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elementary particle
- The Elementary Particles is also a novel by Michel Houellebecq, translated into English by Frank Wynne
In particle physics, an elementary particle is 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.
Standard Model
(main article with table of particles: Standard Model)
The Standard Model of particle physics contains 12 flavours of elementary fermions ("matter particles"), plus their corresponding antiparticles, as well as elementary bosons ("radiation particles") that mediate the forces and the still undiscovered Higgs boson. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is fundamentally incompatible with Einstein's general relativity. There are likely to be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force or the sparticles, supersymmetric partners of the ordinary particles.
Fundamental fermions
The 12 fundamental fermionic flavours are divided into three generations 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 tau.
First generation
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Second generation
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Third generation
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Antiparticles
There are also 12 fundamental fermionic antiparticles which correspond to these 12 particles. The positron e<sup>+</sup> corresponds to the electron and has an electric charge of +1 and so on:
First generation
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Second generation
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Third generation
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Quarks
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. neutral). 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 since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. 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.
Evidence for quarks comes from firing electrons at hydrogen nuclei (essentially a proton) to determine the distribution of charge within a proton. If the charge is uniform, the electrostatic field around the proton should be uniform and the electron should scatter elastically. Low energy electrons do scatter in this way and the protons recoil, but above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and a jet of fundamental particles is emitted. If protons can do this to electrons, it suggests that the charge in the proton is not uniform but split in between even smaller charged particles, i.e quarks.
Fundamental bosons
In the Standard model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, while the Higgs Boson is responsible for particles having mass.
Gluons
Gluons are the mediators of the strong force, and carry both a color and an anticolor. Although gluons are massless, they are never observed in detectors due to confinement; rather, they produce jets of hadrons, similarly to single quarks.
Electroweak bosons
There are three weak gauge bosons: W<sup>+</sup>, W<sup>-</sup>, and Z<sup>0</sup>; these mediate the weak force. The massless photon mediates the electromagnetic force.
Higgs boson
Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to be unified as a single electroweak force at high energies. The reason for this difference at low energies is thought to be due to the existence of the higgs boson. Through the process of spontaneous symmetry breaking, the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain massless (the electromagnetic photon). Although the Higgs mechanism has become an accepted part of the Standard Model, the boson itself has never been observed in detectors. This is thought to be due to the particle's great mass, but its continuing absence is a major cause of concern for particle physicists.
Beyond the Standard Model
Supersymmetry
One major extension of the standard model involves supersymmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos and charginos. Each particle in the Standard Model would have a superpartner whose spin differs by 1/2 from the ordinary particle. In addition, the sparticles are heavier than their ordinary counterparts: they are so heavy that existing particle colliders would not be powerful enough to be able to detect them. However, some physicists believe that sparticles will be detected by 2008 in the Large Hadron Collider at CERN.
String theory
According to string theorists, each kind of fundamental particle corresponds to a different resonant vibrational pattern of a fundamental 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). 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.
String theory also predicts the existence of gravitons. Gravitons are practically impossible to detect experimentally, because the gravitational force is so weak compared to the other forces.
Preon theory
According to Preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called "Preons" for which is derived from "pre-quarks". In essence, Preon theory tries to do for the Standard Model what the Standard Model did for the particle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to half a dozen more fundamental particles and the rules that govern their interactions.
While the methodology in string theory is typically to attempt to build a complete mathematical structure from the ground up, a Preon Theory typically looks for patterns in the Standard Model itself and tries to find simple models that can mimic those patterns.
Links and References
Reference
- Brian Greene, The Elegant Universe, W.W.Norton & Company, 1999, ISBN 0-393-05858-1.
See also
External links
Informational
- 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
- University of California: Particle Data Group
- particleadventure.org: Particle chart
- CERNCourier: Season of Higgs and melodrama
- Pentaquark information page
