The atom, although small in size and great in number, is one of the greatest enigmas in the science world today. Over 200 different subatomic particles have been found, and scientists are still looking for more. The most basic parts of the atom are the electron, the proton and the neutron. These three make up a small group of the know subatomic particles. Of these three only the electron is actually a fundamental particle. The proton and neutron are both hadrons composed of different smaller particles called quarks.

Any of the subatomic particles that are built from quarks, and thus react through strong nuclear force, are hadrons. The hadrons include mesons and baryons. All known subatomic particles except bosons and leptons, are hadrons. Except for protons and for neutrons that are bound in nuclei, all hadrons have short lives and are produced in the high-energy collisions of subatomic particles (Carrigan 35).

The other three basic forces of nature also affect hadron behavior: all are subject to gravitation; charged hadrons obey electromagnetic laws; and some hadrons break up by way of the weak nuclear force, while others decay via the strong electromagnetic forces. Mesons are any member of a family of subatomic particles composed of an even number of quarks and antiquarks. Mesons are sensitive to the strong force because their constituent quarks are strongly interacting. Mesons consist of an even number of quarks with half-integral spin, and so they have integral spin (Martin 157). They vary widely in mass, ranging from 140 MeV to nearly 10 GeV. Various types of mesons have been discovered since their existence was first predicted in 1935 by the Japanese physicist Yukawa Hideki.

Of those so far identified, the pi meson and the K meson are the most important. Pi mesons, also known as pions, are chiefly responsible for the strong interactions between the protons and neutrons in atomic nuclei. mesons, or kaons, have several competing decay modes. Investigations of these processes have led to a better understanding of parity and its non conservation (Carrigan 143). Mesons serve as a useful tool for studying the properties and interactions of quarks, the fundamental units of matter that constitute all hadrons (any of the subatomic particles that react by the force of strong interaction). Although mesons are unstable, many last long enough (a few billionths of a second) to be observed with particle detectors, making it possible for researchers to reconstruct the motions of quarks.

Any model attempting to explain quarks must correctly interpret the behavior of mesons. One of the early successes of the Eightfold Way, a forerunner of modern quark models devised by the physicists Murray Gell-Mann and Yuval Ne " eman, was the prediction and subsequent discovery of the eta meson (1962) (Carrigan 156). Some years later the decay rate of the pi meson into two photons was used to support the hypothesis that quarks can take on one of three colours. Surprises in meson behavior are also important, as attested by the study of CP violation (the violation of the combined conservation laws associated with charge [C] and parity [P]) in the K-meson system. Quarks are any of a group of subatomic particles believed to be among the fundamental constituents of matter. In much the same way that protons and neutrons make up atomic nuclei, these particles themselves are thought to consist of quarks (Martin 187).

Quarks constitute all hadrons (baryons and mesons) all particles that interact by means of the strong force, the force that binds the components of the nucleus. According to prevailing theory, quarks have mass and exhibit a spin equal to one-half the basic quantum mechanical unit of angular momentum. The latter property implies that they obey the Pauli exclusion principle, which states that no two particles having half-integral spin can exist in exactly the same quantum state. Quarks appear to be truly fundamental.

They have no apparent structure; that is, they cannot be resolved into something smaller (Carrigan 113). Quarks always seem to occur in combination with other quarks or antiquarks, never alone. For years physicists have attempted to knock a quark out of a baryon in experiments with particle accelerators to observe it in a free state but have not yet succeeded in doing so. Throughout the 1960 s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, Murray Gell-Mann of the United States and Yuval Ne " eman of Israel, proposed a particle classification scheme called the Eightfold Way, based on the mathematical symmetry group SU (3), that described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, adopting the term from a passage in James Joyce's novel Finnegan's Wake.

(The American physicist George Zweig developed a similar theory independently that same year and called his fundamental particles "aces." ) Gell-Mann's model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It postulated the existence of three types of quarks, distinguished by distinctive "flavors." These three quark types are now commonly designated as "up" (u), "down" (d), and "strange" (s). Each carries a fractional electric charge (i. e. , a charge less than that of the electron) (Martin 132). The up and down quarks are thought to make up protons and neutrons and are thus the ones observed in ordinary matter.

Strange quarks occur as components of K mesons and various other extremely short-lived subatomic particles that were first observed in cosmic rays but that play no part in ordinary matter. The interpretation of quarks as actual physical entities posed two major problems. First, quarks had to have half-integral spin for the model to work, but at the same time they seemed to violate the Pauli exclusion principle. In many of the baryon configurations constructed of quarks, sometimes two or even three identical quarks had to be set in the same quantum state-an arrangement prohibited by the exclusion principle. Second, quarks appeared to defy being freed from the particles they made up. Although the forces binding quarks were strong, it seemed improbable that they were powerful enough to withstand bombardment by high-energy electrons and neutrinos from particle accelerators (Fraser 75).

