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Title: 14.1Early Discoveries


1
CHAPTER 14Elementary Particles
  • 14.1 Early Discoveries
  • 14.2 The Fundamental Interactions
  • 14.8 Accelerators
  • 14.3 Classification of Elementary Particles
  • 14.4 Conservation Laws and Symmetries
  • 14.5 Quarks
  • 14.6 The Families of Matter
  • 14.7 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, Id be a botanist. Enrico Fermi
2
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?

3
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)
4
But particle physics measurements were happening
Energetic particles collide with stationary
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.
5
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)
Electron positron
Vacuum
0
E
6
Anti-particles
  • Diracs 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
7
The Positron
Cosmic rays are highly energetic particles,
mostly protons, that cross interstellar space and
enter the Earths atmosphere, where their
interaction with particles creates cosmic
showers of many distinct particles.
  • Carl Anderson identified the positron in cosmic
    rays. It was easy it had positive charge and was
    light.

Andersons cloud chamber photo of positron track
Carl Anderson (1905-1991)
8
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 Treks dilithium crystals supposedly
contain anti-matter, which powers the Enterprise.
9
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 space-time diagram.
Electrons interact through mediation of a photon.
The axes are normally omitted.
10
Yukawas Meson
  • The Japanese physicist Hideki Yukawa had the idea
    of developing a quantum field theory that would
    describe the force between nucleonsanalogously
    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)
11
Yukawas Meson
  • Yukawas meson, called a pion (or pi-meson or
    p-meson), was identified in 1947 by C. F. Powell
    (19031969) and G. P. Occhialini (19071993)
  • 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
(Yukawas meson) between a neutron and a proton.
12
Other Mesons, Quarks, and Gluons
  • Yukawas pion is responsible for the nuclear
    force.
  • Later well 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) its massless
    and has spin 1, just like the photon.

Computed image of quarks and gluons in a nucleon
13
The Weak Interaction
  • 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.

Abdus Salam (1926-1996)
Sheldon Glashow (1932- )
14
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.

15
The Fundamental Interactions
One of the main goals of particle physics is to
unify these forces (to show that theyre all just
different aspects of the same force), just as
Maxwell did for the electric and magnetic forces
many years earlier.
16
The Fundamental Interactions
A finite range effectively confines the particle,
which, by the uncertainty principle, gives it a
minimal momentum and hence a minimum kinetic
energy and mass. Photons and gravitons are
massless. W and Z bosons are heavy.
17
14.8 Accelerators
Particle accelerators generate high enough
energies to create particles 1 GeV/c2 or greater.
18
Accelerators
  • There are three main types of accelerators used
    presently in particle physics experiments
    synchrotrons, linear accelerators, and colliders.

19
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.

20
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.

21
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.

22
Large Hadron Collider
Counter-propagating 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.
23
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 dont.

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.
24
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 hasnt
    been observed yet.
  • The search for the Higgs boson is of the highest
    priority in elementary particle physics.

25
Boson Properties
26
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.
27
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

28
Neutrinos
Picture of the sun, taken not with light, but
with neutrinos, made at the Japanese neutrino
observatory Super-Kamiokande.
  • Neutrinos have zero charge.
  • The electron neutrino occurs in the beta decay
    of the neutron.
  • Their masses are known to be very small. The
    precise mass of neutrinos may have a bearing on
    current cosmological theories of the universe
    because of the gravitational attraction of mass.
  • Like all other leptons, they have spin 1/2, and
    all three neutrinos have been identified
    experimentally.
  • Neutrinos are particularly difficult to detect
    because they have no charge and little mass, and
    they interact very weakly (they easily pass
    through the earth!).

29
Neutrino Oscillations
  • One of the most perplexing problems over the
    last three decades has been the solar neutrino
    problem the number of neutrinos reaching Earth
    from the sun is a factor of 2 to 3 too small if
    our understanding of the energy-producing
    (nuclear fusion) is correct.
  • Neutrinos come in three varieties or flavors
    electron, muon, and tau. The solution was found
    when researchers saw neutrinos generated in the
    Earths atmosphere (from cosmic rays) changing or
    oscillating into another flavor (the sun only
    emits electron neutrinos).
  • Also, this could only happen if neutrinos have
    mass.

30
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 quarksa
    quark and an anti-quark.) Theyre unstable and
    rare.
  • Baryons have masses at least as large as the
    proton and have half-integral 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 ßa??? (barys), meaning "heavy," because
    at the time of their naming it was believed that
    baryons were characterized by having greater mass
    than other particles. (Theyre made up of three
    quarks.) All baryons decay into protons.

