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Encyclopedia > Standard Model
For the Standard Model in Cryptography, see Standard Model (cryptography).
For the Standard Model in Cosmology, see the article on the Big Bang.

It is understood that the entire dynamics of the universe can be explained in terms of matter and by the forces that act on it. For pedagogical purposes, in this article, the Standard Model is divided in a similar manner: ordinary matter particles (fermions), force mediating particles (bosons), and the Higgs particle (also a boson). Fermions, named after Enrico Fermi, are particles which form totally-antisymmetric composite quantum states. ... Boson (game) Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. ... The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ...

Technically, quantum field theory provides the mathematical framework for the Standard Model. Consequently, each type of particle is described in terms of a mathematical field.

### Particles of Ordinary Matter

The matter particles described by the Standard Model all have an intrinsic spin whose value is determined to be 1/2, making them fermions. For this reason, they follow the Pauli Exclusion Principle. Apart from their antiparticle partners, a total of twelve different matter particles are known as of early 2007. Six of these are classified as quarks (up, down, strange, charm, top and bottom), and the other six as leptons (electron, muon, tau, and their corresponding neutrinos). The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925, which states that no two identical fermions may occupy the same quantum state simultaneously. ... Corresponding to each kind of particle, there is an associated antiparticle with the same mass and opposite charges. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the &#931;c++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... The up quark is a first-generation quark with a charge of +(2/3)e. ... The down quark is a first-generation quark with a charge of -(1/3)e. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the &#931;c++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... The charm quark is a second-generation quark with a charge of +(2/3)e. ... The top quark is the third-generation up-type quark with a charge of +(2/3)e. ... The bottom quark is a third-generation quark with a charge of -(1/3)e. ... A lepton is also a unit of currency. ... The electron is a fundamental subatomic particle that carries an electric charge. ... The moons shadow, as seen in muons 700m below ground at the Soudan 2 detector. ... The tau lepton (often called the tau or occasionally the tauon) is a negatively charged elementary particle with a lifetime of 3Ã—10âˆ’13 seconds and a high mass of 1777 MeV (compared to 939 MeV for protons and 0. ... Neutrinos are elementary particles. ...

Organization of Fermions
Generation 1 Generation 2 Generation 3
Quarks Up
Quark
( $u,$) Charm
Quark
( $c,$) Top
Quark
( $t,$)
Down
Quark
( $d,$) Strange
Quark
( $s,$) Bottom
Quark
( $b,$)
Leptons Electron
Neutrino
( $nu_e,$) Muon
Neutrino
( $nu_mu,$) Tau
Neutrino
( $nu_tau,$)
Electron ( $e,$) Muon ( $mu,$) Tau
Lepton
( $tau,$)

These particles carry charges which make them susceptible to the fundamental forces (described in the next subsection).

• Each quark carries any one of three color charges – red, green or blue, enabling them to participate in strong interactions.
• The up-type quarks (up, charm, and top quarks) carry an electric charge of +2/3, and the down-type quarks (down, strange, and bottom) carry an electric charge of –1/3, enabling both types to participate in electromagnetic interactions.
• Leptons do not carry any color charge – they are color neutral, preventing them from participating in strong interactions.
• The down-type leptons (the electron, the muon, and the tau lepton) carry an electric charge of –1, enabling them to participate in electromagnetic interactions.
• The up–type leptons (the neutrinos) carry no electric charge, preventing them from participating in electromagnetic interactions
• Both, quarks and leptons carry a handful of flavor charges (see flavor), including the weak isospin, enabling all particles to interact via the weak nuclear interaction.

Pairs from each group (one up-type quark, one down-type quarks, a lepton and its corresponding neutrino) form a generation. Corresponding particles within each generation are identical to each other apart from their masses and flavors. In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... In particle physics, flavor is a property of a fermion that identifies it, a label that specifies the name of the particle. ... The weak isospin in theoretical physics parallels the idea of the isospin under the strong interaction, but applied under the weak interaction. ... According to the standard model of particle physics, all the elementary particles seen in particle collision experiments can be divided into three generations. ...

