Subatomic Particle | Research & Encyclopedia Articles

Subatomic Particle

For nearly a century after John Dalton announced his atomic theory in 1803, the concept that matter consists of tiny, indivisible particles (atoms) was widely accepted among scientists. That theory explained nearly all physical phenomena involving matter then known.

However, the theory was profoundly shaken in the 1890s with J. J. Thomson's discovery of the electron. It immediately became obvious that atoms are not indivisible, but, in fact, consist of even smaller particles, the electron being one of them. The existence of the electron implied, furthermore, that at least one other subatomic particle existed. Since electrons are negatively charged and atoms are electrically neutral, it follows that some positively charged subatomic particle must also exist. That particle, the proton, was discovered by Ernest Rutherford in 1919.

Strong evidence also existed for the presence of yet a third subatomic particle. The discrepancy between atomic number (number of protons in an atom) and atomic weight suggested that atomic nuclei might contain a particle approximately equal in mass to the proton, but electrically neutral. This particle, the neutron, was discovered by James Chadwick in 1932 as a result of the bombardment of beryllium atoms with alpha particles.

A new atomic model that included only protons, neutrons, and electrons was attractive to scientists because of its simplicity and because it explained most of the known physical phenomena. Yet, they had almost no time at all to indulge in the luxury of such a simple view of nature. Even before Chadwick's discovery of the neutron, a suggestion had been made that at least one more subatomic particle existed. In 1928, Paul Dirac predicted the existence of a positively-charged electron, the positron. Four years later, at nearly the same time of Chadwick's discovery, Carl David Anderson found Dirac's hypothesized particle in a cosmic ray shower.

Dirac's theory suggested that, like the electron, all particles should have an antiparticle from which they differed only in their electrical charge. The search for the antiproton involved far more difficult technical problems than did the search for the positron. In fact, it was not until 1955 that Owen Chamberlain and Emilio Segré found the antiproton in the products of a reaction conducted with the University of California's bevatron.

The family of subatomic particles was soon to be expanded even more. The Japanese physicist, Hideki Yukawa, developed a theory to explain the force of attraction that holds protons and neutrons together in the nucleus (the "strong" force). His calculations showed that such a force would be carried by a particle whose mass lay between that of an electron and that of a proton.

The search for Yukawa's "medium" particle began almost immediately. Carl Anderson and Seth Neddermeyer started examining cosmic ray photographs for evidence of the particle. In 1938, they thought they had found the particle and suggested the name mesotron, later shortened to meson, for it. Unfortunately, studies showed that the new particle did not interact with protons and neutrons and could not, therefore, be the carrier of the strong force. Anderson and Neddermeyer had, however, discovered another subatomic particle, one that was eventually renamed the mu meson, or muon.

The story of Yukawa's prediction does have a happy ending. In 1947, the English physicist, Cecil Powell, discovered another type of meson in cosmic ray showers. This type of meson did interact with protons and neutrons and did meet Yukawa's criteria. The particle was given the name pi meson, or pion.

At the very time that Yukawa was developing his meson theory, a technological breakthrough was taking place that was to revolutionize the study of subatomic particles. That breakthrough began with the invention of machines that can accelerate particles to very high energies by John Douglas Cockroft and Ernest Walton in England and Ernest Orlando Lawrence in the United States. Experiments conducted with these machines soon revealed the existence of well over 100 new subatomic particles.

Before long, physicists were overwhelmed with the apparent chaos of the "particle zoo" that was being revealed to them. The particles that made up this "zoo" had a bewildering variety of properties. Some, for example, had half-integer spins (1/2, 3/2, 5/2, or 7/2), while others had only integral spins (0, 1, 2). Some existed only in neutral states (for example, the &Lgr;0 and 0), while others existed in one or more charged and, sometimes, neutral states. The pions (&Pgr;+, &Pgr;-, and &Pgr;0) and delta particles (++, +, -, and 0) are examples. Some had very short lives (as short as 10-23 second), while others had lifetimes much longer than would have been expected. Because of their unusually long lifetimes, the latter particles were said to have a property called strangeness . Finally, many of the new particles decayed in ways that suggested the existence of certain basic properties that were (usually) conserved during decay. For example, certain types of heavy particles (baryons ) always decayed in such a way that a property described as baryon number seemed to be conserved.