These problems were resolved by the introduction of the concept of colour, as formulated in quantum chromodynamics (QCD). In this theory of strong interactions, developed in 1977, the term colour has nothing to do with the colours of the everyday world but rather represents a special quantum property of quarks. The colours red, green, and blue are ascribed to quarks, and their opposites, minus-red, minus-green, and minus-blue, to antiquarks. According to QCD, all combinations of quarks must contain equal mixtures of these imaginary colours so that they will cancel out one another, with the resulting particle having no net colour.

A baryon, for example, always consists of a combination of one red, one green, and one blue quark. The property of colour in strong interactions plays a role analogous to an electric charge in electromagnetic interactions (Martin 190). Charge implies the exchange of photons between charged particles. Similarly, colour involves the exchange of massless particles called gluons among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution.

The binding forces carried by the gluons tend to be weak when quarks are close together. At a distance of approximately 10-13 cm-about the diameter of a proton-quarks behave as though they were free (Fraser 217). This condition is called asymptotic freedom. When one begins to draw the quarks apart, however, as if attempting to knock them out of a proton, the force grows stronger. This is in direct contrast to the electromagnetic force, which grows weaker with the square of the distance between the interacting bodies.

As explained by QCD, gluons have the ability to create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an accelerated particle, the gluons utilize energy that they draw from the quark's motion to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines. Although QCD cogently explains the behavior of quarks and provides a means of calculating their basic properties, it does not account for the flavors of "charm" and "bottom" associated with two types of heavy quarks that were found in the late 1970 s. The discovery of the charmed (c) and bottom (b) quarks and their associated antiquarks, achieved through the creation of mesons, strongly suggests that quarks occur in pairs.

This speculation led to efforts to find a sixth type of quark called "top" (t), after its proposed flavor. According to theory, the top quark carries a + 2/3 electric charge; its partner, the bottom quark, has a charge of - 1/3. In 1995 two independent groups of scientists at Fermi National Accelerator Laboratory, Batavia, Illinois, reported that they had found the top quark. A weighted average of their results gives the top quark a mass of 176 +/- 12 GeV (billion electron volts). (The next heaviest quark, the bottom, has a mass of 4. 8 GeV.

) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the prevailing theoretical scheme of nature's fundamental building blocks. Baryons are any member of one of two classes of hadrons (particles built from quarks and thus experiencing the strong nuclear force). Baryons are heavy subatomic particles that are made up of three quarks. Both protons and neutrons, as well as other particles, are baryons.

(The other class of hadronic particle is built from a quark and an antiquark and is called a meson. ) Baryons are characterized by a baryon number, B, of 1 (Martin 89). Their antiparticles, called antibaryons, have a baryon number of -1. An atom containing, for example, one proton and one neutron (each with a baryon number of 1) has a baryon number of 2. In addition to their differences in composition, baryons and mesons can be distinguished from one another by spin: the three quarks that make up a baryon can only produce half-integer values, while meson spins always add up to integer values.

Gluons are the so-called messenger particle of the strong nuclear force, which binds subatomic particles known as quarks within the protons and neutrons of stable matter as well as within heavier, short-lived particles created at high energies. Quarks interact by emitting and absorbing gluons, just as electrically charged particles interact through the emission and absorption of photons. In quantum chromodynamics (QCD), the theory of the strong force, the interactions of quarks are described in terms of eight types of massless gluon, which, like the photon, all carry one unit of intrinsic angular momentum, or spin. Like quarks, the gluons carry a "strong charge" known as colour; this means that gluons can interact between themselves through the strong force. In 1979 confirmation of the conception came with the observation of the radiation of gluons by quarks in studies of high-energy particle collisions at the German national laboratory, Deutsches Elektro nen-Synchrotron (DESY;" German Electron-Synchrotron), in Hamburg.

Leptons are any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Like all fermions, leptons have a half-integral spin. (In quantum-mechanical terms, spin constitutes the property of intrinsic angular momentum. ) Leptons obey the Pauli exclusion principle, which prohibits any two identical fermions in a given population from occupying the same quantum state.

Leptons are said to be fundamental particles; that is, they do not appear to be made up of smaller units of matter. Leptons can either carry one unit of electric charge or be neutral. The charged leptons are the electrons, muons, and taus. Each of these types has a negative charge and a distinct mass.

Electrons, the lightest leptons, have a mass only 0. 0005 that of a proton. Muons are heavier, having more than 200 times as much mass as electrons (Schwarz 22). Taus, in turn, are approximately 3, 700 times more massive than electrons. Each charged lepton has an associated neutral partner, or neutrino (i. e.

