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antiparticle

antiparticle,elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment. Every elementary particle has a corresponding antiparticle; the antiparticle of an antiparticle is the original particle. In a few cases, such as the photon and the neutral pion, the particle is its own antiparticle, but most antiparticles are distinct from their ordinary counterparts.

When a particle and its antiparticle collide, both can be annihilated and other particles such as photons or pions produced. In some cases this represents the total conversion of mass into energy. For example, the collision between an electron and its antiparticle, a positron, results in the conversion of their combined masses into the energy of two photons. The reverse process, pair production, is the simultaneous creation of a particle and its antiparticle from the particles that result from their mutual annihilation.

The existence of antiparticles for electrons was predicted in 1928 by P. A. M. Dirac's relativistic quantum theory of the electron. According to the theory both positive and negative values are possible for the total relativistic energy of a free electron. In 1932, Carl D. Anderson, while studying cosmic rays, discovered the predicted positron, the first known antiparticle. About 23 years passed before the discovery of the next antiparticles—the antiproton was discovered by Owen Chamberlain and Emilio Segrè in 1955 at the Univ. of California, and the antineutron was discovered the following year—but the existence of antiparticles for all known particles was by then firmly established in theory.

The existence of antiparticles makes possible the creation of antimatter, composed of atoms made up of antiprotons and antineutrons in a nucleus surrounded by positrons. A very simple type of "atom" incorporating antiparticles is positronium, a brief pairing of a positron and an electron that may occur before their annihilation; it was first created and identified in the laboratory in 1951. Di-positronium, a molecule consisting of two positronium, was created in 2007. A few simple nuclei of antimatter have been created in the laboratory, such as the antideuteron (see deuterium). In 1995 nine atoms of antihydrogen (a single positively charged positron orbiting a single negatively charged antiproton) were created at CERN (near Geneva, Switzerland) by an Italian-German team headed by Walter Oelert.

Any antimatter in our part of the universe is necessarily very short-lived (the antihydrogen atoms, for example, survived for only 40 billionths of a second) because of the overwhelming preponderance of ordinary matter, by which the antimatter is quickly annihilated. Although scientists for a time considered the possibility that entire galaxies of antimatter could have evolved in a part of the universe far removed from our own, observations now indicate that this is not the case. The experimental work of Val L. Fitch and James W. Cronin in 1964 demonstrated an asymmetry in matter-antimatter reactions involving neutral K mesons (kaons) that may explain why the universe is composed mostly of matter. For their discovery, they shared the 1980 Nobel Prize in Physics. Later studies at Fermi National Accelerator Laboratory and CERN concerning the decay of other neutral mesons have found a matter-antimatter asymmetry in their decay.

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Antiparticle

Antiparticle

Antiparticles are subatomic particles that are the opposite of other subatomic particles in some way or another. In the case of the antielectron and the antiproton, this difference is a matter of charge. The electron is negatively charged, and the antielectron is positively charged; the proton is positively charged, and the antiproton is negatively charged. Since the neutron carries no electric charge, its antiparticle, the antineutron, is characterized has having a spin opposite to that of the neutron.

Dirac's hypothesis

The discovery of antiparticles is a rather remarkable scientific detective story. In the late 1920s, British physicist Paul Dirac (19021984) was working to improve the model of the atom then used by scientists. As he performed his mathematical calculations, he found that electrons should be expected in two energy states, one positive and one negative. However, the concept of negative energy was unknown to scientists at that time.

Dirac suggested that some electrons might carry a positive electrical charge, the opposite of that normally found in an electron. Scientists were skeptical about the idea. Electrons were well known, and the only form in which they had ever been observed was with a negative charge.

The dilemma was soon resolved, however. Only five years after Dirac proposed the concept of a positive electron, just such a particle was found by American physicist Carl Anderson (19051991). Anderson named the newly found particle a positron, for posi tive electron.

Other antiparticles and antimatter

Anderson's discovery raised an obvious question: If an antielectron exists, could there also be an antiproton, a proton with a negative charge? That question took much longer to answer than did Dirac's original problem. It was not until 1955 that Italian-American physicist Emilio Segrè (19051989) and American physicist Owen Chamberlain (1920 ) produced antiprotons by colliding normal protons with each other inside a powerful cyclotron (atom-smashing machine).