31
The Hadrons
32
Particles and Lifetimes
  • The lifetimes of particles are also indications
    of their force interactions.
  • Particles that decay through the strong
    interaction are usually the shortest-lived,
    normally decaying in less than 10-20 s.
  • The decays caused by the electromagnetic
    interaction generally have lifetimes on the order
    of 10-16 s, and
  • The weak interaction decays are even slower,
    longer than 10-10 s.

33
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.

34
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.
35
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.

36
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 Lt for the tau and its neutrino
    similarly.
  • Thus three additional conservation laws.

37
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 ? and S 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.

38
Strangeness is strange
  • p0 can decay into two photons by the strong
    interaction, it is not possible for K0 to decay
    at all by the strong interaction. The K0 is the
    lightest S 0 particle, and there is no other
    strange particle to which it can decay. It can
    decay only by the weak interaction, which
    violates strangeness conservation.
  • Because the typical decay times of the weak
    interaction are on the order of 10-10 s, this
    explains the longer decay time for K0.
  • Only ?S 1 violations are allowed by the weak
    interaction.

39
Hypercharge
  • One more quantity, called hypercharge, has also
    become widely used as a quantum number.
  • The hypercharge quantum number Y is defined by Y
    S B.
  • Hypercharge, the sum of the strangeness and
    baryon quantum numbers, is conserved in strong
    interactions.
  • The hypercharge and strangeness conservation laws
    hold for the strong and electromagnetic
    interactions, but are violated for the weak
    interaction.

40
Symmetries
  • Symmetries lead directly to conservation laws.
  • Three symmetry operators called parity, charge
    conjugation, and time reversal are considered.

41
The Conservation of Parity P
  • The conservation of parity P describes the
    inversion symmetry of space,
  • Inversion, if valid, does not change the laws of
    physics.
  • The conservation of parity is valid for the
    strong and electromagnetic interactions, but not
    for the weak interaction (experimentally).

42
Charge conjugation C
  • Charge conjugation C reverses the sign of the
    particles charge and magnetic moment.
  • It has the effect of interchanging every particle
    with its antiparticle.
  • Charge conjugation is valid for the strong and
    electromagnetic interactions, but it is also not
    conserved in the weak interactions.
  • Even though both C and P are violated for the
    weak interaction it was believed that when both
    charge conjugation and parity operations are
    performed (called CP), conservation was still
    valid.

43
Time Reversal T
  • Here time t is replaced with t.
  • When all three operations are performed (CPT),
    where T is the time reversal symmetry,
    conservation holds.
  • We speak of the invariance of the symmetry
    operators, such as T, CP, and CPT.

44
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.
45
The Weak Interaction The Electroweak Theory
  • In the 1960s Sheldon Glashow, Steven Weinberg,
    and Abdus Salam unified the electro-magnetic 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.

Abdus Salam (1926-1996)
Sheldon Glashow (1932- )
46
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.

47
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.
48
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).

49
Quark Properties
The spin of all quarks (and anti-quarks) is 1/2.
50
Quark Description of Particles
  • Baryons normally consist of three quarks or
    anti-quarks.
  • A meson consists of a quark-anti-quark pair,
    yielding the required baryon number of 0.

51
Quark Description of Particles
  • A meson consists of a quark-anti-quark pair,
    which gives the required baryon number of 0.
    Baryons normally consist of three quarks.
  • The structure is quite simple. For example, a p -
    consists of ud, which gives a charge of (-2e/3)
    (-e/3) -e, and the two spins couple to give 0
    (-1/2 1/2 0).
  • A proton is uud, which gives a charge of (2e/3)
    (2e/3) (-e/3) e its baryon number is 1/3
    1/3 1/3 1 and two of the quarks spins
    couple to zero, leaving a spin 1/2 for the proton
    (1/2 1/2 - 1/2 1/2).

52
Other Particles
  • What about the quark composition of the O-, 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.

53
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 O-!
  • 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-
54
Color
  • There are three colors for quarks we call red
    (R), green (G), and blue (B) with anti-quark
    color antired ( ) antigreen ( ) and antiblue
    ( ). (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.

