Elementary Particles
| I | INTRODUCTION |
Elementary
Particles, in physics, particles that cannot be broken down into any
other particles. The term elementary particles also is used more loosely to
include some subatomic particles that are composed of other particles. Particles
that cannot be broken further are sometimes called fundamental particles to
avoid confusion. These fundamental particles provide the basic units that make
up all matter and energy in the universe.
Scientists and philosophers have sought to
identify and study elementary particles since ancient times. Aristotle and other
ancient Greek philosophers believed that all things were composed of four
elementary materials: fire, water, air, and earth. People in other ancient
cultures developed similar notions of basic substances. As early scientists
began collecting and analyzing information about the world, they showed that
these materials were not fundamental but were made of other substances.
In the 1800s British physicist John Dalton
was so sure he had identified the most basic objects that he called them
atoms (from the Greek word for “indivisible”). By the early 1900s
scientists were able to break apart these atoms into particles that they called
the electron and the nucleus. Electrons surround the dense nucleus of an atom.
In the 1930s, researchers showed that the nucleus consists of smaller particles,
called the proton and the neutron. Today, scientists have evidence that the
proton and neutron are themselves made up of even smaller particles, called
quarks.
Scientists now believe that quarks and three
other types of particles—leptons, force-carrying bosons, and the Higgs boson—are
truly fundamental and cannot be split into anything smaller. In the 1960s
American physicists Steven Weinberg and Sheldon Glashow and Pakistani physicist
Abdus Salam developed a mathematical description of the nature and behavior of
elementary particles. Their theory, known as the standard model of particle
physics, has greatly advanced understanding of the fundamental particles and
forces in the universe. Yet some questions about particles remain unanswered by
the standard model, and physicists continue to work toward a theory that would
explain even more about particles.
| II | WHAT MAKES UP THE UNIVERSE? |
Everything in the universe, from elementary
particles and atoms to people, houses, and planets, can be classified into one
of two categories: fermions (pronounced FUR-me-onz) or bosons (pronounced
BO-zonz). The behavior of a particle or group of particles, such as an atom or a
house, determines whether it is a fermion or boson. The distinction between
these two categories is not noticeable on the large scale of people or houses,
but it has profound implications in the world of atoms and elementary particles.
Fundamental particles are classified according to whether they are fermions or
bosons. Fundamental fermions combine to form atoms and other more unusual
particles, while fundamental bosons carry forces between particles and give
particles mass.
In 1925 Austrian-born American physicist
Wolfgang Pauli formulated a rule of physics that helped define fermions. He
suggested that no two electrons can have the same properties and locations. He
proposed this exclusion principle to explain why all of the electrons in atoms
have slightly different amounts of energy. In 1926 Italian-born American
physicist Enrico Fermi and British physicist Paul Dirac developed equations that
describe electron behavior, providing mathematical proof of the exclusion
principle. Physicists call particles that obey the exclusion principle fermions
in honor of Fermi. Protons, neutrons, and the quarks that comprise them are all
examples of fermions.
Some particles, such as particles of light
called photons, do not obey the exclusion principle. Two or more photons can
have the exact same characteristics. In 1925 German-born American physicist
Albert Einstein and Indian mathematician Satyendra Bose developed a set of
equations describing the behavior of particles that do not obey the exclusion
principle. Particles that obey the equations of Bose and Einstein are called
bosons, in honor of Bose.
Classifying particles as either fermions or
bosons is similar to classifying whole numbers as either odd or even. No number
is both odd and even, yet every whole number is either odd or even. Similarly,
particles are either fermions or bosons. Sums of odd and even numbers are either
odd or even, depending on how many odd numbers were added. Adding two odd
numbers together yields an even number, but adding a third odd number makes the
sum odd again. Adding any number of even numbers yields an even sum. In a
similar manner, adding an even number of fermions yields a boson, while adding
an odd number of fermions results in a fermion. Adding any number of bosons
yields a boson.
For example, a hydrogen atom contains two
fermions: an electron and a proton. But the atom itself is a boson because it
contains an even number of fermions. According to the exclusion principle, the
electron inside the hydrogen atom cannot have the same properties as another
electron nearby. However, the hydrogen atom itself, as a boson, does not follow
the exclusion principle. Thus, one hydrogen atom can be identical to another
hydrogen atom.
A particle composed of three fermions, on the
other hand, is a fermion. An atom of heavy hydrogen, also called a deuteron, is
a hydrogen atom with a neutron added to the nucleus. A deuteron contains three
fermions: one proton, one electron, and one neutron. Since the deuteron contains
an odd number of fermions, it too is a fermion. Just like its constituent
particles, the deuteron must obey the exclusion principle. It cannot have the
same properties as another deuteron atom.
The differences between fermions and bosons
have important implications. If electrons did not obey the exclusion principle,
all electrons in an atom could have the same energy and be identical. If all of
the electrons in an atom were identical, different elements would not have such
different properties. For example, metals conduct electricity better than
plastics do because the arrangement of the electrons in their atoms and
molecules differs. If electrons were bosons, their arrangements could be
identical in these atoms, and devices that rely on the conduction of
electricity, such as televisions and computers, would not work. Photons, on the
other hand, are bosons, so a group of photons can all have identical properties.
This characteristic allows the photons to form a coherent beam of identical
particles called a laser.
The most fundamental particles that make up
matter fall into the fermion category. These fermions cannot be split into
anything smaller. The particles that carry the forces acting on matter and
antimatter are bosons called force carriers. Force carriers are also fundamental
particles, so they cannot be split into anything smaller. These bosons carry the
four basic forces in the universe: the electromagnetic, the gravitational, the
strong (force that holds the nuclei of atoms together), and the weak (force that
causes atoms to radioactively decay). Scientists believe another type of
fundamental boson, called the Higgs boson, gives matter and antimatter mass.
Scientists have yet to discover definitive proof of the existence of the Higgs
boson.
| III | PARTICLES OF MATTER |
Ordinary matter makes up all the objects
and materials familiar to life on Earth, including people, cars, buildings,
mountains, air, and clouds. Stars, planets, and other celestial bodies also
contain ordinary matter. The fundamental fermions that make up matter fall into
two categories: leptons and quarks. Each lepton and quark has an antiparticle
partner, with the same mass but opposite charge. Leptons and quarks differ from
each other in two main ways: (1) the electric charge they carry and (2) the way
they interact with each other and with other particles. Scientists usually state
the electric charge of a particle as a multiple of the electric charge of a
proton, which is 1.602 × 10-19 coulombs (C). Leptons have electric
charges of either -1 or 0 (neutral), with their antiparticles having charges of
+1 or 0. Quarks have electric charges of either +’ or -€. Antiquarks have electric
charges of either -’ or
+€. Leptons interact
rather weakly with one another and with other particles, while quarks interact
strongly with one another.
