons exist in three distinct sets.
One of each charge type of quark
and lepton make up a generation.
All visible matter in the universe is
made from the first generation;
second- and third-generation
particles are unstable and decay
into first-generation particles.
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Three families: 1897-2000
Particle masses in MeV; 1 MeV  1.81027 gram