If antielectrons and antiprotons exist, is it possible that antimatter also exists? Antimatter would consist of antiatoms made of antiprotons and antielectrons. The idea may seem bizarre because we have no experience with antimatter in our everyday lives. Scientists now believe that antimatter is common in the universe, but we don't have any direct contact with it.

If antimatter does exist, locating it may be a problem. Scientists know that the collision of an antiparticle with its mirror imagean electron with a positron, for exampleresults in the annihilation of both, with the release of huge amounts of energy. Thus, any time matter comes into contact with antimatter, both are destroyed and converted into energy.

[See also Subatomic particles ]

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antimatter

antimatter Matter made up of antiparticles, identical to ordinary particles in every way except the charge, spin and magnetic moment are reversed. Its existence is predicted by the quantum mechanics. When an antiparticle, such as a positron (anti-electron), antiproton, or antineutron meets its respective particle, both are annihilated. The possibility exists that there are stars or galaxies composed entirely of antimatter. See also subatomic particles

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antimatter

an·ti·mat·ter / ˈantēˌmatər; ˈantī-/ • n. Physics molecules formed by atoms consisting of antiprotons, antineutrons, and positrons. Stable antimatter does not appear to exist in our universe.

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antimatter

antimatter: see antiparticle.

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Antimatter

Antimatter

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Antimatter is a form of matter made of particles that are equal in mass to the particles comprising ordinary matterneutrons, protons, electrons, and so forthbut with opposite electrical properties. That is, an antiproton has the same mass as the proton, but negative charge; an antielectron (positron) has the same mass as an electron, but negative charge; and an antineutron has the same mass as a neutron, but with a magnetic moment opposite in sign to the neutrons. Antiparticles are built up of antiquarks, which come in six varieties that mirror the six varieties of quarks in ordinary matter. In theory, antimatter has almost exactly the same properties as ordinary matter.

When particles and antiparticles approach closely, they annihilate each other in a burst of high-energy photons. A complementary or reverse process also occurs: high-energy photons can produce particle-antiparticle pairs. Antiparticles are also given off by the nuclei of certain isotopes undergoing fission.

The existence of antimatter was proposed by British physicist Paul Dirac (19021984) in 1927. The positron was first observed in 1932 and the antiproton and antineutron in the 1950s. The mutual annihilation process that occurs when matter and antimatter meet provides a 100% matter-to-energy conversion, as opposed to the low efficiencies achieved in nuclear fission and fusion reactions. However, antimatter cannot be used as a practical energy source because there are no bulk sources of antimatter. Antimatter can only be collected painstakingly, a few particles at a time.

The production of antimatter from energy must always, so far as present-day physics can determine, produce matter and antimatter in equal quantities. Therefore, the big bangthe explosion in which the Universe originatedshould have yielded equal quantities of matter and antimatter. However, the universe seems to consist entirely of matter. Where did all the antimatter go? What broke the symmetry between matter and antimatter?

This remains one of the great unsolved problems of physics. In general terms, most physicists argue that some subtle aspect of quantum physics led to the production of very slightly greater numbers of ordinary particles than of antiparticles at the origin of the universe. Most particles and antiparticles annihilated each other, yielding energy, but the slight majority of ordinary particles persisted to yield the matter that comprises the universe today.

For decades, some physicists theorized that it might still be out therethat some galaxies might consist of antimatter. Because antimatter should absorb and emit photons just like normal matter, it would be impossible to tell a matter galaxy from an antimatter galaxy by observing it through the telescope.

Theoretical physicists now argue that this is unlikely, and that the entire universe almost certainly does consist entirely of ordinary matteralthough they still dont know why. If matter and antimatter had segregated into separate galactic domains after the big bang, these domains must have gone through a process of mutual annihilation along their contacting borders; this would have produced large quantities of gamma rays, which would still be observable as a gamma-ray background everywhere in the sky. A cosmic gamma-ray background of uncertain origin does exist, but is only about one fifth as intense as it would be if the formation of matter and antimatter domains were responsible for it. Nevertheless, if antimatter galaxies do exist, some of the very-high-speed atoms from space that are known as cosmic rays should consist of antimatter. A device to detect antimatter cosmic rays was tested briefly on a space shuttle flight in 1998 and found no antimatter. It was scheduled to make a more thorough investigation, operating on the International Space Station, in 2003, but as of October 2006 it still had not flown again in space. It was scheduled for launch in 2007.

