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.
(main article with table of particles: Standard Model)
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 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 other 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.
The 12 fundamental
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.
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 , down antiquark , charm antiquark , strange antiquark , top antiquark , and bottom antiquark . The antineutrinos are: the electron-antineutrino , the muon-antineutrino , and the tauon-antineutrino .
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). A more recent discovery is the five quark baryon state, created in Jefferson lab. It consists of two up quarks, two down quarks, and one anti-strange quark. These colors cancel out to form white. 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.
Out of the 12 bosonic fundamental particles, eight of them are 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 like single quarks.
Out of the remaining four fundamental bosons, three are weak gauge bosons: W+, W-, and Z0; these mediate the weak force. The last fundamental boson is the photon, which mediates the electromagnetic force.
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
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.
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.
Links and References
- Brian Greene, The Elegant Universe, W.W.Norton & Company, 1999, ISBN 0-393-05858-1.