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In 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 particles of the Universe from which all larger particles are made. In the Standard Model , the Quark s, Lepton s, and Gauge Boson s are elementary particles.12 Historically, the Hadron s ( Meson s and Baryon s such as the Proton and Neutron ) and even whole Atom s 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 Radiation and brought about Quantum Mechanics . __TOC__ OVERVIEW All elementary particles are either are Fermion s, having Half-integer spin; they are divided into twelve Flavour s. Particles associated with Fundamental Force s are Boson s, having Integer spin.3 :: Lepton s — Electron , Muon , Tau , Electron Neutrino , Muon Neutrino , Tau Neutrino
:: Gauge Boson s – Gluon , W And Z Bosons , Photon ::Other bosons — Higgs Boson , Graviton STANDARD MODEL See Also: Standard Model The Standard Model of particle physics contains 12 flavours of elementary Fermion s, plus their corresponding Antiparticle s, as well as elementary Boson s 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 Sparticle s, Supersymmetric partners of the ordinary particles. Fundamental fermions See Also: fermion The 12 fundamental fermionic flavours are divided into three and the Tau Lepton . Antiparticles See Also: antimatter 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 See Also: quark Quarks and antiquarks have never been detected to be isolated, a fact explained by Confinement . Every quark carries one of three Color Charge s of the Strong Interaction ; antiquarks similarly carry anticolor. Color charged particles interact via Gluon exchange in the same way that charged particles interact via Photon exchange. 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; effectively, they very rarely separate from one another (and when they do they create an energy carrier particle which later converts to two new quarks of different type). However, color charged particles may combine to form color neutral . Quarks also carry fractional Electric Charge s, 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 s at Nuclei to determine the distribution of charge within Nucleon s (which are baryons). If the charge is uniform, the Electric Field around 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. Fundamental bosons See Also: boson In the Standard Model, vector ( Spin -1) bosons ( Gluon s, Photon s, and the W And Z Bosons ) mediate forces, while the Higgs Boson (spin-0) is responsible for particles having intrinsic Mass . Gluons See Also: gluon Gluons are the mediators of the Strong Interaction and carry both Colour and anticolour. Although gluons are massless, they are never observed in Detectors due to Colour Confinement ; rather, they produce Jets of Hadron s, similar to single Quark s. 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. Electroweak bosons See Also: W and Z bosons There are three . The massless Photon mediates the Electromagnetic Interaction . Higgs boson See Also: Higgs boson Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single experiments will 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 Boson and other more theoretical considerations such as the Hierarchy Problem . There are many speculative theories beyond the Standard Model which attempt to rectify these deficiencies. Grand unification See Also: grand unification theory One extension of the Standard Model attempts to combine the Electroweak Interaction with the Strong Interaction into 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-Kamiokande rules out the simplest GUTs, including SU(5) and SO(10). Supersymmetry See Also: supersymmetry Supersymmetry extends the Standard Model by adding an additional class of symmetries to the Lagrangian . These symmetries exchange Fermion ic particles with Boson ic ones. Such a symmetry predicts the existence of supersymmetric particles, abbreviated as ''' Sparticle s''', which include the Slepton s, Squark s, Neutralino s and Chargino s. 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 Collider s would not be powerful enough to produce them. However, some physicists believe that sparticles will be detected when the Large Hadron Collider at CERN begins running. String theory See Also: string theory String Theory is a theory of physics where all normal matter is comprised of strings (measuring at the Planck length) that exist in an 11 dimensional (according to M-theory , the leading version) universe of the 3 extended space,1 extended time, and 6 extra curled up dimensions. The elementary particles are simply strings vibrating at different frequencies which determines mass,electric charge,strong charge,spin and weak charge.A string can be open (a line) or closed,in a loop (a one dimensional sphere), as a string moves through space it sweeps out something called a world sheet. String theory predicts 1-10 branes (a one Brane being a string and a 10 brane being a 10 dimensional object) which prevent tears in the "fabric" of space using the uncertainty principle (eg. the electron orbiting a hydrogen atom is also every where else in the universe). One particular prediction of string theory is the existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string. Another important prediction of string theory is the existence of a massless spin-2 particle behaving like the Graviton . Preon theory See Also: preon 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 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. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s. SEE ALSO REFERENCES EXTERNAL LINKS
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