Antimatter, despite its scarcity, has practical uses. Radioactive isotopes that give off positrons are the basis of PET (positron-emission tomography) scanning technique. When administered to a patient, a positron-emitting isotope of (say) oxygen is utilized chemically just like ordinary oxygen; when it breaks down, it emits positrons. Each positron cannot travel far before it encounters an electron. The two particles annihilate, giving off two identical, back-to-back gamma rays. These can be picked up by a detector array, and a map of positron-emission activity can be created by a computer that deduces the locations of large numbers of positron-electron annihilations from these gamma-ray detections. PET produces real-time images of brain activity of importance to neurology, and has also been used to probe the structure of nonliving systems (engines, semiconductor crystals, etc.).

The latest goal of antimatter research is the creation of cool antihydrogen atoms (composed of an antiproton orbited by a positron). These early antihydrogen atoms, however, were formed moving at high speed, and quickly struck normal atoms in the apparatus and were destroyed. In 2002, researchers at CERN (Conseil Europée pour la Recherche Nucléaire) produced thousands of antihydrogen atoms, which were cooled to about 50 degrees Kelvin. The goal of such research is to isolate the antihydrogen from ordinary matter long enough to observe its properties, such as its absorption and emission spectra; cool atoms, which are moving more slowly, are easier to confine. The properties of antihydrogen are predicted by theory to be identical to those of ordinary hydrogen; therefore, if they are not, revisions to basic physics will be required. As of late 2006, CERN researchers were still working toward the goal of precision spectroscopy of antihydrogen.

See also Acceleration; Accelerators; Atomic theory; Electromagnetic spectrum; Particle detectors; Quantum mechanics; Subatomic particles.

Resources

BOOKS

Fraser, Gordon. Antimatter: The Ultimate Mirror. Cambridge, UK: Cambridge University Press, 2002.

PERIODICALS

Stone, Richard. Neutrino Hunters Go Nuclear to Tackle Antimatter Deficit. Science. 313 (2006): 291.

Wilczek, Frank. In Search of Symmetry Lost. Nature. 433 (2005): 239-247.

Williams, Gary. Antimatter and 20th Century Science. Physics Education. 40 (2005): 238-244.

Larry Gilman

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Antimatter

ANTIMATTER

The proton in a hydrogen atom carries one unit of positive charge, the electron one unit of negative charge. Imagine reversing the signs of the charges, producing a negative proton and a positive electron. One would have a sample of antimatter, in this case an atom of antihydrogen.

Concept and Confirmation

The existence of antimatter was predicted by Paul Dirac in 1928. While attempting to make quantum mechanics consistent with special relativity, Dirac encountered negative-energy solutions. In special relativity, when a free particle of mass m moves with momentum p , its energy E is given by E2 = (mc2)2 + (pc )2 where c denotes the speed of light in a vacuum. The square root yields a spectrum of positive and negative energies (Figure 1): E = ±[(mc2)2 + (pc2]1/2

FIGURE 1

With no ground state, the negative energies offered no immediate physical interpretation. But for spin-1/2 particles (fermions) such as the electron, which are constrained by the Pauli exclusion principle, Dirac conceptualized a way to make sense of these states, with observable consequences. Dirac supposed that the vacuum consists of no positive- energy particles and the negative-energy states filled with electrons. The exclusion principle prohibits the negative-energy electrons from dropping further into the bottomless energy well. This vacuum thus carries infinite negative charge and energy, but Dirac postulated that only departures from this situation are observable.