#### Spin and Chirality

Main article: Spin (physics)

All particles in the Standard Model have an intrinsic spin, allowing us to roughly visualize each particle as a miniature top spinning in space. Technically, we associate a quantum number to this property called spin, and is fixed for all matter particles (quarks and leptons, alike) at 1/2. Hence, the particles always spin with constant speed. The spin of all particles is described by a vector, $mathbf{s}$, with cartesian components $(s_x,,s_y,,s_z)$ whose length never changes. At length scales dominated by quantum mechanics, the orientation of the spin is limited in a very non-intuitive way. The component of $mathbf{s}$ along any given direction (say, along the x-axis) is limited to take on one of two values: $s_x=+textstylefrac{1}{2}hbar$, in which case the particle is said to be "spin-up in the x-direction", or $s_x=textstyle-frac{1}{2}hbar$, in which case is "spin-down." Here, $hbar$ is Planck's constant -- a very tiny unit of angular momentum. Even more bizarre is that despite this limitation, any component of $mathbf{s}$ may be in a mixture (superposition) between the spin-up case and the spin-down case. Only probing the particle would cause the value of the x-component of the spin to snap to exclusively one of the two allowed values. In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... This article is about vectors that have a particular relation to the spatial coordinates. ... Fig. ... A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ... Gyroscope. ... Quantum superposition is the application of the superposition principle to quantum mechanics. ...

There is nothing special about the x-, y- and the z-axis. In principle and in practice, one may be interested in the component of spin along a direction that lies between the x- and y-axis, or between the x- and z- axis, or between all three axis. The allowed values are still limited to $+textstylefrac{1}{2}hbar$ and $-textstylefrac{1}{2}hbar$. Physicists are often interested in the particle's spin is along its momentum, p (direction of motion). The component of spin along the particle's momentum is chirality. As usual, a particles chirality are limited to $s_mathbf{p}=+textstylefrac{1}{2}hbar$, in which case the particle is right-handed, or $s_mathbf{p}=-textstylefrac{1}{2}hbar$, in which case the particle is left-handed. An understanding of chirality is crucial to understand the selective nature of the weak nuclear force. In classical mechanics, momentum (pl. ... A phenomenon is said to be chiral if it is not identical to its mirror image (see Chirality (mathematics)). The spin of a particle may be used to define a handedness for that particle. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...

### Force Mediating Particles

The force mediating particles described by the Standard Model all have an intrinsic spin whose value is 1, making them bosons. As a result, they do not follow the Pauli Exclusion Principle. The different types of force mediating particles are described below. Image File history File links Interactions. ...

• The photons mediate the familiar electromagnetic force between electrically charged particles (these are the quarks, electrons, muons, tau, W+ and W). They are massless and are described by the theory of quantum electrodynamics.
• The W+, W, and Z0 gauge bosons mediate the weak nuclear interactions between particles of different flavors (all quarks and leptons). They are massive, with the Z0 being more massive than the equally massive W+ and W. An interesting feature of the weak force is that interactions involving the W+ and W gauge bosons act on exclusively left-handed particles (those particles whose chirality is $s_mathbf{p}=-textstylefrac{1}{2}hbar$). The right-handed particles are completely neutral to the W bosons. Furthermore, the W+ and W bosons carry an electric charge of +1 and –1 making those susceptible to electromagnetic interactions. The electrically neutral Z0 boson acts on particles of both chiralities, but preferentially on left-handed ones. The weak nuclear interaction is unique in that it is the only one that selectively acts on particles of different chiralities; the photons of electromagnetism and the gluons of the strong force act on particles without such prejudice. These three gauge bosons along with the photons are grouped together which collectively mediate the electroweak interactions.
• The eight gluons mediate the strong nuclear interactions between color charged particles (the quarks). They are massless. But, each of the eight carry combinations of color and an anticolor charge enabling them to interact among themselves. The gluons and their interactions are described by the theory of quantum chromodynamics.

The interactions between all the particles described by the Standard Model are summarized in the illustration immediately above and to the right. In physics, the photon (from Greek Ï†Ï‰Ï‚, phÅs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... Electromagnetism is the force observed as static electricity, and causes the flow of electric charge (electric current) in electrical conductors. ... Quantum electrodynamics (QED) is a relativistic quantum field theory of electromagnetism. ... In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... A phenomenon is said to be chiral if it is not identical to its mirror image (see Chirality (mathematics)). The spin of a particle may be used to define a handedness for that particle. ... In physics, the electroweak theory presents a unified description of two of the four fundamental forces of nature: electromagnetism and the weak nuclear force. ... In physics, gluons are the bosonic particles which are responsible for the strong nuclear force. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... Quantum chromodynamics (QCD) is the theory of the strong interaction, a fundamental force describing the interactions of the quarks and gluons found in nucleons (such as the proton and neutron). ...

