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In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s (s, s, s, and s), which generally are " particles" and " particles", as well as the fundamental s (s and the ), which generally are "" that mediate among fermions. A particle containing two or more elementary particles is a . Ordinary matter is composed of s, once presumed to be elementary particles—''atomos'' meaning "unable to be cut" in Greek—although the atom's existence remained controversial until about 1905, as some leading physicists regarded molecules as mathematical illusions, and matter as ultimately composed of . Subatomic constituents of the atom were first identified in the early 1930s; the and the , along with the , the particle of . At that time, the recent advent of was radically altering the conception of particles, as a single particle could seemingly span a field , a paradox still eluding satisfactory explanation. Via quantum theory, and s were found to contain s – s and s – now considered elementary particles. And within a , the electron's three (, , ) can separate via the into three (, , and ). Yet a free electron – one which is ''not'' orbiting an and hence lacks – appears unsplittable and remains regarded as an elementary particle. Around 1980, an elementary particle's status as indeed elementary – an ''ultimate constituent'' of substance – was mostly discarded for a more practical outlook, embodied in particle physics' , what's known as science's most experimentally successful theory. Many elaborations upon and theories , including the popular , double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive, although all such s remain undiscovered. Meanwhile, an elementary boson mediating – the – remains hypothetical. Also, according to some hypotheses, spacetime is quantized, so within these hypotheses there probably exist "atoms" of space and time themselves.


Overview

All elementary particles are either s or s. These classes are distinguished by their : fermions obey and bosons obey . Their is differentiated via the : it is for fermions, and for bosons. In the , elementary particles are represented for as s. Though extremely successful, the Standard Model is limited to the microcosm by its omission of and has some parameters arbitrarily added but unexplained.


Cosmic abundance of elementary particles

According to the current models of , the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made up of one up and two down quarks, while protons are made of two up and one down quark. Since the other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all s, in turn consist of up quarks and down quarks. Some estimates imply that there are roughly baryons (almost entirely protons and neutrons) in the observable universe. The number of protons in the observable universe is called the . In terms of number of particles, some estimates imply that nearly all the matter, excluding , occurs in neutrinos, which constitute the majority of the roughly elementary particles of matter that exist in the visible universe. Other estimates imply that roughly elementary particles exist in the visible universe (not including ), mostly photons and other massless force carriers.


Standard Model

The Standard Model of particle physics contains 12 flavors of elementary s, plus their corresponding s, as well as elementary bosons that mediate the forces and the , which was reported on July 4, 2012, as having been likely detected by the two main experiments at the ( and ). 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 's . There may be hypothetical elementary particles not described by the Standard Model, such as the , the particle that would carry the , and s, partners of the ordinary particles.


Fundamental fermions

The 12 fundamental fermions are divided into 3  of 4 particles each. Half of the fermions are s, three of which have an electric charge of −1, called the electron (), the (), and the (); the other three leptons are s (, , ), which are the only elementary fermions with neither electric nor color charge. The remaining six particles are s (discussed below).


Generations


Mass

The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: relative to square of light speed (MeV/c2). For example, the most accurately known quark mass is of the top quark () at or , estimated using the . Estimates of the values of quark masses depend on the version of used to describe quark interactions. Quarks are always confined in an envelope of which confer vastly greater mass to the and where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of the surrounding gluons, slight differences in the calculation make large differences in the masses.


Antiparticles

There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, the (positron) ' is the electron's antiparticle and has an electric charge of +1.