Leptons and quarks each come in 6
varieties. Scientists divide these 12 basic types into 3 groups, called
generations. Each generation consists of 2 leptons and 2 quarks. All ordinary
matter consists of just the first generation of particles. The particles in the
second and third generation tend to be heavier than their counterparts in the
first generation. These heavier, higher-generation particles decay, or
spontaneously change, into their first generation counterparts. Most of these
decays occur very quickly, and the particles in the higher generations exist for
an extremely short time (a millionth of a second or less). Particle physicists
are still trying to understand the role of the second and third generations in
nature.
| A | Leptons |
Scientists divide leptons into two
groups: particles that have electric charges and particles, called neutrinos,
that are electrically neutral. Each of the three generations contains a charged
lepton and a neutrino. The first generation of leptons consists of the electron
(e-) and the electron neutrino (νe); the second generation, the muon
(µ) and the muon neutrino (νµ); and the third generation, the tau
(t) and the tau neutrino (νt;).
 |
The electron is probably the most
familiar elementary particle. Electrons are about 2,000 times lighter than
protons and have an electric charge of –1. They are stable, so they can exist
independently (outside an atom) for an infinitely long time. All atoms contain
electrons, and the behavior of electrons in atoms distinguishes one type of atom
from another. When atoms radioactively decay, they sometimes emit an electron in
a process called beta decay.
Studies of beta decay led to the
discovery of the electron neutrino, the first generation lepton with no electric
charge. Atoms release neutrinos, along with electrons, when they undergo beta
decay. Electron neutrinos might have a tiny mass, but their mass is so small
that scientists have not been able to measure it or conclusively confirm that
the particles have any mass at all.
Physicists discovered a particle heavier
than the electron but lighter than a proton in studies of high-energy particles
created in Earth’s atmosphere. This particle, called the muon (pronounced
MYOO-on), is the second generation charged lepton. Muons have an electric charge
of -1 and an average lifetime of 1.52 microseconds (a microsecond is
one-millionth of a second). Unlike electrons, they do not make up everyday
matter. Muons live their brief lives in the atmosphere, where heavier particles
called pions decay into muons and other particles. The electrically neutral
partner of the muon is the muon neutrino. Muon neutrinos, like electron
neutrinos, have either a tiny mass too small to measure or no mass at all. They
are released when a muon decays.
The third generation charged lepton is
the tau. The tau has an electric charge of -1 and almost twice the mass of a
proton. Scientists have detected taus only in laboratory experiments. The
average lifetime of taus is extremely short—only 0.3 picoseconds (a picosecond
is one-trillionth of a second). The tau has an electrically neutral partner
called the tau neutrino. Scientists have detected tau neutrinos directly during
experiments. Like the other neutrinos, the tau neutrino has an extremely small
mass.
| B | Quarks |
The fundamental particles that make up
protons and neutrons are called quarks. Like leptons, quarks come in six
varieties, or “flavors,” divided into three generations. Unlike leptons,
however, quarks never exist alone—they are always combined with other quarks. In
fact, quarks cannot be isolated even with the most advanced laboratory equipment
and processes. Scientists have had to determine the charges and approximate
masses of quarks mathematically by studying particles that contain quarks.
Quarks are unique among all elementary
particles in that they have fractional electric charges—either +’ or -€. In an observable particle,
the fractional charges of quarks in the particle add up to an integer charge for
the combination.
The first generation quarks are
designated up (u) and down (d); the second generation, charm (c) and strange
(s); and the third generation, top (t) and bottom (b). The odd names for quarks
do not describe any aspect of the particles; they merely give scientists a way
to refer to a particular type of quark.
 |
The up quark and the down quark make up
protons and neutrons in atoms, as described below. The up quark has an electric
charge of +’, and the
down quark has a charge of -€. The second generation
quarks have greater mass than those in the first generation. The charm quark has
an electric charge of +’, and the strange quark has
a charge of -€. The
heaviest quarks are the third generation top and bottom quarks. Some scientists
originally called the top and bottom quarks truth and beauty, but those names
have dropped out of use. The top quark has an electric charge of +’, and the bottom quark has a
charge of -€. The up
quark, the charm quark, and the top quark behave similarly and are called
up-type quarks. The down quark, the strange quark, and the bottom quark are
called down-type quarks because they share the same electric charge.
Particles made of quarks are called
hadrons (pronounced HA-dronz). Hadrons are not fundamental, since they consist
of quarks, but they are commonly included in discussions of elementary
particles. Two classes of hadrons can be found in nature: mesons (pronounced
ME-zonz) and baryons (pronounced BARE-ee-onz).
Mesons contain a quark and an antiquark
(the antiparticle partner of the quark). Since they contain two fermions, mesons
are bosons. The first meson that scientists detected was the pion. Pions exist
as intermediary particles in the nuclei of atoms, forming from and being
absorbed by protons and neutrons. The pion comes in three varieties: a positive
pion (p+), a negative pion
(p-), and an electrically
neutral pion (p0). The
positive pion consists of an up quark and a down antiquark. The up quark has
charge +’ and the down
antiquark has charge +€, so the charge on the
positive pion is +1. Positive pions have an average lifetime of 26 nanoseconds
(a nanosecond is one-billionth of a second). The negative pion contains an up
antiquark and a down quark, so the charge on the negative pion is -’ plus –€, or -1. It has the same
mass and average lifetime as the positive pion. The neutral pion contains an up
quark and an up antiquark, so the electric charges cancel each other. It has an
average lifetime of 9 femtoseconds (a femtosecond is one-quadrillionth of a
second).
Many other mesons exist. All six quarks
play a part in the formation of mesons, although mesons containing heavier
quarks like the top quark have very short lifetimes. Other mesons include the
kaons (pronounced KAY-ons) and the D particles. Kaons (Κ) and Ds come in several
different varieties, just as pions do. All varieties of kaons and some varieties
of Ds contain either a strange quark or a strange antiquark. All Ds contain
either a charm quark or a charm antiquark.
Three quarks together form a baryon. A
baryon contains an odd number of fermions, so it is a fermion itself. Protons,
the positively charged particles in all atomic nuclei, are baryons that consist
of two up quarks and a down quark. Adding the charges of two up quarks and a
down quark, +’ plus
+’ plus -€, produces a net charge of
+1, the charge of the proton. Protons have never been observed to decay.