Suppose a negative-energy electron receives energy from an external source, such as an electromagnetic field, sufficient to jump the 2mc2 energy gap. This boosts the electron into a positive-energy state, leaving a hole in a negative-energy state (Figure 2). The absence of a negative-energy electron (the hole) would be observed in the laboratory as the presence of a particle of positive energy and positive charge. In the laboratory, this process is observed as pair production: Electromagnetic Energye- + e+ where e denotes the electron and e the antielectron (the positron). The electron will eventually drop back into the hole in pair annihilation: e- + e+Electromagnetic Energy The electromagnetic energy is carried by gamma-ray photons . The positron was first observed in cloud chamber tracks of cosmic rays, photographed by Carl Anderson in 1932.

Pair production and annihilation respect the conservation of energy, linear and angular momentum, and electric charge. Charge conservation requires particle-antiparticle pairs in production and annihilation, and momentum conservation requires more than one gamma ray.

Because the proton is a fermion, Dirac's argument requires the antiproton to exist. In 1955 Emilio Segrè and others demonstrated its existence with an accelerator at Berkeley. Since the proton is a bound state of constituent fermions called quarks, anti- quarks evidently exist as well. Today antiprotons are

FIGURE 2

produced in the accelerators of Fermilab in Batavia, Illinois, and the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, in collisions that convert kinetic energy into mass, such as p + pp + p + p + where denotes the antiproton. The antiprotons are stored and then collided head-on with a proton beam. The pair annihilations result in gamma rays and other final states because the proton and antiproton participate in the weak and strong interactions, in addition to the electromagnetic interction.

The exclusion principle does not apply to integer spin particles (bosons) such as photons or pions. In 1948 Richard Feynman created another way to think of the negative energy states that applies to fermions and bosons. In Feynman's version of relativistic quantum electrodynamics, the negative-energy particles move backward in time. This concept is interpreted physically as a positive-energy antiparticle moving forward in time. This interpretation is analogous to the description of electric current. In an electric circuit, positive charges moving from A to B (the so-called conventional current) and the same amount of negative charge moving from B to A produce identical changes in the charge distribution. Likewise, when the relativistic quantum mechanics equations have a negative-energy particle moving backward in time, the phenomena show a positive- energy particle of the opposite charge moving ahead in time—the antiparticle (Figures 3a and 3b).

For every particle species P, there is a corresponding antiparticle P

FIGURE 3(a)

FIGURE 3(b)

have identical mass but opposite charge, opposite magnetic dipole moment, and (if applicable) opposite signs of all other conserved quantum numbers (such as strangeness or baryon number that extend the charge concept to nonelectromagnetic interactions). What about uncharged bosons such as the photon or the Z0? For these species the particle and antiparticle are indistinguishable—they are their own antiparticles.

What Happened to All the Antimatter?

In high-energy collisions, where kinetic energies exceed particle masses, photons and particle- antiparticle pairs readily change into one another. The ultimate high-energy "laboratory" was the early universe immediately following the Big Bang. That ultra-relativistic environment witnessed dynamic equilibrium between the photons and particle- antiparticle pairs (here denoted generically as ) through the reaction P + ↔ γ + γ

As the universe expanded and cooled, the production of particles and antiparticles of mass m was quenched when the photon energy dropped below mc2. This occurred for the protons and neutrons (collectively called baryons) about 10-6 s after the Big Bang. However, pair annihilations continued (until the onset of nucleosynthesis, about 3 minutes later) without pair production to replenish the supply of baryon-antibaryon pairs. Since particles and antiparticles appear and annihilate as pairs, why did not all baryons annihilate with antibaryons, leaving the universe a gas of photons? That the universe today consists of matter with little anti- matter suggests a lack of symmetry in their reactions. The decay channels available to antiparticles differ slightly from those for the corresponding particles. For example, suppose that, left to themselves, particle P is stable, but its antiparticle can decay, with a small probability , into something else: → a + b . The probability of not decaying is 1-δ . Every that decays is unavailable to annihilate with a P particle, so when all the antiparticles have disappeared through decay or annihilation, some P particles are left over. Such reactions are indeed observed in the laboratory, as in the decay of the kaons. They demonstrate matter-antimatter asymmetry, a mechanism for producing a universe dominated by matter.

See also:Annihilation and Creation; Antiproton, Discovery of; Big Bang Nucleosynthesis; Positron, Discovery of

Bibliography

Berstein, J.; Fishbane, P. M.; and Gasiorowicz, S. Modern Physics (Prentice Hall, Upper Saddle River, NJ, 2000).