To add even more confusion to the study of subatomic particles, evidence had long pointed to the existence of certain massless particles. The term itself is difficult to comprehend and has meaning only if one recalls the interconvertability of mass and energy. Albert Einstein 's use of the concept of "packages" of energy-- photons--in 1905 was one of the early references to a particle with no mass. The invention by Wolfgang Pauli in 1931 of the neutrino to explain away mathematical problems with beta decay was another example. Where did these chargeless, massless particles fit into the "zoo"?

Scientists are traditionally uncomfortable with disorganized, "messy " collections of data, as the "particle zoo" had become. So a number of efforts were launched to bring some organization to the " zoo." Some proposals were relatively simple, merely grouping and organizing particles according to common properties. For example, particles were classified according to their spin. Those with half-integral spins were called fermions, while those with integral spins were classified as bosons . All particles that feel the strong force were grouped together as hadrons.

Particles were also grouped according to their mass. The lightest particles were named leptons, those with medium mass, mesons , heavier particles were called baryons, and the heaviest particles of all, hyperons.

These classification systems and this terminology have changed somewhat with time. As scientists have learned more about the composition of various particles, the basis on which they are classified has also been modified.

Probably the most significant effort at organizing the " particle zoo" was the work of Murray Gell-Mann, Yuval Ne'eman, and George Zweig in the 1960s and 1970s. These researchers suggested that hadrons were composed of various combinations of three basic particles. Gell-Mann suggested the whimsical name quark for these particles. In order to account for all known properties of the hadrons, three types of quarks were hypothesized. They were eventually named the up (u), down (d), and strange (s) quarks.

In 1961, Gell-Mann and Zweig independently proposed a scheme for organizing all known subatomic particles, a scheme that became known as the eightfold way. According to this strategy, particles are grouped according to various properties, such as their spin and strangeness. The validity of the Gell-Mann-Ne'eman system was established in 1964 with the discovery of the -particle at the Brookhaven National Laboratory. The -particle had been predicted by Gell-Mann and Ne'eman in order to fill a gap in one of the eightfold way categories. The general theory that hadrons are bound states of smaller particles held together by the strong force is also known as the bootstrap model.

Over the past two decades, physicists have refined their methods of classifying subatomic particles to produce what is now commonly known as the Standard Model. According to the Standard Model, two basic types of particles exist: quarks and leptons. Quarks and leptons are thought to be fundamental particles, in that they have no dimensions. They are similar to geometric points in space. Six kinds of each particle exist, arranged in pairs according to energy level.

Energy level 1 refers to normal, everyday conditions with which we are all familiar. At this energy level, only up and down quarks, the electron and the electron neutrino exist. Energy level 2 is characteristic of cosmic ray events and can be produced by most particle accelerators. At this energy level, two more quarks, called strange and charm, can exist. Two more leptons, the muon and muon neutrino, also exist at energy level 2.

Finally, energy level 3 is attainable in the most powerful of existing particle accelerators. It is characteristic of the energy level that existed at the creation of the universe. At this level, the bottom (or beauty) and top (or truth) quark may exist, along with the tau particle and its companion neutrino. Of all the particles, only the top (or truth) quark has not been completely confirmed by experiment.

According to the Standard Model, all other subatomic particles consist of some combination of quarks and their antiparticles. A proton, for example, is thought to consist of two up quarks and one down quark (uud). Also, the neutral pi meson is made up of an up quark and its antiparticle (uu) or a down quark and its antiparticle (dd). In general, baryons consist of three quarks and mesons of one quark and its corresponding antiquark.

In addition to quarks and leptons, there are groups of other fundamental particles. These mediating particles (the graviton, the photon, the gluons, and W+, W-, and Z⁰) are said to be mediating or virtual particles, or carriers of the four basic forces. Of the mediating particles, one (the photon) has been known for nearly a century, three (the W+, W-, and Z⁰) were discovered in 1983, another group (the gluons) is widely believed to exist, and the last (the graviton) is still a purely speculative concept.

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