The neutrons found inside atoms are
baryons as well. A neutron consists of one up quark and two down quarks. Adding
these charges gives +’
plus -€ plus -€ for a net charge of 0,
making the neutron electrically neutral. Neutrons have a slightly greater mass
than protons and an average lifetime of 930 seconds.
Many other baryons exist, and many
contain quarks other than the up and down flavors. For example, lambda and sigma
(S) particles contain strange, charm,
or bottom quarks. For lambda particles, the average lifespan ranges from 200
femtoseconds to 1.2 picoseconds. The average lifetime of sigma particles ranges
from 0.0007 femtoseconds to 150 picoseconds.
| IV | PARTICLES OF ANTIMATTER |
British physicist Paul Dirac proposed an
early theory of particle interactions in 1928. His theory predicted the
existence of antiparticles, which combine to form antimatter. Antiparticles have
the same mass as their normal particle counterparts, but they have several
opposite quantities, such as electric charge and color charge. Color charge
determines how particles react with one another under the strong force (the
force that holds the nuclei of atoms together, just as electric charge
determines how particles react to one another under the electromagnetic force).
The antiparticles of fermions are also fermions, and the antiparticles of bosons
are bosons.
All fermions have antiparticles. The
antiparticle of an electron is called the positron (pronounced POZ-i-tron). The
antiparticle of the proton is the antiproton. The antiproton consists of
antiquarks—two up antiquarks and one down antiquark. Antiquarks have the
opposite electric and color charges of their counterparts. The antiparticles of
neutrinos are called antineutrinos. Both neutrinos and antineutrinos have no
electric charge or color charge, but physicists still consider them distinct
from one another. Neutrinos and antineutrinos behave differently when they
collide with other particles and in radioactive decay. When a particle decays,
for example, an antineutrino accompanies the production of a charged lepton, and
a neutrino accompanies the production of a charged antilepton. In addition,
reactions that absorb neutrinos do not absorb antineutrinos, giving further
evidence of the distinction between neutrinos and antineutrinos.
When a particle and its associated
antiparticle collide, they annihilate, or destroy, each other, creating a tiny
burst of energy. Particle-antiparticle collisions would provide a very efficient
source of energy if large numbers of antiparticles could be harnessed cheaply.
Physicists already make use of this energy in machines called particle
accelerators. Particle accelerators increase the speed (and therefore energy) of
elementary particles and make the particles collide with one another. When
particles and antiparticles (such as protons and antiprotons) collide, their
kinetic energy and the energy released when they annihilate each other converts
to matter, creating new and unusual particles for physicists to study.
Particle-antiparticle collisions could
someday fuel spacecraft, which need only a slight push to change their speed or
direction in the vacuum of space. The antiparticles and particles would have to
be kept away from each other until the spacecraft needed the energy of their
collisions. Finely tuned magnetic fields could be used to trap the particles and
keep them separate, but these magnetic fields are difficult to set up and
maintain. At the end of the 20th century, technology was not advanced enough to
allow spacecraft to carry the equipment and particles necessary for using
particle-antiparticle collisions as fuel.
| V | FORCE CARRIERS |
All of the known forces in our universe can
be classified as one of four types: electromagnetic, strong, weak, or
gravitational. These forces affect everything in the universe. The
electromagnetic force binds electrons to the atoms that compose our bodies, the
objects around us, the Earth, the planets, and the Moon. The strong nuclear
force holds together the nuclei inside the atoms that compose matter. Reactions
due to the weak nuclear force fuel the Sun, providing light and heat. Gravity
holds people and objects to the ground.
Each force has a particular property
associated with it, such as electric charge for the electromagnetic force.
Elementary particles that do not have electric charge, such as neutrinos, are
electrically neutral and are not affected by the electromagnetic force.
Mechanical forces, such as the force used to
push a child on a swing, result from the electrical repulsion between electrons
and are thus electromagnetic. Even though a parent pushing a child on a swing
feels his or her hands touching the child, the atoms in the parent’s hands never
come into contact with the atoms of the child. The electrons in the parent’s
atoms repel those in the child while remaining a slight distance away from them.
In a similar manner, the Sun attracts Earth through gravity, without Earth ever
contacting the Sun. Physicists call these forces nonlocal, because the
forces appear to affect objects that are not in the same location, but at a
distance from one another.
Theories about elementary particles, however,
require forces to be local—that is, the objects affecting each other must come
into contact. Scientists achieved this locality by introducing the idea of
elementary particles that carry the force from one object to another.
Experiments have confirmed the existence of many of these particles. In the case
of electromagnetism, a particle called a photon travels between the two
repelling electrons. One electron releases the photon and recoils, while the
other electron absorbs it and is pushed away.
Each of the four forces has one or more
unique force carriers, such as the photon, associated with it. These force
carrier particles are bosons, since they do not obey the exclusion principle—any
number of force carriers can have the exact same characteristics. They are also
believed to be fundamental, so they cannot be split into smaller particles.
Other than the fact that they are all fundamental bosons, the force carriers
have very few common features. They are as unique as the forces they carry.
| A | The Electromagnetic Force and Photons |
For centuries, electricity and magnetism
seemed distinct forces. In the 1800s, however, experiments showed many
connections between these two forces. In 1864 British physicist James Clerk
Maxwell drew together the work of many physicists to show that electricity and
magnetism are actually different aspects of the same electromagnetic force. This
force causes particles with similar electric charges to repel one another and
particles with opposite charges to attract one another. Maxwell also showed that
light is a traveling form of electromagnetic energy. The founders of quantum
mechanics took Maxwell’s work one step further. In 1925 German-British physicist
Max Born, and German physicists Ernst Pascual Jordan and Werner Heisenberg
showed mathematically that packets of light energy, later called photons, are
emitted and absorbed when charged particles attract or repel each other through
the electromagnetic force.
Any particle with electric charge, such as
a quark or an electron, is subject to, or “feels,” the electromagnetic force.
Electrically neutral particles, such as neutrinos, do not feel it. The electric
charge of a hadron is the sum of the charges on the quarks in the hadron. If the
sum is zero, the electromagnetic force does not affect the hadron, although it
does affect the quarks inside the hadron. Photons carry the electromagnetic
force between particles but have no mass or electric charge themselves. Since
photons have no electric charge, they are not affected by the force they
carry.
Unlike neutrinos and some other
electrically neutral particles, the photon does not have a distinct
antiparticle. Particles that have antiparticles are like positive and negative
numbers—they are each the other’s additive inverse. Photons are like the number
zero, which is its own additive inverse. In effect, a photon is its own
antiparticle.
