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In particle physics, an elementary particle or fundamental particle is a particle not known to have any substructure, thus 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 universe from which all other particles are made. In the Standard Model of particle physics, the elementary particles include the fundamental fermions (including quarks, leptons, and their antiparticles), and the fundamental bosons (including gauge bosons and the Higgs boson). Although elementary particles are not made up of smaller particles, some of them may change to lighter particles (according to specific rules).
Historically, atoms were once regarded as elementary particles. The word "atom" means "that which cannot be cut" from the Greek word atomos. Later, hadrons such as protons and neutrons were considered elementary. A central feature in elementary particle theory is the early 20th century idea of "quanta", which revolutionized the understanding of electromagnetic radiation and brought about quantum mechanics. For mathematical purposes, elementary particles are normally treated as point particles, although some particle theories, such as string theory, posit that elementary particles have a finite nonzero size.
According to the Standard Model, all elementary particles are either bosons or fermions (depending on their spin). The spin-statistics theorem identifies the resulting quantum statistics that differentiates fermions from bosons. According to this methodology: Particles normally associated with matter are fermions. They have half-integer spin and are divided into twelve flavours. Particles associated with fundamental forces are bosons and they have integer spin.
- Elementary fermions (matter particles):
- Elementary bosons (force-carrying particles):
- Other bosons
Of these, a boson that could be of the larger family of Higgs boson was the last discovered and was claimed as the heaviest boson ever found. Additional elementary particles may exist, such as the graviton, which would mediate gravitation. Such particles lie beyond the Standard Model.
Common elementary particles 
Several estimates imply that practically all the matter, when measured by mass, in the visible universe (not including dark matter) is in the protons of hydrogen atoms, and that roughly 1080 protons exist in the visible universe (Eddington number), and roughly 1080 atoms exist in the visible universe. Each proton is, in turn, composed of 3 elementary particles: two up quarks and one down quark. Neutrons and other particles heavier than protons, as well as helium and other atoms with more than one proton, are so rare that their total mass in the visible universe is much less than the total mass of protons in hydrogen atoms. Lighter particles of matter, although equally (electrons) or vastly more (neutrinos) numerous than protons, are so much lighter than protons, that their total mass in the visible universe is again much less than the total mass of all protons.
Some estimates imply that practically all the matter, when measured by numbers of particles, in the visible universe (not including dark matter) is in the form of neutrinos, and that roughly 1086 elementary particles of matter exist in the visible universe, mostly neutrinos. Some estimates imply that roughly 1097 elementary particles exist in the visible universe (not including dark matter), mostly photons, gravitons, and other massless force carriers.
Standard Model 
The Standard Model of particle physics contains 12 flavors of elementary fermions, plus their corresponding antiparticles, as well as elementary bosons that mediate the forces and the Higgs boson, which was reported on July 4, 2012, as having been likely detected by the two main experiments at the LHC (ATLAS and CMS). However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is not known if it is compatible with Einstein's general relativity. There may be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force, and 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||Second generation||Third generation|
|First generation||Second generation||Third generation|
|up quark||u||charm quark||c||top quark||t|
|down quark||d||strange quark||s||bottom quark||b|
There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, the antielectron (positron) e+ is the electron's antiparticle and has an electric charge of +1.
|First generation||Second generation||Third generation|
|First generation||Second generation||Third generation|
|up antiquark||u||charm antiquark||c||top antiquark||t|
|down antiquark||d||strange antiquark||s||bottom antiquark||b|
Isolated quarks and antiquarks have never been detected, 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 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.
However, color-charged particles may combine to form color neutral composite particles called hadrons. A quark may pair up with 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 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 
In the Standard Model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, whereas the Higgs boson (spin-0) is responsible for the intrinsic mass of particles. Bosons differ from fermions in the fact that multiple bosons can occupy the same quantum state(Pauli exclusion principle). Also, Bosons can be either elementary, like photons, or a combination, like mesons. The spin of bosons are integers instead of half integers.
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 hadrons, similar to single quarks. The first evidence for gluons came from annihilations of electrons and antielectrons at high energies, which sometimes produced three jets — a quark, an antiquark, and a gluon.
Electroweak bosons 
There are three weak gauge bosons: W+, W−, and Z0; these mediate the weak interaction. The W bosons are known for their mediation in nuclear decay. The W− converts a neutron into a proton then decay into an electron and electron antineutrino pair. The Z0 does not convert charge but rather changes momentum and is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction. These four gauge bosons form the electroweak interaction among elementary particles.