Feynman, R. P. Quantum Electrodynamics (W. A. Benjamin, Reading, MA, 1962).

Feynman, R. P.; Leighton, R. B.; and Sands, M. The Feynman Lectures on Physics (Addison-Wesley, Reading, MA, 1965).

Hawking, S. A Brief History of Time: From the Big Bang to Black Holes (Bantam, New York, 1988).

Segrè, E. From X-Rays to Quarks: Modern Physicists and Their Discoveries (W.H. Freeman, San Francisco, CA, 1980).

Weinberg, S. The First Three Minutes: A Modern View of the Origin of the Universe (Basic Books, New York, 1988).

Wilczek, F. "The Cosmic Asymmetry between Matter and Antimatter." Scientific American 243 , 82–90 (1980).

Dwight E. Neuenschwander

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Antimatter

Antimatter

Antimatter is matter comprising particles that are equal in mass to the particles comprising ordinary matter—neutrons, protons, electrons, and so forth—but with opposite electrical properties. An antiproton has the same mass as the proton , but negative charge, an antielectron (positron) has the same mass as an electron , but negative charge, and an antineutron has the same mass as a neutron , but with a magnetic moment opposite in sign to the neutron's. Antiparticles are built up of antiquarks, which come in six varieties that mirror the six varieties of quarks that make up ordinary matter. In theory, antimatter has almost exactly the same properties as ordinary matter.

When particles and antiparticles approach closely, they annihilate each other in a burst of high-energy photons. A complementary or reverse process also occurs: high-energy photons can produce particle-antiparticle pairs. Antiparticles are also given off by the fissioning nuclei of certain isotopes.

The existence of anti-matter was proposed by British physicist Paul Dirac (1902–1984) in 1927. The positron was first observed in 1932 and the antiproton and anti-neutron in the 1950s. Antimatter does yield energy ; the annihilation process that occurs when matter and anti-matter meet provides a 100% matter-to-energy conversion, as opposed to the extremely small efficiencies achieved in nuclear fission and fusion reactors. However, antimatter cannot be used as a practical energy source because there are no bulk sources of antimatter. Antimatter can only be collected painstakingly, a few particles at a time.

Antimatter's rarity is one of the great mysteries of cosmology . The production of antimatter from energy must always, so far as present-day physics can determine, produce matter and antimatter in equal quantities. Therefore, the big bang—the explosion in which the Universe originated—should have yielded equal quantities of matter and antimatter. However, the Universe seems to consist entirely of matter. Where did all the antimatter go?

For decades, some physicists theorized that it might still be out there—that some galaxies might consist of antimatter. Because antimatter should absorb and emit photons just like normal matter, it would be impossible to tell a matter galaxy from an antimatter galaxy by observing it through the telescope .

Theoretical physicists now argue that this is unlikely, and that the entire Universe almost certainly does consist entirely of ordinary matter—although they still don't know why. If matter and antimatter had segregated into separate galactic domains after the big bang, these domains must have gone through a process of mutual annihilation along their contacting borders; this would have produced large quantities of gamma rays, and these which would still be observable as a gamma-ray background everywhere in the sky. A cosmic gamma-ray background of uncertain origin does exist, but is only about one fifth as intense as it would be if the formation of matter and antimatter domains were responsible for it. Nevertheless, if antimatter galaxies do exist, some of the very-high-speed atoms from space that are known as cosmic rays should consist of antimatter. A device to detect antimatter cosmic rays was tested briefly on a space shuttle flight in 1998, and found no antimatter; it is scheduled to make a more thorough investigation, operating on the International Space Station , in 2003.

Antimatter, despite its scarcity, has practical uses. Radioactive isotopes that give off positrons are the basis of PET (positron-emission tomography) scanning technique. When administered to a patient, a positron-emitting isotope of (say) oxygen is utilized chemically just like ordinary oxygen; when it breaks down, it emits positrons. Each positron cannot travel far before it encounters an electron. The two particles annihilate, giving off two identical, back-to-back gamma rays. These can be picked up by a detector array, and a map of positron-emission activity can be created by a computer that deduces the locations of large numbers of positron-electron annihilations from these gamma-ray detections. PET produces real-time images of brain activity of importance to neurology, and has also been used to probe the structure of nonliving systems (engines, semiconductor crystals, etc.).