In one example of the electromagnetic
force, two electrons repel each other because they both have negative electric
charges. One electron releases a photon, and the other electron absorbs it. Even
though photons have no mass, their energy gives them momentum, a property that
enables them to affect other particles. The momentum of the photon pushes the
two electrons apart, just as the momentum of a basketball tossed between two ice
skaters will push the skaters apart. For more information about electromagnetic
radiation and particle physics, see Quantum Electrodynamics.
| B | The Strong Force and Gluons |
Quarks and particles made of quarks attract
each other through the strong force. The strong force holds the quarks in
protons and neutrons together, and it holds protons and neutrons together in the
nuclei of atoms. If electromagnetism were the only force between quarks, the two
up quarks in a proton would repel each other because they are both positively
charged. (The up quarks are also attracted to the negatively charged down quark
in the proton, but this attraction is not as great as the repulsion between the
up quarks.) However, the strong force is stronger than the electromagnetic
force, so it glues the quarks inside the proton together.
A property of particles called color charge
determines how the strong force affects them. The term color charge has
nothing to do with color in the usual sense; it is just a convenient way for
scientists to describe this property of particles. Color charge is similar to
electric charge, which determines a particle’s electromagnetic interactions.
Quarks can have a color charge of red, blue, or green. Antiquarks can have a
color charge of antired (also called cyan), antiblue (also called yellow), or
antigreen (also called magenta). Quark types and colors are not linked—up
quarks, for example, may be red, green, or blue.
All observed objects carry a color charge
of zero, so quarks (which compose matter) must combine to form hadrons that are
colorless, or color neutral. The color charges of the quarks in hadrons
therefore cancel one another. Mesons contain a quark of one color and an
antiquark of the quark’s anticolor. The color charges cancel each other out and
make the meson white, or colorless. Baryons contain three quarks, each with a
different color. As with light, the colors red, blue, and green combine to
produce white, so the baryon is white, or colorless.
The bosons that carry the strong force
between particles are called gluons. Gluons have no mass or electric charge and,
like photons, they are their own antiparticle. Unlike photons, however, gluons
do have color charge. They carry a color and an anticolor. Possible gluon color
combinations include red-antiblue, green-antired, and blue-antigreen. Because
gluons carry color charge, they can attract each other, while the colorless,
electrically neutral photons cannot. Colors and anticolors attract each other,
so gluons that carry one color will attract gluons that carry the associated
anticolor.
Gluons carry the strong force by moving
between quarks and antiquarks and changing the colors of these particles. Quarks
and antiquarks in hadrons constantly exchange gluons, changing colors as they
emit and absorb gluons. Baryons and mesons are all colorless, so each time a
quark or antiquark changes color, other quarks or antiquarks in the particle
must change color as well to preserve the balance. The constant exchange of
gluons and color charge inside mesons and baryons creates a color force field
that holds the particles together.
The strong force is the strongest of the
four forces in atoms. Quarks are bound so tightly to each other that they cannot
be isolated. Separating a quark from an antiquark requires more energy than
creating a quark and antiquark does. Attempting to pull apart a meson, then,
just creates another meson: The quark in the original meson combines with a
newly created antiquark, and the antiquark in the original meson combines with a
newly created quark.
In addition to holding quarks together in
mesons and baryons, gluons and the strong force also attract mesons and baryons
to one another. The nuclei of atoms contain two kinds of baryons: protons and
neutrons. Protons and neutrons are colorless, so the strong force does not
attract them to each other directly. Instead, the individual quarks in one
neutron or proton attract the quarks of its neighbors. The pull of quarks toward
each other, even though they occur in separate baryons, provides enough energy
to create a quark-antiquark pair. This pair of particles forms a type of meson
called a pion. The exchange of pions between neutrons and protons holds the
baryons in the nucleus together. The strong force between baryons in the nucleus
is called the residual strong force.
| C | The Weak Force and Vector Bosons |
While the strong force holds the nucleus of
an atom together, the weak force can make the nucleus decay, changing some of
its particles into other particles. The weak force is so named because it is far
weaker than the electromagnetic or strong forces. For example, an interaction
involving the weak force is 10 quintillion (10 billion billion) times less
likely to occur than an interaction involving the electromagnetic force. Three
particles, called vector bosons, carry the weak force. The weak force equivalent
to electric charge and color charge is a property called weak hypercharge. Weak
hypercharge determines whether the weak force will affect a particle. All
fermions possess weak hypercharge, as do the vector bosons that carry the weak
force.
All elementary particles, except the force
carriers of the other forces and the Higgs boson, interact by means of the weak
force. But the effects of the weak force are usually masked by the other,
stronger forces. The weak force is not very significant when considering most of
the interactions between two quarks. For example, the strong force completely
overwhelms the weak force when a quark bounces off another quark. Nor does the
weak force significantly affect interactions between two charged particles, such
as the interaction between an electron and a proton. The electromagnetic force
dominates those interactions.
The weak force becomes significant when an
interaction does not involve the strong force or the electromagnetic force. For
example, neutrinos have neither electric charge nor color charge, so any
interaction involving a neutrino must be due to either the weak force or the
gravitational force. The gravitational force is even weaker than the weak force
on the scale of elementary particles, so the weak force dominates in neutrino
interactions.
One example of a weak interaction is beta
decay involving the decay of a neutron. When a neutron decays, it turns into a
proton and emits an electron and an electron antineutrino. The neutron and
antineutrino are electrically neutral, ruling out the electromagnetic force as a
cause. The antineutrino and electron are colorless, so the strong force is not
at work. Beta decay is due solely to the weak force.
The weak force is carried by three vector
bosons. These bosons are designated the W+, the W-, and
the Z0. The W bosons are electrically charged (+1 and –1), so they
can feel the electromagnetic force. These two bosons are each other’s
antiparticle counterparts, while the Z0 is its own antiparticle. All
three vector bosons are colorless. A distinctive feature of the vector bosons is
their mass. The weak force is the only force carried by particles that have
mass. These massive force carriers cannot travel as far as the massless force
carriers of the three long-range forces, so the weak force acts over shorter
distances than the other three forces.
When the weak force affects a particle,
the particle emits one of the three weak vector bosons—W+,
W-, or Z0—and changes into a different particle. The weak
vector boson then decays to produce other particles. In interactions that
involve the W+ and W-, a particle changes into a particle
with a different electric charge. For example, in beta decay, one of the down
quarks in a neutron changes into an up quark and the neutron releases a W boson.
This change in quark type converts the neutron (two down quarks and an up quark)
to a proton (one down quark and two up quarks). The W boson released by the
neutron could then decay into an electron and an electron antineutrino. In
Z0 interactions, a particle changes into a particle with the same
electric charge.
