- Elementary particle
particle physics, an elementary particle or fundamental particle is a not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universefrom which all other particles are made. In the Standard Model, the quarks, leptons, and gauge bosons are elementary particles. [cite book | author=Gribbin, John | title=Q is for Quantum - An Encyclopedia of Particle Physics | publisher=Simon & Schuster | year=2000 | id=ISBN 0-684-85578-X] [cite book | author=Clark, John, E.O. | title=The Essential Dictionary of Science | publisher=Barnes & Noble | year=2004 | id=ISBN 0-7607-4616-8]
hadrons ( mesons and baryons such as the protonand neutron) and even whole atoms were once regarded as elementary particles. A central feature in elementary particle theory is the early 20th century idea of "quanta", which revolutionised the understanding of electromagnetic radiationand brought about quantum mechanics.
All elementary particles are either
bosons or fermions (depending on their spin). The spin-statistics theoremidentifies the resulting quantum statisticsthat differentiates fermions from bosons. According to this methodology: particles normally associated with matterare fermions, having half-integerspin; they are divided into twelve flavours. Particles associated with fundamental forces are bosons, having integerspin. [cite book | author=Veltman, Martinus | title=Facts and Mysteries in Elementary Particle Physics | publisher=World Scientific | year=2003 | id=ISBN 981-238-149-X]
Fermions::: Quarks — up, down, charm, strange, top, bottom:: Leptons — electron neutrino, electron, muon neutrino, muon, tau neutrino, tau
Bosons::: Gauge bosons — gluon, W and Z bosons, photon::Other bosons — Higgs boson, graviton
The Standard Model of particle physics contains 12 flavours of elementary
fermions, plus their corresponding antiparticles, as well as elementary bosons 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.
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 muonand the tau lepton.
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 and so on:
Quarks and antiquarks have never been detected to be isolated, a fact explained by confinement. Every quark carries one of three
color charges of the strong interaction; antiquarks similarly carry anticolor. Color charged particles interact via gluonexchange in the same way that charged particles interact via photonexchange. However, gluons are themselves color charged, resulting in an amplification of the strong force as color charged particles are separated. Unlike the electromagnetic force which diminishes as charged particles separate, color charged particles feel increasing force.
However, color charged particles may combine to form color neutral
composite particles called hadrons. A quark may pair up to an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral meson. Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form a color-neutral baryon. Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral antibaryon.
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 the existence of quarks comes from
deep inelastic scattering: firing electrons at nuclei to determine the distribution of charge within nucleons (which are baryons). If the charge is uniform, the electric fieldaround the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and a jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks.
In the Standard Model, vector (spin-1) bosons (
gluons, photons, and the W and Z bosons) mediate forces, while the Higgs boson(spin-0) is responsible for particles having intrinsic mass.
Gluons are the mediators of the
strong interactionand carry both colour and anticolour. Although gluons are massless, they are never observed in detectors due to colour confinement; rather, they produce jets of hadrons, similar to single quarks. The first evidence for gluons came from annihilations of electrons and positrons at high energies which sometimes produced three jets — a quark, an antiquark, and a gluon.
There are three
weak gauge bosons: "W+", "W−", and "Z0"; these mediate the weak interaction. The massless photonmediates the electromagnetic interaction.
Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single
electroweak forceat high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY. The differences at low energies is a consequence of the high masses of the "W" and "Z" bosons, which in turn are a consequence of the Higgs mechanism. 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 photon). Although the Higgs mechanism has become an accepted part of the Standard Model, the Higgs bosonitself has not yet been observed in detectors. Indirect evidence for the Higgs boson suggests its mass lies below 200-250 GeV. [ [http://www.newscientist.com/article.ns?id=dn5095 Quark experiment predicts heavier Higgs] ] In this case, the LHC experiments may be able to discover this last missing piece of the Standard Model.
Beyond the Standard Model
Although all experimental evidence confirms the predictions of the
Standard Model, many physicists find this model to be unsatisfactory due to its many undetermined parameters, many fundamental particles, the non-observation of the Higgs bosonand other more theoretical considerations such as the hierarchy problem. There are many speculative theories beyond the Standard Model which attempt to rectify these deficiencies.
One extension of the Standard Model attempts to combine the
electroweak interactionwith the strong interactioninto a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism. The most dramatic prediction of grand unification is the existence of X and Y bosons, which cause proton decay. However, the non-observation of proton decay at Super-Kamiokanderules out the simplest GUTs, including SU(5) and SO(10).
Supersymmetry extends the Standard Model by adding an additional class of symmetries to the
Lagrangian. These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of 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. Due to the breaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. However, some physicists believe that sparticles will be detected when the Large Hadron Colliderat CERNbegins running.
String Theory is a theory of physics where all "particles" that make up matter and energy are comprised of strings (measuring at the Planck length) that exist in an 11-dimensional (according to
M-theory, the leading version) universe. These strings vibrate at different frequencies which determine mass, electric charge, color charge, and spin. A string can be open (a line) or closed in a loop (a one-dimensional sphere, like a circle). As a string moves through space it sweeps out something called a "world sheet". String theory predicts 1- to 10-branes (a 1- branebeing a string and a 10-brane being a 10-dimensional object) which prevent tears in the "fabric" of space using the uncertainty principle(e.g. the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in the universe at any given moment).
String theory posits that our universe is merely a 4-brane, inside which exist the 3 space dimensions and the 1 time dimension that we observe. The remaining 6 theoretical dimensions are either very tiny and curled up (and too small to affect our universe in any way) or simply do not/cannot exist in our universe (because they exist in a grander scheme called the "multiverse" outside our known universe).
Some of predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like the
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, which is derived from "pre-quarks". In essence, preon theory tries to do for the Standard Modelwhat the Standard Model did for the particle zoothat 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. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s.
List of particles
Feynman, R.P. & Weinberg, S. (1987). "Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures", New York: Cambridge University Press.
* Greene, Brian, " [http://www.pbs.org/wgbh/nova/elegant/part-flash.html Elementary particles] ". The Elegant Universe, NOVA (
* [http://particleadventure.org/frameless/standard_model.html particleadventure.org: The Standard Model]
* [http://particleadventure.org/frameless/beyond_start.html Unsolved Mysteries. Beyond The Standard Model]
* [http://particleadventure.org/frameless/quarknaming.html What is the World Made of? The Naming of Quarks]
* [http://particleadventure.org/frameless/chart.html particleadventure.org: Particle chart]
* [http://pdg.lbl.gov/ University of California: Particle Data Group]
* [http://www.cerncourier.com/main/article/41/2/17 CERNCourier: Season of Higgs and melodrama]
* [http://plato.phy.ohiou.edu/~hicks/thplus.htm Pentaquark information page]
* [http://www.interactions.org Interactions.org] Particle physics news
* [http://www.symmetrymagazine.org Symmetry Magazine] , a joint
* [http://abctsau.org/RelationshipGravitonPhoton.pdf Energy relationship between photons and gravitons]
* [http://www-personal.umich.edu/~janhande/sizedmatter/sizedmatter_images.htm "Sized Matter: perception of the extreme unseen"] — Michigan University project for artistic visualisation of subatomic particles.
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