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In Particle Physics , an elementary particle is one of a wide variety of particles simpler than Atom s. For example, atoms are made up of smaller particles known as Electron s, Protons , and Neutron s. The proton and neutron, in turn, are composed of more elementary particles known as Quark s. One of the outstanding problems of particle physics is to find the most elementary particles, 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 See Also: Standard Model The Standard Model of particle physics contains 12 flavours of elementary Fermion s (" Matter particles"), 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 can never separate from one another. 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 Color and anticolor. Although gluons are massless, they are never observed in Detectors due to 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 Electroweak Force at 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 Boson itself has not yet been observed in detectors. Indirect evidence for the Higgs boson suggests its mass lies below about 200 GeV. In this case, the LHC 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 Particle s, 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 Boson s, 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 According to String Theorists , each kind of fundamental particle corresponds to a different pattern of fundamental string. All strings are essentially the same, although they may be open (lines) or closed (loops). Different particles differ in the coordination of their strings. Modern string theories include Supersymmetry , making them Superstring Theories . 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 . By predicting Gravity , string theory unifies Quantum Mechanics with General Relativity , making it the first consistent theory of Quantum Gravity . One problem with string theory is that it predicts that the number of Dimension s for Spacetime much greater than 4 (the number of observed dimensions). These Extra Dimensions are supposedly Compactified or rolled-up. Other related theories such as Brane theories contain extended extra dimensions, which are hidden from us by our confinement to a brane. 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 1980's. LINKS AND REFERENCES Reference
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