A quark or lepton can change into a
different quark or lepton from another generation only by the weak interaction.
Thus the weak force is the reason that all stable matter contains only first
generation leptons and quarks. The second and third generation leptons and
quarks are heavier than their first generation counterparts, so they quickly
decay into the lighter first generation leptons and quarks by exchanging W and Z
bosons. The first generation particles have no lighter counterparts into which
they can decay, so they are stable.
| D | The Gravitational Force and Gravitons |
The gravitational force is probably the
most familiar force, yet it is the only force not described by the standard
model of particle physics. In 1915 German-born American physicist Albert
Einstein developed a significant new approach to the concept of gravity: the
general theory of relativity. While general relativity successfully described
many phenomena, the theory was framed differently than were theories of particle
physics, making relativity difficult to reconcile with particle physics. Through
the end of the 20th century, all efforts to develop a theory of gravitation
entirely consistent with particle physics failed.
Physicists call their goal of an overall
theory a “theory of everything,” because it would explain all four known forces
in the universe and how these forces affect particles. In such a theory, the
particles that carry the gravitational force would be called gravitons.
Gravitons should share many characteristics with photons because, like
electromagnetism, gravitation is a long-range force that gets weaker with
distance. Gravitons should be massless and have no electric charge or color
charge. The graviton is the only force carrier not yet observed in an
experiment.
Gravitation is the weakest of the four
forces on the atomic scale, but it can become extremely powerful on a cosmic
scale. For instance, the gravitational force between Earth and the Sun holds
Earth in orbit. Gravity can have large effects, because, unlike the
electromagnetic force, it is always attractive. Every particle in your body has
some tiny gravitational attraction to the ground. The innumerable tiny
attractions add up, which is why you do not float off into space. The negative
charge on electrons, however, cancels out the positive charge on the protons in
your body, leaving you electrically neutral.
Another unique feature of gravitation is
its universality—every object is gravitationally attracted to every other
object, even objects without mass. For example, the theory of relativity
predicted that light should feel the gravitational force. Before Einstein,
scientists thought that gravitational attraction depended only on mass. They
thought that light, being massless, would not be attracted by gravitation.
Relativity, however, holds that gravitational attraction depends on the energy
of an object and that mass is just one possible form of energy. Einstein was
proven correct in 1919, when astronomers observed that the gravitational
attraction between light from distant stars and the Sun bends the path of the
light around the Sun (Gravitational Lens).
| VI | THE HIGGS BOSON |
The standard model of particle physics
includes an elementary boson that is not a force carrier: the Higgs boson.
Scientists have not yet detected the Higgs boson in an experiment, but they
believe it gives elementary particles their mass. Composite particles receive
their mass from their constituent particles, and in some cases, the energy
involved in holding these particles together. For example, the mass of a neutron
comes from the mass of its quarks and the energy of the strong force holding the
quarks together. The quarks themselves, however, have no such source of mass,
which is why physicists introduced the idea of the Higgs boson. Elementary
particles should obtain their mass by interacting with the Higgs boson.
Scientists expect the mass of the Higgs
boson to be large compared to that of most other fundamental particles.
Physicists can create more massive particles by forcing smaller particles to
collide at high speeds. The energy released in the collisions converts to
matter. Producing the Higgs boson, with its relatively large mass, will require
a tremendous amount of energy. Many scientists are searching for the Higgs boson
using machines called particle colliders. Particle colliders shoot a beam of
particles at a target or another beam of particles to produce new, more massive
particles.
| VII | UNIFICATION THEORIES |
Scientific progress often occurs when
people find connections between apparently unconnected phenomena. For example,
19th-century British physicist James Clerk Maxwell made a connection between
electric forces on charged objects and the force on a moving charge due to a
magnet. He deduced that the electric force and the magnetic force were just
different aspects of the same force. His discovery led to a deeper understanding
of electromagnetism.
The unification of electricity and
magnetism and the discovery of the strong and weak nuclear forces in the
mid-20th century left physicists with four apparently independent forces:
electromagnetism, the strong force, the weak force, and gravitation. Physicists
believe they should be able to connect these forces with one unified theory,
called a theory of everything (TOE). A TOE should explain all particles and
particle interactions by demonstrating that these four forces are different
aspects of one universal force. The theory should also explain why fermions come
in three generations when all stable matter contains fermions from just the
first generation.
Scientists also hope that in explaining the
extra generations, a TOE will explain why particles have the masses they do.
They would like an explanation of why the top quark is so much heavier than the
other quarks and why neutrinos are so much lighter than the other fermions. The
standard model does not address these questions, and scientists have had to
determine the masses of particles by experiment rather than by theoretical
calculations.
Unification of all of the forces, however,
is not an easy task. Each force appears to have distinctive properties and
unique force carriers. In addition, physicists have yet to describe successfully
the gravitational force in terms of particles, as they have for the other three
forces. Despite these daunting obstacles, particle physicists continue to seek a
unified theory and have made some progress. Starting points for unification
include the electroweak theory and grand unification theories.
| A | Electroweak Unification |
American physicists Sheldon Glashow and
Steven Weinberg and Pakistani physicist Abdus Salam completed the first step
toward finding a universal force in the 1960s with their standard model theory
of particle physics. Using a branch of mathematics called group theory, they
showed how the weak force and the electromagnetic force could be combined
mathematically into a single electroweak force. The electromagnetic force seems
much stronger than the weak force at low energies, but that disparity is due to
the differences between the force carriers. At higher energies, the difference
between the W and Z bosons of the weak force, which have mass, and the massless
photons of the electromagnetic force becomes less significant, and the two
forces become indistinguishable.
| B | Grand Unified Theories and Beyond |
The standard model also uses group theory
to describe the strong force, but scientists have not yet been able to unify the
strong force with the electroweak force. The next step toward finding a TOE
would be a grand unified theory (GUT), a theory that would unify the strong,
electromagnetic, and weak forces (the forces currently described by the standard
model). A GUT should describe all three forces as different aspects of one
force. At high energies, the distinctions between the three aspects should
disappear. The only force remaining would then be the gravitational force, which
scientists have not been able to describe with particle theory. See also
Unified Field Theory.