Force Mediating Particles
Electromagnetic Force Weak Nuclear Force Strong Nuclear Force
Photon γ W+, W-, and Z0
Gauge Bosons
W + , W ,
Z0
Gluons g

### The Higgs Boson

Main article: Higgs Boson

The Higgs particle described by the Standard Model has no intrinsic spin, and thus is also classified as a boson. As of January 2007, the experimental evidence for the Higgs boson has not been found; so far it is a theoretical particle. It is hoped that upon the completion of the Large Hadron Collider, experiments conducted at CERN would bring experimental evidence confirming the existence for the particle. The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ... The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland ( ). Currently under construction, the LHC is scheduled to begin operation (at reduced energies) in November 2007. ... CERN logo The Organisation EuropÃ©enne pour la Recherche NuclÃ©aire (English: European Organization for Nuclear Research), commonly known as CERN, pronounced (or in French), is the worlds largest particle physics laboratory, situated just west of Geneva on the border between France and Switzerland. ...

The Higgs boson plays a unique role in the Standard Model.

### Table

Left handed fermions in the Standard Model
Generation 1
Fermion (Left-handed) Symbol Electric charge Weak charge* Weak isospin Hypercharge Color charge* Mass**
Electron e −1 $bold{2}$ −1/2 −1/2 $bold{1}$ 0.510999 MeV
Electron neutrino νe 0 $bold{2}$ +1/2 −1/2 $bold{1}$ < 2 eV
Positron ec +1 $bold{1}$ 0 +1 $bold{1}$ 0.510999 MeV
Electron antineutrino $nu_e^c$ 0 $bold{1}$ 0 0 $bold{1}$ < 2 eV
Up quark u +2/3 $bold{2}$ +1/2 +1/6 $bold{3}$ ~3 MeV ***
Down quark d −1/3 $bold{2}$ −1/2 +1/6 $bold{3}$ ~6 MeV ***
Anti-up antiquark uc −2/3 $bold{1}$ 0 −2/3 $bold{bar{3}}$ ~3 MeV ***
Anti-down antiquark dc +1/3 $bold{1}$ 0 +1/3 $bold{bar{3}}$ ~6 MeV ***

Generation 2
Fermion (Left-handed) Symbol Electric charge Weak charge* Weak isospin Hypercharge Color charge* Mass**
Muon μ −1 $bold{2}$ −1/2 −1/2 $bold{1}$ 105.658 MeV
Muon neutrino νμ 0 $bold{2}$ +1/2 −1/2 $bold{1}$ < 2 eV
Anti-Muon μc +1 $bold{1}$ 0 +1 $bold{1}$ 105.658 MeV
Muon antineutrino $nu_mu^c$ 0 $bold{1}$ 0 0 $bold{1}$ < 2 eV
Charm quark c +2/3 $bold{2}$ +1/2 +1/6 $bold{3}$ ~1.3 GeV
Strange quark s −1/3 $bold{2}$ −1/2 +1/6 $bold{3}$ ~100 MeV
Anti-charm antiquark cc −2/3 $bold{1}$ 0 −2/3 $bold{bar{3}}$ ~1.3 GeV
Anti-strange antiquark sc +1/3 $bold{1}$ 0 +1/3 $bold{bar{3}}$ ~100 MeV

Generation 3
Fermion (Left-handed) Symbol Electric charge Weak charge* Weak isospin Hypercharge Color charge* Mass**
Tau lepton τ −1 $bold{2}$ −1/2 −1/2 $bold{1}$ 1.777 GeV
Tau neutrino ντ 0 $bold{2}$ +1/2 −1/2 $bold{1}$ < 2 eV
Anti-Tau τc +1 $bold{1}$ 0 +1 $bold{1}$ 1.777 GeV
Tau antineutrino $nu_tau^c$ 0 $bold{1}$ 0 0 $bold{1}$ < 2 eV
Top quark t +2/3 $bold{2}$ +1/2 +1/6 $bold{3}$ 171.4 GeV
Bottom quark b −1/3 $bold{2}$ −1/2 +1/6 $bold{3}$ ~4.2 GeV
Anti-top antiquark tc −2/3 $bold{1}$ 0 −2/3 $bold{bar{3}}$ 171.4 GeV
Anti-bottom antiquark bc +1/3 $bold{1}$ 0 +1/3 $bold{bar{3}}$ ~4.2 GeV

