Fundamental Particles

Fundamental particles are the elementary entities from which all matter is made. They have no known smaller parts. As recently as 1900 most people believed that atoms were the tiniest particles in the universe.

By the 1930s, however, it was clear that atoms were made up of even smaller particles—protons, neutrons, and electrons, then considered to be the fundamental particles of matter. (A proton is a positively charged particle that weighs about one atomic mass unit [1.0073 AMU]; a neutron has about the same mass [1.0087 AMU] but no charge; and an electron has a much smaller mass [0.0005 AMU] and a negative charge.) Protons and neutrons make up the tiny nucleus of an atom, while electrons exist outside the atomic nucleus in discrete energy levels within an electron "cloud."

By 1970 it began to appear that matter might contain even smaller particles, an idea suggested in 1963 by American physicist Murray Gell-Mann (who called the particles quarks ) and independently by American physicist George Zweig (who called them aces ). There are in actuality hundreds of subatomic particles that have been observed, but many of them are unstable.

Fermions

At the start of the twenty-first century, scientists believe that all matter is made up of tiny particles called fermions (named after American physicist Enrico Fermi). Fermions include quarks and leptons. Leptons include electrons (along with muons and neutrinos); they have no measurable size, and they are not affected by the strong nuclear force. Quarks, on the other hand, are influenced by the strong nuclear force. They are the fundamental particles that make up protons and neutrons (as well as mesons and some other particles). Both protons and neutrons are classified as baryons, composite particles each made up of three quarks.

Quarks come in six different types, or "flavors": up and down, top and bottom, and charm and strange. Protons and neutrons are made of up (u) quarks (which have a charge of +⅔) and down (d) quarks (which have a charge of −⅓). A proton is made from two u quarks (+⅔)(+⅔) and one d quark (−⅓), giving a total charge of +1. A neutron contains one u quark (+⅔) and two d quarks (−⅓)(−⅓) for a total charge of zero.

Fundamental Forces

There are also fundamental forces acting on matter; these have their own sets of fundamental particles. The forces are the strong nuclear force (or strong interaction), the weak nuclear force (or weak interaction), and electromagnetism (which includes light, x rays, and all the other electromagnetic forces). All these forces are transmitted by particles called fundamental bosons (named after Indian physicist S. N. Bose).

Fundamental bosons differ from fermions in spin and the number of quarks they contain. Fermions have spins measured in half numbers, and they contain an odd number of quarks. Bosons have whole integer spins, and they contain an even number of quarks. The bosons that transmit the strong nuclear force are called gluons, those that transmit electromagnetic forces are photons, and those transmitting the weak force are known as weak bosons. A fourth force, the gravitational force, is believed to be transmitted by particles called gravitons; however, the particles have not yet been observed. Still another kind of boson, called a Higgs boson, is thought to be the source of mass in other particles, but this particle also has not actually been observed.

Particle Accelerators

The study of fundamental particles often involves speeding up charged particles, such as protons or electrons, and then letting them collide with targets so as to produce other particles for further study. The particle accelerators used to do this are devices that force the charged particles to jump over longer and longer space gaps per unit of time, until the particles are moving at speeds approaching the speed of light.

The earliest of such devices were the linear Cockcroft-Walton accelerator (1929), the circular cyclotron (1930), and the Van de Graaff generator (1931). Modern synchrotrons are large machines that have both linear and curved sections. The most powerful synchrotron is the Tevatron proton accelerator at the Fermilab located near Batavia, Illinois (just outside of Chicago); it lies inside an underground circular tunnel that measures almost 6.4 kilometers (4.0 miles) around. The longest accelerator is the collider at the CERN research center in Geneva, Switzerland—it has a circumference of about 27.3 kilometers (17.0 miles).

Particle Detectors

Detection of fundamental particles is difficult because the particles are so extremely tiny. The earliest detector was just photographic film, since particles passing through would expose the film and become evident when it was developed. The first device designed for the purpose of detecting tiny particles was the "cloud chamber" (invented by Scottish physicist Charles Wilson in 1911). It was a glass container filled with air saturated with water (or alcohol) vapor. Charged particles passing through the chamber formed ions leaving fog tracks—the heavier the particles, the wider their tracks.

The "bubble chamber" (invented by American physicist Donald Glaser in 1952) was similar to a cloud chamber, except that it was filled with a liquid (usually liquefied helium or hydrogen) held at a temperature just below its BOILING POINT . Moving particles would disturb the liquid, causing bubbles to form along their paths. There was also a "spark chamber" (invented in Japan in 1959) that contained a series of parallel metal plates and produced an electrical discharge along the ion trail left by a charged particle. Although all of these devices were once important for detecting subatomic particles, they have largely been replaced by more modern detectors.

In the twenty-first century fundamental particles are studied using detectors such as tracking chambers (which trace the path of a particle with electrical signals), sampling calorimeters (which track the particle's path by its energy of motion), scintillators (which give off light when particles strike them), or magnetic detectors (which cause charged particles to move in curved paths). Many instruments use combinations of these various kinds of detectors.

As high-energy particles pass through a bubble chamber, bubbles are formed, which leave tracks as they move. This is important in the study of subatomic collisions, which normally are not visible to the naked eye.

Antimatter

To further complicate the subject of subatomic particles, each kind of particle has an antiparticle. For example, for each kind of quark there is an antiquark of the same mass and spin, but of opposite charge. The first antiparticle to be observed was the positron, an electron with a positive charge. An antiproton is like a proton, but it has a negative charge. Antiparticles can be observed, and molecules of antimatter can even be generated. A positron orbiting an antiproton, for example, is an antihydrogen atom.

Many scientists believe that there must be some areas of the universe that are completely made up of antimatter, the exact opposite of the kind of matter found on Earth. If that is true, such areas would not be very compatible with areas made of matter—when a particle and its antiparticle make contact, they destroy each other and are converted into energy. According to Einstein's special theory, E = mc ² , which means that energy is equivalent to mass times the speed of light, squared. In other words, a tiny speck of matter can be converted to a considerable amount of energy.

The conversion can also go the other way. Large releases of energy that occur when high-energy particles collide can produce new particles and antiparticles of matter. Much modern research in particle physics involves high-energy collisions between beams of particles, such as protons, so as to generate other kinds of particles. Some collisions involve interactions of particles with antiparticles (e.g., electrons with positrons). Particle accelerators have been turned into giant colliders in which beams of particles moving at speeds approaching the speed of light collide with each other, producing other kinds of particles.

By the early 2000s several hundred subatomic particles were known, almost all of them being made of quarks. The few remaining ones are the leptons, the electron being the best known. The other leptons are still rather mysterious. The muons and tau particles are negatively charged like the electron, while the neutrinos (which have no detectable mass but often pair up with the heavier leptons) have no charge. The electron was discovered in 1897, but there is still much to learn about other fundamental particles.

SEE ALSO Atomic Structure ; Fermi, Enrico .

Doris K. Kolb

Bibliography

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