One type of GUT contains a theory called
supersymmetry (SUSY), first suggested in 1971. Supersymmetric theories set rules
for new symmetries, or pairings, between particles and interactions. The
standard model, for example, requires that every particle have an associated
antiparticle. In a similar manner, SUSY requires that every particle have an
associated supersymmetric partner. While particles and their associated
antiparticles are either both fermions or bosons, the supersymmetric partner of
a fermion should be a boson, and the supersymmetric partner of a boson should be
a fermion. For example, the fermion electron should be paired with a boson
called a selecton, and the fermion quarks with bosons called squarks. The
force-carrying bosons, such as photons and gluons, should be paired with
fermions, such as particles called photinos and gluinos. Scientists have yet to
detect these supersymmetric partners, but they believe the partners may be
massive compared to known particles, and therefore require too much energy to
create with current particle accelerators.
Another approach to grand unification
involves string theories. British physicist Paul Dirac developed the first
string theory in 1950. String theories describe elementary particles as loops of
vibrating string. Scientists believe these strings are currently invisible to us
because the vibrations do not occur in the four familiar dimensions of space and
time—some string theories, for example, need as many as 26 dimensions to explain
particles and particle interactions. Incorporating supersymmetry with string
theory results in theories of superstrings. Superstring theories are one of the
leading candidates in the quest to unify gravitation with the other forces. The
mathematics of superstring theories incorporates gravity into particle physics
easily. Many scientists, however, do not believe superstrings are the answers,
because they have not detected the additional dimensions required by string
theory.
| VIII | STUDYING ELEMENTARY PARTICLES |
Studying elementary particles requires
specialized equipment, the skill of deduction, and much patience. All of the
fundamental particles—leptons, quarks, force-carrying bosons, and the Higgs
boson—appear to be “point particles.” A point particle is infinitely small—it
exists at a certain point in space without taking up any space. These
fundamental particles are therefore impossible to see directly, even with the
most powerful microscopes. Instead, scientists must deduce the properties of a
particle from the way it affects other objects.
In a way, studying an elementary particle
is like tracking a white polar bear in a field of snow: The polar bear may be
impossible to see, but you can see the tracks it left in the snow, you can find
trees it clawed, and you can find the remains of polar bear meals. You might
even smell or hear the polar bear. From these observations, you could determine
the position of the polar bear, its speed (from the spacing of the paw prints),
and its weight (from the depth of the paw prints). No one can see an elementary
particle, but scientists can look at the tracks it leaves in detectors, and they
can look at materials with which it has interacted. They can even measure
electric and magnetic fields caused by electrically charged particles. From
these observations, physicists can deduce the position of an elementary
particle, its speed, its weight, and many other properties.
Most particles are extremely unstable,
which means they decay into other particles very quickly. Only the proton,
neutron, electron, photon, and neutrinos can be detected a significantly long
time after they are created. Studying the other particles, such as mesons, the
heavier baryons, and the heavier leptons, requires detectors that can take many
(250,000 or more) measurements per second. In addition, these heavier particles
do not naturally exist on the surface of Earth, so scientists must create them
in the laboratory or look to natural laboratories, such as stars and Earth’s
atmosphere. Creating these particles requires extremely high amounts of
energy.
Particle physicists use large, specialized
facilities to measure the effects of elementary particles. In some cases, they
use particle accelerators and particle colliders to create the particles to be
studied. Particle accelerators are huge devices that use electric and magnetic
fields to speed up elementary particles. Particle colliders are chambers in
which beams of accelerated elementary particles crash into one another.
Scientists can also study elementary particles from outer space, from sources
such as the Sun. Physicists use large particle detectors, complex machines with
several different instruments, to measure many different properties of
elementary particles. Particle traps slow down and isolate particles, allowing
direct study of the particles’ properties.
| A | Particle Accelerators and Colliders |
When energetic particles collide, the
energy released in the collision can convert to matter and produce new
particles. The more energy produced in the collision, the heavier the new
particles can be. Particle accelerators produce heavier elementary particles by
accelerating beams of electrons, protons, or their antiparticles to very high
energies. Once the accelerated particles reach the desired energy, scientists
steer them into a collision. The particles can collide with a stationary object
(in a fixed target experiment) or with another beam of accelerated particles (in
a collider experiment).
Particle accelerators come in two basic
types—linear accelerators and circular accelerators. Devices that accelerate
particles in a straight line are called linear accelerators. They use electric
fields to speed up charged particles. Traditional (not flat screen) television
sets and computer monitors use this method to accelerate electrons toward the
screen (Television: Picture Tube). Linear accelerators have two main
uses: They can produce a beam of particles for a fixed target experiment, or
they can feed particles into a circular accelerator.
Circular accelerators, or synchrotrons
(pronounced SIN-krow-trons), use magnetic fields to accelerate charged particles
in a circle. The particles can circle many times, gaining energy each time they
travel around the circle. Thus synchrotrons can accelerate particles to
extremely high energies. Synchrotrons can be used in fixed target experiments,
or they can accelerate two beams simultaneously for use in a collider
experiment.
Positively charged particles bend a
different way in a magnetic field than do negatively charged particles, so a
synchrotron can accelerate electrons in one direction and positrons in the
other. A synchrotron can also accelerate protons in one direction and
antiprotons in the other. Scientists are even considering building a synchrotron
to accelerate less stable particles, such as muons and antimuons.
Once particles reach the desired energy,
experimenters slightly change the magnetic field controlling the particles,
bringing the two beams into a collision. The particles and antiparticles
annihilate each other. The resulting energy produces numerous other particles
for the scientists to study.
| B | Extraterrestrial Particle Sources |
Many great discoveries in particle
physics have been made by looking to the heavens. The universe is a natural
particle accelerator, and particles from outer space continually bombard Earth’s
atmosphere. Extraterrestrial particles called cosmic rays—and their collisions
with other particles in the atmosphere—produce many unusual and unstable
particles. Scientists first discovered the muon and the pion in cosmic rays, as
well as the positron. Mesons made up of the strange quark were also first
spotted in cosmic ray experiments before modern large accelerator facilities
were built.
Neutrinos stream to Earth from cosmic
sources. Nuclear reactions in the Sun produce incredibly large numbers of
electron neutrinos that can then be detected on Earth. Experiments studying
these solar neutrinos suggest that the mass of the neutrino is very small but
that it is not zero.
| C | Particle Detectors |
Every particle experiment needs particle
detectors. Particle detectors come in many shapes, sizes, and types. Some
detectors track particles, some count the number of particles passing by, some
measure the energy left in the detector by a particle, and some are even more
specialized. In addition, many detectors contain large magnets to bend the paths
of charged particles. The direction the path bends indicates the electric charge
of the particle, and the amount the path bends indicates the mass and speed of
the particle.