* - These are not ordinary abelian charges, which can be added together, but are labels of group representations of lie groups. Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... The weak isospin in theoretical physics parallels the idea of the isospin under the strong interaction, but applied under the weak interaction. ... In particle physics, the hypercharge (represented by Y) is the sum of the baryon number B and the flavor charges: strangeness S, charm C, bottomness and topness T, although the last one can be omitted given the extremely short life of the top quark (it decays to other quarks before... In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The electron is a fundamental subatomic particle that carries an electric charge. ... Neutrinos are elementary particles. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... In particle physics, the hypercharge (represented by Y) is the sum of the baryon number B and the flavor charges: strangeness S, charm C, bottomness and topness T, although the last one can be omitted given the extremely short life of the top quark (it decays to other quarks before... In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The moons shadow, as seen in muons 700m below ground at the Soudan 2 detector. ... Neutrinos are elementary particles. ... An antimuon is the antiparticle of the muon. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... In particle physics, the hypercharge (represented by Y) is the sum of the baryon number B and the flavor charges: strangeness S, charm C, bottomness and topness T, although the last one can be omitted given the extremely short life of the top quark (it decays to other quarks before... In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The tau lepton (often called the tau or occasionally the tauon) is a negatively charged elementary particle with a lifetime of 3Ã—10âˆ’13 seconds and a high mass of 1777 MeV (compared to 939 MeV for protons and 0. ... Neutrinos are elementary particles. ... The tau lepton (often called the tau or occasionally the tauon) is a negatively charged elementary particle with a lifetime of 3Ã—10âˆ’13 seconds and a high mass of 1777 MeV (compared to 939 MeV for protons and 0. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... These are the 6 quarks and their most likely decay modes. ... Abelian, in mathematics, is used in many different definitions: In group theory: Abelian group, a group in which the binary operation is commutative Category of abelian groups Ab has abelian groups as objects and group homomorphisms as morphisms Metabelian group is a group where the commutator subgroup is contained in... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... Group representation theory is the branch of mathematics that studies properties of abstract groups via their representations as linear transformations of vector spaces. ... In mathematics, a Lie group is a group whose elements can be continuously parametrized by real numbers, such as the rotation group, which can be parametrized by the Euler angles. ...

** - Mass is really a coupling between a left-handed fermion and a right-handed fermion. For example, the mass of an electron is really a coupling between a left-handed electron and a right-handed electron, which is the antiparticle of a left-handed positron. Also neutrinos show large mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left-handed electron neutrino and a right-handed electron neutrino have the same mass as this table seems to suggest. Corresponding to each kind of particle, there is an associated antiparticle with the same mass and opposite charges. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... In particle physics, flavor is a property of a fermion that identifies it, a label that specifies the name of the particle. ...

*** - What is actually measured experimentally are the masses of baryons and hadrons and various cross-sections. Since quarks can't be isolated because of QCD confinement, the quantity here is supposed to be the mass of the quark at the renormalization scale of the QCD phase transition. In order to compute this quantity, physicists have to compute the hadron spectrum using lattice gauge theory and try out various masses for the quarks until the model comes up with a close fit with experimental data. Since the masses of the first-generation quarks are significantly below the QCD scale, the uncertainties are pretty large. In fact, current lattice QCD models seem to suggest a significantly lower mass of these quarks from that of this table. In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. ... The initialism QCD can mean: Quantum chromodynamics Quintessential Player, formerly known as Quintessential CD Quality, Cost, Delivery, A three-letter acronym used in lean manufacturing This page concerning a three-letter acronym or abbreviation is a disambiguation page â€” a navigational aid which lists other pages that might otherwise share the... Colour confinement (often just confinement) is the physics phenomenon that color charged particles (such as quarks) cannot be isolated. ... Figure 1. ... In physics, a phase transition, (or phase change) is the transformation of a thermodynamic system from one phase to another. ... It has been suggested that this article or section be merged into Lattice field theory. ... It has been suggested that lattice field theory be merged into this article or section. ...

