ELEMENTRY PARTICLES

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.

Structure of Matter Modern physics has revealed successively deeper layers of structure in ordinary matter. Matter is composed, on a tiny scale, of particles called atoms. Atoms are in turn made up of minuscule nuclei surrounded by a cloud of particles called electrons. Nuclei are composed of particles called protons and neutrons, which are themselves made up of even smaller particles called quarks. Quarks are believed to be fundamental, meaning that they cannot be broken up into smaller particles.© Microsoft Corporation. All Rights Reserved.

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.

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SCIENTIFIC DISCOVERIES
Elementary-Particle Interaction
Physicists have sought for decades to demonstrate that the forces governing the behavior of elementary particles at the atomic level are different aspects of the same fundamental force. Progress toward a unified theory was made in the 1960s and 1970s, when physicists unified the electromagnetic force with the lesser-known weak force (the force responsible for slow nuclear processes, such as beta decay). The two forces are now sometimes referred to collectively as the electroweak interaction. American physicist Steven Weinberg and Pakistani physicist Abdus Salam independently proposed similar unified theories for these two interactions in 1967 and 1968. In 1979 the two scientists shared the Nobel Prize in physics for their contribution. Weinberg described the electroweak theory in this 1974 Scientific American article.
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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.

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SCIENTIFIC DISCOVERIES
Electrons: The First Hundred Years
Theoretical physicist C. Llewellyn Smith discusses the discoveries that scientists have made to date about the electron and other elementary particles—subatomic particles that scientists believe cannot be split into smaller units of matter. Scientists have discovered what Smith refers to as sibling and cousin particles to the electron, but much about the nature of these particles is still a mystery. One way scientists learn about these particles is to accelerate them to high energies, smash them together, and then study what happens when they collide. By observing the behavior of these particles, scientists hope to learn more about the fundamental structures of the universe.
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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.

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SCIENTIFIC DISCOVERIES
Scientists Create Matter Out of Light
German-born American physicist Albert Einstein’s elegant equation E=mc2 predicted that energy could be converted to matter. Using a linear accelerator and high-energy laser light, physicists have done just that. This 1997 Encarta Yearbook article describes their success.
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II WHAT MAKES UP THE UNIVERSE?

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A Brief Introduction to Particle Physics
Physicist Clifford V. Johnson takes readers on a brief introductory tour of the world of particle physics. A leading theoretician in elementary particle physics, Johnson traces the history of this field from its beginnings to the present day. He explains why physicists are currently intrigued with the exotic ideas of superstrings and M-theory.
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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

Family of Major Elementary Particles Elementary particles are thought to be the smallest units of matter. They are classified by mass, spin, and electric charge.© Microsoft Corporation. All Rights Reserved.

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.

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SCIENTIFIC DISCOVERIES
The Number of Families of Matter
According to the currently accepted theory of fundamental particles and their interactions, three generations, or families, of elementary particles exist in nature. The most familiar of these families is the first generation, which includes the electron and the “up” and “down” quarks that form the protons and neutrons in the nucleus of an atom. German-born American physicist and Nobel laureate Jack Steinberger and American physicist Gary J. Feldman participated in experiments in the late 1980s that confirmed the existence of only three families of elementary particles. They described their work in a 1991 Scientific American article. Since the article was published, scientists have verified the existence of the top quark.
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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

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SCIENTIFIC DISCOVERIES
Heavy Leptons
American physicist Martin L. Perl shared the 1995 Nobel Prize in physics for his discovery of an elementary particle known as the tau lepton. He described the detection of the tau lepton in a 1978 article in Scientific American. At the time, several fundamental particles were thought to exist but had not yet been detected. Physicists were unsure if there would be an end to this proliferation of newly identified elementary particles. As of 1998, physicists believe that there are three and only three “families” of matter. The first family of matter includes the electron and the two types of quarks that make up the proton and neutron. The tau lepton belongs to the third family of matter.
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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). Scientists believe the tau has an electrically neutral partner called the tau neutrino. While scientists have never detected a tau neutrino directly, they believe they have seen the effects of tau neutrinos during experiments. Like the other neutrinos, the tau neutrino has a very small mass or no mass at all.