Physicists have extensively studied, and
come to understand, commonly occurring interactions between particles, so most
current particle experiments focus on rare interactions, which are less well
understood. Experiments must generate incredibly large numbers of particle
interactions to produce a few of the desired rare interactions. Scientists are
not interested in studying the majority of interactions produced in an
experiment, so they need fast computers and sophisticated programs to sort the
data and pick out the important interactions.
Each type of particle has distinct
properties, so each type of particle behaves differently in detectors.
Experiments typically have many types of detectors to distinguish between
different particles. Each detector produces such an enormous amount of data on
each interaction that analyzing particle experiments requires a huge amount of
computer time.
| D | Particle Traps |
Scientists use particle traps to study
particles that are more stable and have less energy than particles studied in
accelerators and colliders. Magnetic and electric fields can be used to trap
charged particles. The fields control the movement of the particle, keeping it
confined to a small area. Neutral particles, such as atoms, can also be trapped,
but that task is much more difficult. Lasers, beams of coherent light, are often
used to trap neutral particles. Light carries energy, and when light strikes an
object, it exerts a small force on the object. Shining lasers on atoms or other
neutral particles causes the particles to gradually slow down and be trapped.
The rules of quantum theory prevent any
particle trap from being perfect. A perfect trap would enable a physicist to
precisely determine a particle’s position and speed. A rule called the
uncertainty principle states that a particle’s location and speed cannot be
precisely measured at the same time. Increasing the precision in one measurement
increases the uncertainty in the other. If a particle trap was infinitely small,
the location of the particle would be known precisely, but this would make
measurement of the particle’s speed infinitely uncertain: The scientist would
not be able to determine anything about the particle’s speed. Likewise, if the
particle trap slowed the particle to a complete rest, its speed would be known
precisely, which would make the particle’s location infinitely uncertain: The
scientist would not be able to determine anything about position, or whether the
particle was even in the trap.
Scientists use particle traps to compare
the properties of particles and antiparticles. Scientists are also trying to
create antihydrogen using particle traps. Antiparticles, such as antiprotons and
positrons, usually exist for just a brief time before they combine with their
counterpart particles in ordinary matter and are annihilated. A particle trap,
however, can confine an antiproton without letting it contact its ordinary
matter counterpart, the proton. Positrons can be confined in a similar manner.
Researchers are currently using particle traps to bring positrons close enough
to antiprotons so these particles can bind and make antihydrogen, just as
electrons and protons make hydrogen.
| IX | HISTORY |
The history of particle physics began in
the early 20th century with the discovery of the parts of the atom and the
photon. Theories explaining the behavior of these particles led physicists to
propose the existence of neutrinos in 1928 and antimatter in 1931. Antimatter
was discovered in 1933, but it took experimenters almost 30 years to confirm the
existence of neutrinos. Physicists were aided in their studies of particles by
the first particle accelerator, invented in 1928, and by its successor, which
was developed in the 1940s.
During the 1950s scientists discovered
mesons and pions in cosmic rays from space. They did not yet understand,
however, that these particles, as well as the protons and neutrons inside atoms,
were composed of quarks.
Two important advances in the theory of
elementary particles occurred in the 1960s: Physicists proposed the existence of
quarks, and they introduced the standard model, a theory that explains how the
strong and weak nuclear forces work. The standard model predicted the existence
of many more particles, which scientists later detected in experiments.
According to the standard model, the number of truly elementary particles is now
30: 6 quarks, 6 antiquarks, 6 leptons, 6 antileptons, the photon, the gluon, the
3 bosons of the weak force, and the Higgs boson. (The graviton, while it may
exist, is not included in the standard model.) Particle physicists continue to
revise their theories and often propose new particles to explain different
phenomena. Some of the particles that have been suggested, but not yet detected,
are the axion, the squark, and the magnetic monopole.
| A | Identifying Parts of the Atom |
In seeking to explain the behavior of
atoms, physicists of the late 1800s searched for the source of negative electric
charge in atoms. British physicist Sir Joseph John Thomson is credited with the
discovery of the electron. Although many others had studied electricity and
streams of electrons, Thomson was the first to measure the properties of
individual electrons and to suggest that electrons existed within atoms. He
measured the ratio of electron mass to electron charge and, in 1897, claimed
that electrons could be found in all matter.
Matter is not made up entirely of
electrons–atoms also contain protons and neutrons. No one person is given credit
for discovering the proton. Many experiments around the turn of the century
examined its properties, but it was not named proton until 1920. The
discovery of the neutron came much later, because the neutron is electrically
neutral and therefore much harder to detect. British physicist James Chadwick
discovered the neutron in 1932. He won the 1935 Nobel Prize in physics for this
discovery.
| B | Einstein and Particles of Light |
Before the development of particle
physics, scientists had a difficult time explaining the behavior of light. Light
often behaves like a wave, such as a wave of sound or a wave on the surface of
water. Other times, however, light behaves more like a beam of particles. To
explain this behavior, Albert Einstein proposed in 1905 that light came in
little packets, or particles, of energy. He was awarded the 1921 Nobel Prize in
physics for his explanation. In 1926 scientists named these particles of light
photons.
| C | Pauli and Neutrinos |
In the early part of the 20th century,
scientists studying beta decay noticed that the sum of the mass and energy
before the decay was greater than the sum of mass and energy present after the
decay. To account for this missing energy, Austrian-born American physicist
Wolfgang Pauli proposed the existence of a new particle in 1928. Pauli called
his suggestion a drastic measure because scientists by then did not expect more
elementary particles. His hypothesis proved correct, however, and this particle
is now known as the electron neutrino. The neutrino was escaping unseen because
it has no electric charge, no color charge, and only a very small mass. American
physicists Fred Reines and Clyde Cowen were the first to experimentally detect
the neutrino in 1956, almost 30 years after Pauli first proposed its existence.
Reines shared the 1995 Nobel Prize in physics for his part in this
experiment.