Image File history File links Download high resolution version (841x293, 28 KB) Summary Logarithmic plot of known masses in the Standard Model of Particles. ... Image File history File links Download high resolution version (841x293, 28 KB) Summary Logarithmic plot of known masses in the Standard Model of Particles. ...

## Tests and predictions

The Standard Model predicted the existence of W and Z bosons, the gluon, the top quark and the charm quark before these particles had been observed. Their predicted properties were experimentally confirmed with good precision.

The Large Electron-Positron collider at CERN tested various predictions about the decay of Z bosons, and found them confirmed. CERN logo The Organisation EuropÃ©enne pour la Recherche NuclÃ©aire (English: European Organization for Nuclear Research), commonly known as CERN, pronounced (or in French), is the worlds largest particle physics laboratory, situated just west of Geneva on the border between France and Switzerland. ...

To get an idea of the success of the Standard Model a comparison between the measured and the predicted values of some quantities are shown in the following table:

Quality Measured (GeV) SM prediction (GeV)
Mass of W boson 80.4120±0.0420 80.3900±0.0180
Mass of Z boson 91.1876±0.0021 91.1874±0.0021

## Challenges to the Standard Model

Unsolved problems in physics: Parameters in the Standard Model: What gives rise to the Standard Model of particle physics? Why do its particle masses and coupling constants possess the values we have measured? Does the Higgs boson predicted by the model really exist? Why are there three generations of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality, or fatally flawed?

There is no experimental indication yet for the existence of the Higgs boson. Image File history File links Question_dropshade. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. ... The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ... According to the standard model of particle physics, all the elementary particles seen in particle collision experiments can be divided into three generations. ... The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ...

Although the Standard Model has had great success in explaining experimental results, it has two important defects:

1 - The model contains 19 free parameters, such as particle masses, which must be determined experimentally. Another 10 parameters are needed to include neutrino masses. These parameters cannot be independently calculated.

2 - Connes has shown that the standard model can be derived from general relativity by generalizing Riemannian geometry to noncommutative geometry. The noncommutative standard model has fewer free parameters than the conventional one. However, despite the fact that noncommutative geometry is formulated in the language of quantum mechanics, quantum field theories in noncommutative geometries are still an open problem. General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1915/16. ... In differential geometry, Riemannian geometry is the study of smooth manifolds with Riemannian metrics, i. ... In mathematics, there is a close relationship between spaces, which are geometric in nature, and the numerical functions on them. ... This article or section is not written in the formal tone expected of an encyclopedia article. ...

Since the completion of the Standard Model, many efforts have been made to address the first of these problems.

One attempt to address the first defect is known as grand unification. The so-called grand unified theories (GUTs) hypothesized that the SU(3), SU(2), and U(1) groups are actually subgroups of a single large symmetry group. At high energies (far beyond the reach of current experiments), the symmetry of the unifying group is preserved; at low energies, it reduces to SU(3)×SU(2)×U(1) by a process known as spontaneous symmetry breaking. The first theory of this kind was proposed in 1974 by Georgi and Glashow, using SU(5) as the unifying group. A distinguishing characteristic of these GUTs is that, unlike the Standard Model, they predict the existence of proton decay. In 1999, the Super-Kamiokande neutrino observatory reported that it had not detected proton decay, establishing a lower limit on the proton half-life of 6.7× 1032 years. This and other experiments have falsified numerous GUTs, including SU(5). Another effort to address the first defect has been to develop preon models which attempt to set forth a substructure of more fundamental particles than those set forth in the Standard Model. Grand unification, grand unified theory, or GUT is a theory in physics that unifies the strong interaction and electroweak interaction. ... Spontaneous symmetry breaking in physics takes place when a system that is symmetric with respect to some symmetry group goes into a vacuum state that is not symmetric. ... 1974 (MCMLXXIV) was a common year starting on Tuesday. ... The decay of a proton, a baryon, into non-baryonic matter, does not occur perturbatively in the Standard Model. ... 1999 (MCMXCIX) was a common year starting on Friday, and was designated the International Year of Older Persons by the United Nations. ... Super-Kamiokande, or Super-K for short, is a neutrino observatory in Japan. ... The neutrino is an elementary particle. ... In particle physics, preons are postulated point-like particles, conceived to be subcomponents of quarks and leptons. ...