The latest goal of antimatter research is the creation of cool antihydrogen atoms (composed of an antiproton orbited by a positron). These early antihydrogen atoms, however, were formed moving at high speed, and quickly struck normal atoms in the apparatus and were destroyed. In 2002, researchers at CERN (Conseil Europée pour la Recherche Nnucléaire) produced thousands of antihydrogen atoms cooled to about 50 degrees Kelvin. The goal of such research is to isolate the antihydrogen from ordinary matter long enough to observe its properties, such as its absorption and emission spectra; cool atoms, which are moving more slowly, are easier to confine. The properties of antihydrogen are predicted by theory to be identical to those of ordinary hydrogen ; therefore, if they are not, revisions to basic physics will be required. CERN researchers hope to obtain their first antihydrogen spectra in late 2003.

See also Acceleration; Accelerators; Atomic theory; Electromagnetic spectrum; Particle detectors; Quantum mechanics; Subatomic particles.


Resources

books

Fraser, Gordon. Antimatter: The Ultimate Mirror. Cambridge, UK: Cambridge University Press, 2000.


periodicals

Peskin, Michael, "The Matter with Antimatter." Science. 419 (October 3, 2002):24–25.

Seife, Charles, "CERN Team Produces Antimatter in Bulk." Science. 297 (September 20, 2002):1979–1981.

Hijmans, Tom W., "Cold Antihydrogen." Science. 419 (October 3, 2002):439–440.


Larry Gilman

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antiparticle

an·ti·par·ti·cle / ˈantēˌpärtikəl; ˈantī-/ • n. Physics a subatomic particle having the same mass as a given particle but opposite electric or magnetic properties. Every kind of subatomic particle has a corresponding antiparticle.

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Antiparticle

Antiparticle

Diracs hypothesis

Other antiparticles

Antimatter

Antiparticles and cosmology

Resources

An antiparticle is a subatomic particle identical with familiar subatomic particles such as electrons or protons, but with opposite electrical charge or, in the case of uncharged particles, opposite magnetic moment. For example, an antielectron (also known as a positron) is identical with the more familiar electron, except that the former carries a single unit of positive electrical charge rather than a single unit of negative electrical charge. Antiparticles are not considered unusual or abnormal but are as fundamental a part of the natural world as are non-antiparticles. The main difference between the two classes of particles is that the world with which humans normally deal is constituted of protons, neutrons, and electrons rather than antiprotons, antineutrons, and anti-electrons. To avoid suggesting that non-antiparticles are more normal than antiparticles, the name koinoparticle has been suggested for ordinary particles such as the proton, electron, and neutron.

Diracs hypothesis

During the late 1920s, the British physicist Paul Dirac (19021984) attempted to modify the currently accepted model of the atom by including in it the relativistic properties of electrons. As a result of his analysis, Dirac found that electrons should be expected to exist in two energy states, one positive and one negative. The concept of positive energy presents no problems, of course, but Dirac and other physicists were uncertain as to the meaning of a negative energy state. What did it mean to say that an electron had less than zero energy?

Eventually Dirac concluded that the negative energy state for an electron might imply the existence of a kind of electron that no one had yet imagined, one that is identical with the familiar negatively-charged electron in every respect except its charge. It was, Dirac suggested, an electron with a positive charge, or an antielectron.

Within five years, Diracs hypothesis had been confirmed. In 1932, the American physicist Carl Anderson found in photographs of a cosmic ray shower the tracks of a particle that satisfied all the properties predicted by Dirac for the antielectron. Anderson suggested the name positron for the new particle.

Other antiparticles

The existence of the positron strongly suggested to scientists that other antiparticles might exist. If there was a positively-charged electron, they asked, why could there also not be a negatively-charged proton: the antiproton? The search for the antiproton took much longer than the search for the antielectron. In fact, it was not until 1955 that Emilio Segre and Owen Chamberlain were able to prove that antiprotons are produced when protons from a powerful cyclotron collide with each other.