Pauli received a Nobel Prize as well, but
not for his proposal of neutrinos. He won the 1945 Nobel Prize in physics for
developing the exclusion principle. The exclusion principle is the rule of
quantum theory that says that no two fermions with exactly the same
characteristics can occupy the same space. Pauli proposed the exclusion
principle in 1925. A year later Italian-born American physicist Enrico Fermi
developed the mathematical equations to explain why two fermions cannot occupy
the same state.
| D | Discovery of Antimatter |
In 1931 British physicist Paul Dirac
produced the precursor of modern particle theories. Dirac’s equations described
the known electromagnetic properties of particles well, but to make his theory
work more comprehensively, Dirac had to introduce the idea of antiparticles,
antimatter counterparts of existing particles. The existence of these particles
was confirmed in 1933, when American physicist Carl Anderson saw something
peculiar while looking at tracks made by cosmic rays in a type of particle
detector called a cloud chamber. A particle passing through the cloud chamber
seemed to have the mass of an electron, but it had a positive rather than a
negative charge—he had discovered the positron. Anderson shared the 1936 Nobel
Prize in physics for this confirmation of Dirac’s theory.
| E | Search for Carriers of the Strong Force |
In 1934 Japanese physicist Yukawa Hideki
predicted the existence of a force carrier holding neutrons and protons together
in the nucleus of an atom. He believed this particle should have a mass between
the mass of the electron and that of the proton. Yukawa’s theory attempted to
describe how the strong force affects particle interactions, but it was not
complete because it did not describe the fundamental interactions between quarks
and gluons. It was, however, highly successful at describing the way protons and
neutrons bond inside the nucleus. The theory predicted the existence of the
pion, the meson that holds the particles in an atomic nucleus together.
When Carl Anderson and American physicist
Seth Neddermeyer detected a new particle in cosmic ray experiments two years
later, many thought this new particle was Yukawa’s meson. But some properties of
the new particle did not match Yukawa’s theory. This dilemma appeared to be
solved in 1947 when yet another particle, the pion, was found in cosmic rays.
The pion’s behavior was consistent with predictions in Yukawa’s theory. The
particle that Anderson and Neddermeyer discovered was later found to be the
muon, but in the beginning, no one could tell the purpose of this particle.
Anderson and Neddermeyer’s muon turned out to be the first indication of a new
type of lepton. Scientists detected the muon neutrino in 1962 and thereafter
regarded the muon and its neutrino partner as a second generation of
leptons.
In the same year that the pion was
discovered, physicists detected another particle in cosmic ray experiments. This
particle, now called the lambda, behaved differently than known particles.
Starting in 1953, scientists found many more such unexpected particles. Because
these particles were different, physicists called them “strange.” These
particles were eventually shown to include strange quarks, which received their
name from the description of the particles they compose.
| F | Invention of the Cyclotron |
While cosmic ray experiments revealed a
myriad of particles, scientists also sought ways to create unusual and unstable
particles in laboratories. American physicist Ernest Lawrence invented the
cyclotron, a type of circular accelerator, in 1932. The cyclotron, however,
could not achieve very high energies. Lawrence’s model was improved
(independently) by American physicist Edwin McMillan and Soviet physicist
Vladimir Veksler in the 1940s, resulting in the synchrocyclotron. The high
energies available using the synchrocyclotron led to many important particle
discoveries.
| G | Separating Leptons and Quarks |
By the 1960s hundreds of different
“elementary” particles had been seen. Physicists found they could separate these
particles into two main groups: those that interacted by the strong force and
those that did not. They called the strongly interacting particles hadrons, and
the particles without strong interactions leptons. American physicist Murray
Gell-Mann proposed in 1964 that many of these observed particles might not be
elementary after all. He showed that all of the properties of hadrons could be
explained if they were various combinations of three quarks. Normal matter, such
as protons, neutrons, and pions, contains only up and down quarks, and strange
matter (such as the lambda particles) contains one or more strange quarks along
with up and down quarks. Gell-Mann was honored for his contributions in 1969
with the Nobel Prize in physics. Gell-Mann’s quark theory was confirmed
experimentally by American physicists Jerome Friedman and Henry Kendall and
Canadian physicist Richard Taylor in 1969. Their experiment demonstrated that
protons have internal structure. This experiment earned them the 1990 Nobel
Prize in physics.
In 1964, the same year Gell-Mann
introduced his quark theory, British physicist Peter Higgs proposed the
existence of the Higgs boson, building on the work others had done in the early
1960s. Some scientists also predicted that same year that a fourth quark—the
charm quark—should exist. Hadrons containing the charm quark were finally
detected in 1976, leaving the number of quarks and the number of leptons equal
at four apiece. Scientists divided the leptons and quarks into two generations,
with the up and down quarks and the electron and electron neutrino in the first,
and the strange and charm quarks and muon and muon neutrino in the second.
| H | A Third Generation of Particles |
A third generation of particles entered
the scene in 1975, just a year before the charm quark was discovered. American
physicist Martin Perl and his collaborators detected a third charged lepton, the
tau. Scientists assumed immediately that a third neutrino accompanied the tau,
but it has not yet been directly detected. Perl shared the 1995 Nobel Prize in
physics with American physicist Frederick Reines for his part in discovering the
tau lepton.
Physicists discovered a third generation
of quarks in 1977. American physicist Leon Lederman and his collaborators
discovered mesons that contained a fifth quark: the bottom quark. Scientists
assumed the bottom quark should have a partner, called the top quark, and so the
hunt for this particle was on. This hunt finally ended in 1995, when evidence of
the top quark was detected at the Fermi National Accelerator Laboratory in
Batavia, Illinois. While the existence of the top quark was no surprise, the
mass of it was. The top quark is over 40 times heavier than the bottom quark,
and 174 times heavier than the proton, which contains three first generation
quarks (two up quarks and one down quark).
| I | The Standard Model |
Throughout the 1960s physicists worked on
a comprehensive theory to explain why different types of elementary particles
exist and why they behave the way they do. Building on the work of Fermi, Dirac,
Yukawa, Gell-Mann, and numerous others, three scientists developed what is now
called the standard model of particle physics. American physicist Steven
Weinberg and Pakistani physicist Abdus Salam extended the earlier work of
American physicist Sheldon Glashow and unified the electromagnetic and weak
forces in 1967. These three men shared the 1979 Nobel Prize in physics for their
highly successful theory. When these scientists developed the standard model,
the physics community had not yet discovered the charm quark and did not know of
the third generation of particles. The theory, however, predicted the charm and
worked well with the addition of a third generation.
One of the key predictions of the
standard model was the existence of particles carrying the weak force. In 1983
Italian physicist Carlo Rubbia and his colleagues discovered the W and Z bosons.
Rubbia and Dutch physicist Simon Van der Meer shared the 1984 Nobel Prize for
their work on the discovery of the W and Z bosons.
Particle physics is not finished yet.
Most of the predictions of the standard model have been verified, but physicists
still seek evidence of physics beyond the standard model. They look for new
particles both on Earth and throughout the cosmos. They work on theories that
would explain why particles have the masses scientists have observed. In
particular, they want to understand why the top quark is so much heavier than
the other particles and why the second and third generations of particles exist
at all. They look for connections between the four forces in the universe and
continue their quest for a theory of everything.
Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft
Corporation. All rights reserved.
No comments:
Post a Comment