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). On 4 July 2012, after many years of experimentally searching for evidence of its existence, the Higgs boson was announced to have been observed at CERN's Large Hadron Collider. Peter Higgs who first posited the existence of the Higgs boson was present at the announcement. The Higgs boson is believed to have a mass of approximately 125 GeV. The statistical significance of this discovery was reported as 5-sigma, which implies a certainty of roughly 99.99994%. In particle physics, this is the level of significance required to officially label experimental observations as a discovery. Research into the properties of the newly discovered particle continues.
Non Fundamental Bosons 
The graviton is an unconfirmed boson that would mediate the interaction of gravity. It would have no charge or mass. The antiparticle would be itself and the spin is two, which by definition makes it a boson. Although this particle is not predicted by the Standard Model, quantum field theory points to virtual particles that carry the gravitational force.
In exact unbroken supersymmetry, a fermion should have a boson superpartner. Likewise, each boson should have a fermion superpartner. These superpartners should have the same mass as the corresponding fermion or boson. However, no superpartners have been found in the Standard Model. This problem can be due to supersymmetry being incorrect or not being exact unbroken symmetry. The particles should be able to be recreated using the LHC (Large Hadron Collider) if they do exist.
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, and other more theoretical considerations such as the hierarchy problem. There are many speculative theories beyond the Standard Model that attempt to rectify these deficiencies.
Grand unification 
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 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 by the Large Hadron Collider at CERN.
String theory 
String Theory is a model of physics where all "particles" that make up matter are composed 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 that 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-brane being a string and a 10-brane being a 10-dimensional object) that 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 proposes 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 either are 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 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 graviton.
Technicolor theories try to modify the Standard model in a minimal way by introducing a new QCD-like interaction. This means one adds a new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs-Boson is not an elementary particle but a bound state of these objects.
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, 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.
Acceleron theory 
In theory, neutrinos are influenced by a new force resulting from their interactions with accelerons. Dark energy results as the universe tries to pull neutrinos apart.
See also 
- Asymptotic freedom
- List of particles
- Physical ontology
- Quantum field theory
- Quantum gravity
- Quantum triviality
- UV fixed point
- Mittal, V. Introduction to Nuclear and Particle Physics. PHI Learning Pvt. Ltd. p. 320. ISBN 9788120343115.
- Gribbin, John (2000). Q is for Quantum – An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X.
- Clark, John, E.O. (2004). The Essential Dictionary of Science. Barnes & Noble. ISBN 0-7607-4616-8.
- Veltman, Martinus (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific. ISBN 981-238-149-X.
- Penrose, Roger (1989). The Emperor's New Mind.
- Munafo, Robert. "Notable Properties of Specific Numbers". The Published Data of Robert Munafo. Retrieved 2011-10-12.
- Davies, Lizzy (2011-11-07). "Higgs boson announcement live: Cern scientists discover subatomic particle | Science | guardian.co.uk". Guardian. Retrieved 2012-07-06.
- "Observation of a New Particle with a Mass of 125 GeV | CMS Experiment". Cms.web.cern.ch. Retrieved 2012-07-06.
- "New Theory Links Neutrino's Slight Mass To Accelerating Universe Expansion". www.sciencedaily.com. Retrieved 2008-06-05.
Further reading 
General readers 
- Feynman, R.P. & Weinberg, S. (1987) Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures. Cambridge Univ. Press.
- Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press.
- Brian Greene (1999). The Elegant Universe. W.W.Norton & Company. ISBN 0-393-05858-1.
- John Gribbin (2000) Q is for Quantum – An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X.
- Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume.
- Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ. Press. ISBN 0-8018-7971-X.
- Martinus Veltman (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific. ISBN 981-238-149-X.
- Frank Close (2004). Particle Physics: A Very Short Introduction. Oxford: Oxford University Press. ISBN 0-19-280434-0.
- Seiden, Abraham (2005). Particle Physics – A Comprehensive Introduction. Addison Wesley. ISBN 0-8053-8736-6.
- Bettini, Alessandro (2008) Introduction to Elementary Particle Physics. Cambridge Univ. Press. ISBN 978-0-521-88021-3
- Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.
- Griffiths, David J. (1987) Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4.
- Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5.
- Perkins, Donald H. (2000) Introduction to High Energy Physics, 4th ed. Cambridge Univ. Press.
The most important address about the current experimental and theoretical knowledge about elementary particle physics is the Particle Data Group, where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding.
other pages are:
- Greene, Brian, "Elementary particles", The Elegant Universe, NOVA (PBS)
- particleadventure.org, a well-made introduction also for non physicists
- CERNCourier: Season of Higgs and melodrama
- Pentaquark information page
- Interactions.org, particle physics news
- Symmetry Magazine, a joint Fermilab/SLAC publication
- "Sized Matter: perception of the extreme unseen", Michigan University project for artistic visualisation of subatomic particles
- Elementary Particles made thinkable, an interactive visualisation allowing physical properties to be compared