The antineutron is a fundamentally different kind of antiparticle than the antielectron or antiproton. Since neutrons have no electrical charge, an antineutron could not differ in this respect from its mirror image. Instead, an antineutron is a particle whose direction of spin is opposite that of the neutron. Since a particles spin is expressed in the magnetic field that it generatesits magnetic momentthe antineutron is defined as the antiparticle with a magnetic moment equal in magnitude, but opposite in sign, to that of the neutron.

Other antiparticles also exist. For example, the electron is a member of a group of fundamental particles known as the leptons. Other leptons include the mu neutrino (muon) and the tau neutrino (tauon), electron-like particles that exist only at very high energy levels not observed under circumstances of our ordinary everyday world. Both muons and tauons have their own antiparticles, the antimuon and the antitauon.

Antimatter

Given the existence of the antiproton, antineutron, and antielectron, one might imagine the existence of antiatoms, atoms that are identical to the atoms of everyday life but have mirror image electrical charges and mirror image magnetic moments. Indeed, atoms of antihydrogen were first produced at the European laboratory CERN (Conseil Europée pour la Recherche Nucl´aire) in 2002.

Handling antimatter is a ticklish business, and antihydrogen atoms do not last long. The reason is that when an antiparticle comes into contact with its mirror imagean antielectron with an electron, for examplethe two particles annihilate each other, and their mass is converted to energy. Thus, any time matter comes into contact with antimatter, both are destroyed and converted into energy.

Antiparticles and cosmology

The Swedish physicist Hannes Alfvén (19081995) studied in some detail the possible role of antiparticles in the creation of the universe. At first glance, one would assume that the number of ordinary particles and antiparticles produced during the big bang would be equal, yet the consensus assertion among physicists today is that the entire observable universe consists of matter, not antimatter. This remains one of the great, unsolved problems of physics. In general terms, most physicists argue that some subtle aspect of quantum physics led to the production of very slightly greater numbers of ordinary particles than of antiparticles at the origin of the universe. Most particles and antiparticles

KEY TERMS

Cyclotron A machine used to accelerate particles to very high energies, making possible nuclear reactions that do not take place at the energies of everyday life.

Lepton A particle that undergoes weak interactions. Three classes of leptonselectrons, muons, and tauons and their antiparticlesare known to exist.

Magnetic moment A measure of the strength of a magnetic field.

Neutrino An elementary particle with no electrical charge and virtually no mass.

Subatomic particle An elementary particle smaller than an atom. Protons, neutrons, and electrons are examples of subatomic particles.

annihilated each other, yielding energy, but the slight majority of ordinary particles persisted to yield the matter that comprises the universe today.

Resources

BOOKS

Anderson, Carl D. The Discovery of Anti-Matter: The Autobiography of Carl David Anderson, The Youngest Man to Win the Nobel Prize. River Edge, NJ: World Scientific, 1999.

Martin, Brian. Nuclear and Particle Physics: An Introduction. New York: John Wiley & Sons, 2006.

Penrose, Roger. The Road to Reality: A Complete Guide to the Laws of the Universe. New York: A. A. Knopf, 2005.

PERIODICALS

Ellis, John. Particle Physics: Antimatter Matters. Nature. 424 (2003): 631-634.

David E. Newton

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Antiparticle

Antiparticle

An antiparticle is a subatomic particle identical with more familiar subatomic particles such as electrons or protons, but with the opposite electrical charge or, in the case of uncharged particles, the opposite magnetic moment. For example, an antielectron (also known as a positron) is identical with the more familiar electron , except that the former carries a single unit of positive electrical charge rather than a single unit of negative electrical charge. Antiparticles are not considered to be unusual or abnormal but are as fundamental a part of the natural world as are non-antiparticles. The main difference between the two classes of particles is that the world with which humans normally deal is constituted of protons, neutrons, and electrons rather than antiprotons, antineutrons, and antielectrons. To avoid suggesting that non-antiparticles are more "normal" than antiparticles, the name koinoparticle has been suggested for "ordinary" particles such as the proton , electron, and neutron .


