Term paper on Sub Atomic Particles

Sub Atomic Particles Essays

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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. K mesons, or kaons, have several competing decay modes. Investigations of these processes have led to a better understanding of parity and its nonconservation (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 1960s 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 Finnegans 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 upprotons 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 1970s. 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 Elektronen-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...

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