, electron-, muon-, and tau-neutrino), that has no electric charge and no significant mass. Moreover, all leptons, including the neutrinos, have antiparticles called antileptons. The mass of the antileptons is identical to that of the leptons, but all of the other properties are reversed (Fraser 98). The total number of leptons appears to remain the same in every particle reaction.

Mathematically, total lepton number L (the number of leptons minus the number of antileptons) is constant. In addition, a conservation law for leptons of each type seems to hold. The number of electrons and electron neutrinos, for example, is conserved separately from the number of muons and mu-neutrinos. The current limit of violation of this conservation law is one part per million.

The electroweak theory of electromagnetic and weak interactions, proposed during the late 1960 s, has enabled physicists to better understand the interactions of leptons. This apparent theoretical conquest, however, has also generated a host of new questions. Other, more recent theoretical schemes seeking to intertwine strong interactions with the weak and the electromagnetic have had a similar effect. A law alike to that of the conservation of lepton number exists for strongly interacting fermions, the baryons (e. g. , protons).

The new "grand unified" theories suggest that a proton decays into leptons and other particles, thereby simultaneously violating lepton and baryon number conservation (Schwarz 56). In such theories the quantity B - L, the number of baryons minus the number of leptons, is conserved. Neutrinos are a type of fundamental particle with no electric charge, little or no mass, and one-half unit of spin. Neutrinos belong to the family of particles called leptons, which are not subject to the strong nuclear force. There are three types of neutrino, each associated with a charged lepton-i. e.

, the electron, muon, and tau (Martin 52). The electron-neutrino was proposed in 1930 by the Austrian physicist Wolfgang Pauli to explain the apparent loss of energy in the process of beta decay, a form of radioactivity. The Italian-born physicist Enrico Fermi further elaborated the proposal and gave the particle its name (Schwarz 103). An electron-neutrino is emitted along with a positron in positive beta decay, while an electron-antineutrino is emitted with an electron in negative beta decay. Neutrinos are the most penetrating of subatomic particles because they react with matter only through the weak interaction. Neutrinos do not cause ionization, because they are not electrically charged (Martin 134).

Only 1 in 10 billion, traveling through matter a distance equal to the Earth's diameter, reacts with a proton or neutron. Electron-neutrinos were first experimentally observed in 1956, when a beam of antineutrinos from a nuclear reactor produced neutrons and positrons by reacting with protons (Fraser 110). Another type of neutrino, produced when pi mesons (pions) decay, was conclusively shown (1962) to be a different species: the muon-neutrino. Although they are as un reactive as the other neutrinos, muon-neutrinos were found to produce muons but never electrons when they react with protons and neutrons. The American physicists Leon Lederman, Melvin Schwartz, and Jack Steinberg er received the 1988 Nobel Prize for Physics for having established the identity of muon neutrinos. In the mid-1970 s, particle physicists discovered yet another variety of charged lepton, the tau.

A tau-neutrino and tau-antineutrino are associated with this third charged lepton (Schwarz 78). All types of neutrino have masses much smaller than those of their charged partners, if they have any mass at all (Martin 76). For example, experiments show that the mass of the electron-neutrino must be less than 0. 0004 that of the electron. There is, however, no compelling theoretical reason for the mass of the neutrino to be exactly zero.

Indeed, the shortfall in the number of neutrinos detected on Earth from the nuclear reactions in the core of the Sun may well be able to be explained if one or more of the neutrino types has a small mass. Antimatter are substances composed of atoms made up of elementary particles that have the mass and charge of electrons, protons, or neutrons, their counterparts in ordinary matter, but for which the charge is opposite in sign. Such particles are called positrons (e+), antiprotons (p), and antineutrons (), or, collectively, antiparticles (Fraser 56). Matter and antimatter cannot coexist at close range for more than a small fraction of a second because they annihilate each other with release of large quantities of energy. It has been suggested that some distant galaxies may be composed entirely of antimatter. The concept of antimatter first arose in analysis of the duality between positive and negative charge.

The work of P. A. M. Dirac on the energy states of the electron led to the prediction and, finally, to laboratory production of a particle identical in every respect but one to the electron, that is, with positive instead of negative charge. Such a particle, called the positron (e+), is not found in ordinary matter (Feinberg 45). The life expectancy or duration of the positron in ordinary matter is very short.

Unless the positron is moving extremely fast, it will be drawn close to an ordinary electron by the attraction between opposite charges. A collision between the positron and electron results in their simultaneous disappearance, their masses being converted into energy in accordance with the Einstein relation E = mc 2, where c is the velocity of light. This process is called annihilation, and the resultant energy is emitted in the form of high-energy quanta of electromagnetic radiation or gamma rays. The inverse reaction e+ + e- can also proceed under appropriate conditions, and the process is called electron-positron creation (Martin 143). This last process is the one commonly used to produce positrons in the laboratory. The electrical properties of antimatter are opposite to those of ordinary matter; thus, for example, the antiproton (p) has a negative charge, and the antineutron (), although electrically neutral, has a magnetic moment opposite in sign to that of the neutron (Fraser 65).