Dirac's Hypothesis

During the late 1920s, the British physicist Paul Dirac attempted to modify the currently accepted model of the atom by including in it the relativistic properties of electrons. As a result of his analysis, Dirac found that electrons should be expected to exist in two energy states, one positive and one negative. The concept of positive energy presents no problems, of course, but Dirac and other physicists were uncertain as to the meaning of a negative energy state. What did it mean to say that an electron had less than zero energy?

Eventually Dirac concluded that the negative energy state for an electron might imply the existence of a kind of electron that no one had yet imagined, one that is identical with the familiar negatively-charged electron in every respect except its charge. It was, Dirac suggested, an electron with a positive charge, or an antielectron.

Within five years, Dirac's hypothesis had been confirmed. In 1932, the American physicist Carl Anderson found in photographs of a cosmic ray shower the tracks of a particle that satisfied all the properties predicted by Dirac for the antielectron. Anderson suggested the name positron for the new particle.

Other antiparticles

The existence of the positron strongly suggested to scientists that other antiparticles might exist. If there was a positively-charged electron, they asked, why could there also not be a negatively-charged proton... the antiproton. The search for the antiproton took much longer than the search for the antielectron. In fact, it was not until 1955 that Emilio Segre and Owen Chamberlain were able to prove that antiprotons are produced when protons from a powerful cyclotron collide with each other.

The antineutron is a fundamentally different kind of antiparticle than the antielectron or antiproton. Since neutrons have no electrical charge, an antineutron could not differ in this respect from its mirror image. Instead, an antineutron is a particle whose direction of spin is opposite that of the neutron. Since a particle's spin is expressed in the magnetic field that it generates—its magnetic moment—the antineutron is defined as the antiparticle with a magnetic moment equal in magnitude, but opposite in sign, to that of the neutron.

Other antiparticles also exist. For example, the electron is a member of a group of fundamental particles known as the leptons. Other leptons include the mu neutrino (muon) and the tau neutrino (tauon), electron-like particles that exist only at very high energy levels not observed under circumstances of our ordinary everyday world. Both muons and tauons have their own antiparticles, the antimuon and the antitauon.


Antimatter

Given the existence of the antiproton, antineutron, and antielectron, one might imagine the existence of antiatoms, atoms that are identical to the atoms of everyday life but have mirror image electrical charges and mirror image magnetic moments. In fact, some scientists believe that a whole universe of antimatter made of antiatoms may actually exist in some dimensions of which we are not aware.

Locating the existence of such an antiuniverse would be very difficult, however. When an antiparticle comes into contact with its mirror image—an antielectron with an electron, for example—the two particles annihilate each other and their mass is converted to energy. Thus, any time matter comes into contact with antimatter, both are destroyed and converted into energy.


Antiparticles and cosmology

The Swedish physicist Hans Alfvén has studied in some detail the possible role of antiparticles in the creation of the universe. At first glance, one would assume that the number of koinoparticles and antiparticles produced during the big bang would be equal. As it happens, however, the way in which the two classes of particles decay is very slightly different, a difference that would have become more and more important as the nascent universe aged during the first second of creation. Eventually, the very small difference in decay properties between particles might have produced a larger and larger difference, with koinomatter finally winning a predominance in terms of numbers throughout the universe. Until and if scientists can learn more about the presence of antimatter in other parts of the universe, however, questions such as these will remain unanswered.


Resources

books

Alfvén, Hans. Worlds-Antiworlds: Antimatter in Cosmology. San Francisco: W. H. Freeman and Co., 1966.

Hewitt, Paul. Conceptual Physics. New York: Prentice Hall, 2001.

Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.


David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cyclotron

—A machine used to accelerate particles to very high energies, making possible nuclear reactions that do not take place at the energies of everyday life.

Lepton

—A particle that undergoes weak interactions. Three classes of leptons—electrons, muons, and tauons and their antiparticles—are known to exist.

Magnetic moment

—A measure of the strength of a magnetic field.

Neutrino

—An elementary particle with no electrical charge and virtually no mass.

Subatomic particle

—An elementary particle smaller than an atom. Protons, neutrons, and electrons are examples of subatomic particles.

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