Elementary particle

In particle physics, an elementary particle refers to a particle of which other, larger particles are composed. For example, atoms are made up of smaller particles known as electrons, protons, and neutrons. The proton and neutron, in turn, are composed of more elementary particles known as quarks. One of the outstanding problems of particle physics is to find the most elementary particles - or the so-called fundamental particles - which make up all the other particles found in Nature, and are not themselves made up of smaller particles.

The Standard Model of particle physics contains 12 species of elementary fermions ("matter particles") and 12 species of elementary bosons ("radiation particles"), plus their corresponding antiparticles. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, and it is possible that some or all of its "elementary" particles are actually composite particles. There might also be other elementary particles not described by the Standard Model, the most prominent being the graviton, the hypothetical particle that carries the gravitational force.

The 12 fundamental fermionic particles are divided into three families of four particles each. Six of the particles are quarks. The remaining six are leptons, three of which are neutrinos, and the remaining three of which have an electric charge of -1: the electron and its two cousins, the muon and the tauon. The particles are:

• first family
  • electron (e^-)
  • electron-neutrino (ν_e)
  • up quark (u)
  • down quark (d)
• second family
  • muon (μ^-)
  • muon-neutrino (ν_μ)
  • charm quark (c)
  • strange quark (s)
• third family
  • tauon (τ^-)
  • tauon-neutrino (ν_τ)
  • top quark (t)
  • bottom quark (b)

(The "families" are called generations.)

There are also 12 fundamental fermionic antiparticles which correspond to these 12 particles. The positron (e⁺) corresponds to the electron and has an electric charge of +1. Its cousins are the positive muon, μ⁺, and the positive tauon, τ⁺. The antiquarks are: up antiquark ${\bar {u}}$, down antiquark ${\bar {d}}$, charm antiquark ${\bar {c}}$, strange antiquark ${\bar {s}}$, top antiquark ${\bar {t}}$, and bottom antiquark ${\bar {b}}$. The antineutrinos are: the electron-antineutrino ${\bar {\nu }}_{e}$, the muon-antineutrino ${\bar {\nu }}_{\mu }$, and the tauon-antineutrino ${\bar {\nu }}_{\tau }$.

Quarks and antiquarks have never been detected to be isolated. A quark can exist paired up to an antiquark, forming a meson: the quark has a "color" (see color charge) and the antiquark a corresponding "anticolor". The color and anticolor cancel out, yielding black (i.e. absence of color charge). Or three quarks can exist together forming a baryon: one quark is "red", another "blue", another "green". These three colors together form white (i.e. absence of color charge). (Cf. RGB color space, complementary color.) Or three antiquarks can exist together forming an antibaryon: one antiquark is "antired", another "antiblue", another "antigreen". These three anticolors together form antiwhite (i.e. absence of color charge). The result is that colors (or anticolors) cannot be isolated either, but quarks do carry colors, and antiquarks carry anticolors.

Quarks also carry fractional electric charges, but isolated fractional charges have never been isolated: quarks always combine to form integral electric charges. Note that quarks have electric charges of either +2/3 or -1/3, whereas antiquarks have corresponding electric charges of either -2/3 or +1/3. Reword: hadrons (both mesons and baryons) always have integral electric charges, even though their components do not, and quarks always appear joined together forming hadrons, and never appear isolated.

According to string theorists [Greene, Elegant Universe], each kind of fundamental particle corresponds to a different resonant vibrational pattern of a string (strings are constantly vibrating in standing wave patterns, similar to the way that quantized orbits of electrons in the Bohr model vibrate in standing wave patterns according to the de Broglie hypothesis). All strings are essentially the same, but different particles differ in the way their strings vibrate. More massive particles correspond to more energetic vibrational patterns. But fundamental particles do not contain strings: they are strings.

However, string theorists also predict the existence of supersymmetric particles, abbreviated as sparticles, which include the selectron, smuon, stauon, sneutrinos, and squarks. The sparticles are heavier (and more energetic) than the ordinary particles: they are so heavy that existing particle colliders would not large (and energetic) enough to be able to detect them. But string theorists currently believe that sparticles will be detected by 2008. Such detections would experimentally confirm superstring theory.

See also

• Subatomic particle
• Particle physics
• List of particles

Reference

• Brian Greene, The Elegant Universe, W.W.Norton & Company, 1999, ISBN 0-393-05858-1.

External links

• Greene, Brian, "Elementary particles". The Elegant Universe, NOVA (PBS)
• particleadventure.org: The Standard Model, Unsolved Mysteries. Beyond The Standard Model, What is the World Made of? The Naming of Quarks
• quarkdance.org ("Cute" dancing quarks with music)
• University of California: Particle Data Group
• particleadventure.org: Particle chart
• CERNCourier: Season of Higgs and melodrama
• 6 December, 2001, BBCNews: 'God particle may not exist Citat: "...its giant accelerator which should have shown up the presence of the Higgs found absolutely nothing - and this could mean particle physics having to revisit some of its most cherished ideas..."
• Milo Wolff: The Physical Origin of Electron Spin - using quantum wave particle structure Citat: "...The electron's structure, as well as its spin, had been a mystery. Providing a physical origin of spin for the first time is the purpose of this paper....note that spin, and other properties, are attributes of the underlying quantum space rather than of the individual particle. This is why spin, like charge, has only one value for all particles...This structure settles a century old paradox of whether particles are waves or point-like bits of matter. They are wave structures in space. There is nothing but space. As Clifford speculated 100 years ago, matter is simply, "undulations in the fabric of space". ..."
• Robert Rutkiewicz: Explaining Particle Spin
• Robert Rutkiewicz: Defining Mass Citat: "...The value of mass is not being redefined. But the concept of mass being a fundamental property is reviewed...A new physical law is postulated: All known particles are elements of momentum moving at a velocity c...This extension is based on special relativity and uses SR equation for mass..."
• The Physical Origin of Electron Spin - using quantum wave particle structure Citat: "...note that spin, and other properties, are attributes of the underlying quantum space rather than of the individual particle. This is why spin, like charge, has only one value for all particles...."
• Glimpses of a new paradigm. K.V.K. Nehru Citat: "...Dewey B. Larson introduces the new paradigm that motion is the basic and sole constituent of the physical universe, and space-time is the content—not the container—of the universe...", Dewey B. Larson (1898-1990)