Quarks

Isolated quarks and antiquarks have never been detected, a fact explained by . Every quark carries one of three s of the ; antiquarks similarly carry anticolor. Color-charged particles interact via exchange in the same way that charged particles interact via exchange. However, gluons are themselves color-charged, resulting in an amplification of the strong force as color-charged particles are separated. Unlike the , which diminishes as charged particles separate, color-charged particles feel increasing force. However, color-charged particles may combine to form color neutral s called s. 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 . Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form a color-neutral . Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral . Quarks also carry fractional 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 + or −, whereas antiquarks have corresponding electric charges of either − or +. Evidence for the existence of quarks comes from : firing s at to determine the distribution of charge within s (which are baryons). If the charge is uniform, the 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 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 (-1) bosons (s, s, and the ) mediate forces, whereas the (spin-0) is responsible for the intrinsic of particles. Bosons differ from fermions in the fact that multiple bosons can occupy the same quantum state (). Also, bosons can be either elementary, like photons, or a combination, like s. The spin of bosons are integers instead of half integers.


Gluons

Gluons mediate the , which join quarks and thereby form s, which are either s (three quarks) or s (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form the . Like quarks, gluons exhibit and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons.


Electroweak bosons

There are three s: W+, W, and Z0; these mediate the . The W bosons are known for their mediation in nuclear decay: The W converts a neutron into a proton then decays into an electron and electron-antineutrino pair. The Z0 does not convert particle flavor or charges, but rather changes momentum; it 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 mediates the . 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 at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the collider at . 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 . Through the process of , 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 with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence, the was announced to have been observed at CERN's Large Hadron Collider. 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 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 . Research into the properties of the newly discovered particle continues.


Graviton

The is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due to , it is sometimes included in tables of elementary particles. The conventional graviton is massless, although some models containing massive gravitons exist.


Beyond the Standard Model

Although experimental evidence overwhelmingly confirms the predictions derived from the , some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance the . Theories attempt to resolve these shortcomings.


Grand unification

One extension of the Standard Model attempts to combine the with the into a single 'grand unified theory' (GUT). Such a force would be into the three forces by a . This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification is the existence of , which cause . However, the non-observation of proton decay at the neutrino observatory rules out the simplest GUTs, including SU(5) and SO(10).


Supersymmetry

Supersymmetry extends the Standard Model by adding another class of symmetries to the . These symmetries exchange ic particles with ic ones. Such a symmetry predicts the existence of s, abbreviated as ''s'', which include the s, s, s, and s. Each particle in the Standard Model would have a superpartner whose differs by from the ordinary particle. Due to the , the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing s would not be powerful enough to produce them. However, some physicists believe that sparticles will be detected by the at .


String theory

String theory is a model of physics whereby all "particles" that make up are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according to , the leading version) or 12-dimensional (according to ) 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 '. String theory predicts 1- to 10-branes (a 1- being a string and a 10-brane being a 10-dimensional object) that prevent tears in the "fabric" of space using the (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 7 theoretical dimensions either are very tiny and curled up (and too small to be macroscopically accessible) or simply do not/cannot exist in our universe (because they exist in a grander scheme called the "" 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 .


Technicolor

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 that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to six 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

s are the hypothetical s that integrally link the newfound mass of the to the conjectured to be accelerating the . In this theory, neutrinos are influenced by a new force resulting from their interactions with accelerons, leading to dark energy. Dark energy results as the universe tries to pull neutrinos apart. Accelerons are thought to interact with matter more infrequently than they do with neutrinos.


See also

* * * * * * *


Notes


Further reading


General readers

* & (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. * * (2000) ''Q is for Quantum – An Encyclopedia of Particle Physics''. Simon & Schuster. . *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''. Johns Hopkins University Press. . * * *


Textbooks

* Bettini, Alessandro (2008) ''Introduction to Elementary Particle Physics''. Cambridge Univ. Press. *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. . * *Perkins, Donald H. (2000) ''Introduction to High Energy Physics'', 4th ed. Cambridge Univ. Press.


External links

The most important address about the current experimental and theoretical knowledge about elementary particle physics is the , where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding. * other pages are:
particleadventure.org
a well-made introduction also for non physicists
CERNCourier: Season of Higgs and melodrama

Interactions.org
particle physics news
Symmetry Magazine
a joint / publication
Elementary Particles made thinkable
an interactive visualisation allowing physical properties to be compared {{Authority control Subatomic particles