B Quarks

Some Experimentally Known Elementary Particles The masses of elementary particles are usually given in units of MeV (million electron volts). One MeV mass equivalent is equal to 1.8 x 10-27 g. The mean lives of the unstable particles are in units of seconds.© Microsoft Corporation. All Rights Reserved.
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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).

Constituents of Matter Matter is composed of tiny particles called quarks. Quarks come in six varieties: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). Quarks also have antimatter counterparts called antiquarks (designated by a line over the letter symbol). Quarks combine to form larger particles called baryons, and quarks and antiquarks combine to form mesons. Protons and neutrons, particles that form the nuclei of atoms, are examples of baryons. Positive and negative kaons are examples of mesons.© Microsoft Corporation. All Rights Reserved.

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

Gluons and Color Charge Gluons are particles of energy that carry the strong nuclear force. They hold together particles called quarks and antiquarks, which combine to form hadrons. Examples of hadrons include protons and neutral kaons. As gluons bind together quarks, or quarks and antiquarks, they affect a property of quarks and antiquarks called color charge. The relationship between color charge and the strong force is similar to that between electric charge and the electromagnetic force.© Microsoft Corporation. All Rights Reserved.
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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.

Strong Force and the Creation of a Particle The strong force holds together particles called quarks inside protons. When a fast-moving particle collides with a proton, the strong force can convert the energy of the collision into matter, resulting in the creation of a new particle.© Microsoft Corporation. All Rights Reserved.
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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

Particle Accelerator The big circle marks the location of the Large Hadron Collider (LHC) at the European particle physics laboratory in CERN. The tunnel where the particles are accelerated is located 100m (320 ft) underground and is 27 km (16.7 mi) in circumference. The smaller circle is the site of the smaller proton-antiproton collider. The border of France and Switzerland bisects the CERN site and the two accelerator rings.Photo Researchers, Inc./CERN/Science Source

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 may not be zero. If these experiments are correct, they could provide the first contradiction of the standard model of particle physics.

C Particle Detectors

Mark II Particle Detector Particle accelerators and detectors provide physicists with invaluable information about subatomic particles. Using particle accelerators, physicists accelerate particles to very high energies. Then they smash these particles into each other or into a target and use particle detectors to measure and record the properties of the particles produced. This Mark II particle detector is part of the 3.2 km (2 mi) linear accelerator in California, called the Stanford Linear Accelerator Center.Photo Researchers, Inc./Stanford Liear Accelerator Center

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

dynamic timeline
Cecil Powell Discovers the Pion



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

Enrico Fermi Famous for producing the first controlled nuclear reaction in 1942, physicist Enrico Fermi worked as a consultant for the Manhattan Project during World War II, helping to design the atomic bomb. He won a Nobel Prize in 1938 for his work on artificial radioactivity. He inspired many students and continues to be honored through various awards and institutions that were established in his name, such as the Fermi National Accelerator Laboratory in Batavia, Illinois.Culver Pictures

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 a very small mass (or no mass at all). 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.

Wolfgang Pauli Wolfgang Pauli won the 1945 Nobel Prize in physics for his discovery of the exclusion principle, also called the Pauli principle, which states that no two electrons in an atom can have identical sets of quantum numbers. These numbers define an electron’s energy, and the exclusion principle allowed scientists to describe the arrangement and behavior of electrons in the chemical elements.Hulton Deutsch

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

Yukawa Hideki Japanese physicist Yukawa Hideki won the 1949 Nobel Prize in physics. Based on his research into quantum mechanics and the fields of force affecting elementary particles, he theoretically deduced the existence of mesons, a family of subatomic particles composed of quarks and antiquarks and having intermediate mass.© The Nobel Foundation

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

Murray Gell-Mann American physicist Murray Gell-Mann won the 1969 Nobel Prize in physics. He researched the interactions of elementary particles and advanced the quark theory.© The Nobel Foundation

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

Leon Lederman American physicist Leon Lederman won the 1988 Nobel Prize in physics. He helped discover the muon neutrino, one of the elementary particles of matter. © The Nobel Foundation

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.

further reading
These sources provide additional information on Elementary Particles.

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

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