55
Color
Quantum electrodynamics and quantum
chromodynamics, are similar in structure color
is often called color charge and the force
between quarks is sometimes referred to as color
force. Earlier we saw that gluons are the
particles that hold the quarks together. Below is
a Feynman diagram of two quarks interacting. A
red quark comes in from the left and interacts
with a blue quark coming in from the right. They
exchange a gluon, changing the blue quark into a
red one and the red quark into a blue one.
56
Color
A color and its anti-color cancel out. We call
this colorless (or white). All hadrons are
colorless. In the figure, the gluon itself must
have the color in order for the diagram
to work. Quarks change color when they emit or
absorb a gluon, and quarks of the same color
repel, whereas quarks of different color attract.
57
Color
To finish the story we should mention that the
six different kinds of quarks are referred to as
flavors. There are six flavors of quarks (u, d,
s, c, b, t). Each flavor has three colors.
Finally, how many different gluons are possible?
Using the three colors red, blue, and green,
there are nine possible combinations for a gluon.
They are Note in the diagram that the gluon is
and not . The combination does not
have any net color change and cannot be
independent. Therefore, there are only eight
independent gluons, and that is what quantum
chromodynamics predicts. Gluons can interact
with each other because each gluon carries a
color charge. Note that in this case gluons, as
the mediator of the strong force, are much
different from photons, the mediator of the
electromagnetic force.
58
Quark-anti-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.

59
Confinement
60
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.
61
The Families of Matter
  • Most of the mass in the universe is made from the
    components in the first generation (electrons and
    u and d quarks).
  • The second generation consists of the muon, its
    neutrino, and the charmed and strange quarks. The
    members of this generation are found in certain
    astrophysical objects of high energy and in
    cosmic rays, and are produced in high-energy
    accelerators.
  • The third generation consists of the tau and its
    neutrino and two more quarks, the bottom (or
    beauty) and top (or truth). The members of this
    third generation existed in the early moments of
    the creation of the universe and can be created
    with very high energy accelerators.

62
Grand Unifying Theories (GUTs)
  • There have been several attempts toward a grand
    unified theory (GUT) to combine the weak,
    electromagnetic, and strong interactions and
    explain why
  • Current experimental measurements have shown the
    proton lifetime to be greater than 1032 years.
    Current theory has it at 10-29 to 10-31 years.
  • Neutrinos may have a small, but finite, mass.
    This has been confirmed.
  • Massive magnetic monopoles may exist. If one
    exists anywhere in the universe, it explains why
    charge is quantized. There is presently no
    confirmed experimental evidence for magnetic
    monopoles.
  • The proton and electron electric charges should
    have the same magnitude.

63
Another challengeMatter-Antimatter
Could this galaxy be made entirely of anti-matter?
  • According to the Big Bang theory, matter and
    antimatter should have been created in exactly
    equal quantities. But it appears that matter
    dominates over antimatter now in our universe,
    and the reason for this has puzzled physicists
    and cosmologists for years.
  • Events in the early universe may be responsible
    for this asymmetry. But explanations go far
    beyond the standard model.

64
Including Gravity String Theory
  • For the last two decades there has been a
    tremendous amount of effort by theorists in
    string theory, which has had several variations.
    The addition of super-symmetry resulted in the
    name theory of super-strings.
  • In super-string theory, elementary particles do
    not exist as points, but rather as tiny, wiggling
    loops that are only 10-35 m in length.
  • Further work has revealed that they describe not
    just strings, but other objects including
    membranes and higher-dimensional objects. The
    addition of membranes has resulted in brane
    theories.
  • Presently super-string theory is a promising
    approach to unify the four fundamental forces,
    including gravity.

65
Super-symmetry
  • Super-symmetry is a necessary ingredient in
    many of the theories trying to unify all four
    forces of nature.
  • The symmetry relates fermions and bosons. All
    fermions will have a super-partner that is a
    boson of equal mass, and vice versa.
  • The super-partner spins differ by h / 2.
  • Presently, none of the known leptons, quarks, or
    gauge bosons can be identified with a
    super-partner of any other particle type.

66
M-theory
  • Recently theorists have proposed a successor to
    super-string theory called M-theory.
  • M-theory has 11 dimensions (ten spatial and one
    for time) and predicts that strings coexist with
    membranes, called branes for short.
  • The number of particles that have been predicted
    from a variety of different theories include the
    fancifully named sleptons, squarks, axions,
    winos, photinos, zinos, gluinos, and preons.
  • Only through experiments (which no one currently
    knows how to do) will scientists be able to wade
    through the vast number of unifying theories.
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