The Dirac theory of electrons and positrons predicts that an electron and a positron, because of Coulomb attraction, will bind together into an atom just as an electron and a proton form a hydrogen atom. The e+e- bound system is called positronium; its annihilation into gamma rays has been observed. Its lifetime is of the order of 10-7 second or 10-10 second, depending on the orientation of the two particles. These lifetimes agree well with those computed from Dirac's theory.

Both protons and neutrons are described by the Dirac equation. Antiprotons can be produced by bombarding protons with protons. If enough energy is available, that is, if the incident proton has a kinetic energy of at least 5. 6 GeV (5. 6 109 electron volts), extra particles of proton mass appear according to the formula E = mc 2. Such energies became available in the 1950 s at the Berkeley Bevatron (Feinberg 45).

In 1955 a team of physicists led by Owen Chamberlain and Emilio S egr observed that antiprotons are produced by high-energy collisions. Antineutrons also were discovered at the Berkeley Bevatron by observing their annihilation in matter with a consequent release of high energies. By the time the antiproton was discovered, a host of new subatomic particles had also been discovered; all these particles are now known to have corresponding antiparticles (Fraser 24). Thus, there are positive and negative muons, positive and negative pions (also called pi-mesons), the K-meson and the anti-K-meson, plus a long list of baryons and antibaryons. Most of these newly discovered particles have too short a lifetime for them to be able to combine with electrons. The exception is the positive muon that together with an electron has been observed to form a muonium atom.

In 1995 physicists at the European Laboratory for Particle Physics (CERN) created the first antiatom, the antimatter counterpart of an ordinary atom-in this case, antihydrogen, the simplest antiatom, consisting of a positron in orbit around an antiproton nucleus (Martin 98). They did so by firing antiprotons through a xenon gas jet. Some of the antiprotons collided with protons in the xenon nuclei, creating pairs of electrons and positrons; a few of the positrons thus produced then combined with the antiprotons to form antihydrogen. Each antiatom produced survived for only about forty-billionths of a second before it came into contact with ordinary matter and was annihilated. Many attempts have been made to investigate the importance of antimatter in cosmological problems; theoretical and experimental knowledge of matter and antimatter is relevant to the understanding of the creation and constitution of the universe (Martin 56). Obviously no star can contain a close mixture of matter and antimatter; otherwise it would instantaneously explode with more violence than a supernova.

Interstellar gas, and even intergalactic gas, cannot be a mixture, either. This is because among the annihilation products of proton plus antiproton into pions there is a certain amount of neutral pions (0), which in turn decay into two energetic gamma rays. Satellite experiments have not detected enough of such gamma rays to suggest a significant amount of antimatter annihilation. One could resort to the hypothesis that matter and antimatter are separated on the scale of clusters of galaxies. The creation of baryon-antibaryon pairs, however, is very localized, the particle and antiparticle being created at distances of approximately 10-13 centimetres. No present understanding of the evolution of the universe can explain the un mixing of matter and antimatter if they had been originally created together (Carrigan 214).

But the presence of large amounts of antimatter in the universe cannot be ruled out completely, nor can the possibility that some cosmic sources of intense radiation might be due to the interpenetration of matter and antimatter. But it can be shown that the total relative amount of antimatter in the Milky Way Galaxy must be less than one part in 107. Soon after the discovery of the antiproton the question was raised as to whether antimatter would be subject to gravitational attraction or repulsion from ordinary matter (Schwarz 97). This question is of extreme importance because gravitational repulsion between matter and antimatter is inconsistent with the theory of general relativity. The answers to such questions can be obtained experimentally because of the properties of K 0 and K 0 mesons.

Observation of the interference phenomena between K 01 and K 02 led to the conclusion, by M. L. Good, that the gravitational interaction between matter and antimatter is identical to that between matter and matter. Bibliography Carrigan, Richard. Particles and Forces: At the Heart of Matter. New York: W.

H. Freeman, 1990. Feinberg, Gerald. What is the World Made Of: Atoms, Leptons, Quarks, and Other Tantalizing Particles. Garden City, N. Y.

: Anchor Press/Doubleday, 1977. Fraser, Gordon. The Quark Machines: How Europe Fought the Particle Physics War. Philadelphia, PA: Institute of Physics Pub. , 1997.

Martin, B. R. Particle physics. New York: Wiley, 1992. Schwarz, Cindy.

A Tour of the Subatomic Zoo: A Guide to Particle Physics. New York: American Institute of Physics, 1992.