In addition, there are cosmological reasons why the Standard Model is believed to be incomplete. In the Standard Model, matter and antimatter are related by the CPT symmetry, which suggests that there should be equal amounts of matter and antimatter after the Big Bang. While the preponderance of matter in the universe can be explained by saying that the universe just started out this way, this explanation strikes most physicists as inelegant. Furthermore, the Standard Model provides no mechanism to generate the cosmic inflation that is believed to have occurred at the beginning of the universe. This article or section does not cite its references or sources. ... In particle physics, antimatter extends the concept of the antiparticle to matter, wherein if a particle and its antiparticle come into contact with each other, the two annihilate â€”that is, they may both be converted into other particles with equal energy in accordance with Einsteins equation E = mc2. ... CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). ... Universe is a word derived from the Old French univers, which in turn comes form the Latin roots unus (one) and versus (a form of vertere, to turn). Physicists concept of the Universe is motivated[] by the attempt to describe the whole of space-time, including all matter and energy... In physical cosmology, cosmic inflation is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density. ...

The Higgs boson, which is predicted by the Standard Model, has not been observed as of 2007 (though some phenomena were observed in the last days of the LEP collider that could be related to the Higgs). One of the reasons for building the Large Hadron Collider is that the increase in energy is expected to make the Higgs observable. The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ... 2007 is a common year starting on Monday of the Gregorian calendar. ... The Large Electron-Positron Collider (usually called LEP for short. ... The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland ( ). Currently under construction, the LHC is scheduled to begin operation (at reduced energies) in November 2007. ...

The first experimental deviation from the Standard Model (as proposed in the 1970's) came in 1998, when Super-Kamiokande published results indicating neutrino oscillation. Under the Standard Model, a massless neutrino cannot oscillate, so this observation implied the existence of non-zero neutrino masses. It was therefore necessary to revise the Standard Model to allow neutrinos to have mass; this may be simply achieved by adding 10 more free parameters beyond the initial 19. 1998 (MCMXCVIII) was a common year starting on Thursday of the Gregorian calendar, and was designated the International Year of the Ocean. ... Super-Kamiokande, or Super-K for short, is a neutrino observatory in Japan. ... Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvo whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor. ... Neutrinos are elementary particles. ...

A further extension of the Standard Model can be found in the theory of supersymmetry, which proposes a massive supersymmetric "partner" for every particle in the conventional Standard Model. Supersymmetric particles have been suggested as a candidate for explaining dark matter. Although supersymmetric particles have not been observed experimentally to date, the theory is one of the most popular avenues of research in theoretical particle physics. This article or section is in need of attention from an expert on the subject. ... Dark matter is a term used in astrophysics to refer to that component of the universe that is unaccounted for, unexplained or inconclusively explained, and whose existence at this time is only inferred. ...

## The anthropic principle

Some claim that the vast majority of possible values for the parameters of the Standard Model are incompatible with the existence of life[citation needed] (see fine-tuned universe for more details). According to arguments based on the anthropic principle, the Standard Model has the field content it does and the parameters it has because these are the values that are likely to give rise to the existence of lifeforms intelligent enough to be self-aware. Some physicists argue that if we knew the landscape of possible theories and prior distribution of these theories and also know the probability that any given theory will give rise to life, we would be able to make a statistical prediction of the parameters of the Standard Model.[citation needed] The deepest visible-light image of the cosmos. ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence on Earth of biochemistry, carbon-based life, and eventually human beings to observe such... This article does not cite its references or sources. ... A prior probability is a marginal probability, interpreted as a description of what is known about a variable in the absence of some evidence. ...

This is a detailed description of the standard model (SM) of particle physics. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In physics, Fermis interaction is an old explanation of the weak force, proposed by Enrico Fermi. ... In physics, the electroweak theory presents a unified description of two of the four fundamental forces of nature: electromagnetism and the weak nuclear force. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... Flavour (or flavor) is a quantum number of elementary particles related to their weak interactions. ... In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ... Quantum chromodynamics (QCD) is the theory of the strong interaction, a fundamental force describing the interactions of the quarks and gluons found in nucleons (such as the proton and neutron). ... Quark Matter refers to any of a number of phases of matter built out of quarks and gluons. ... In physics, and specifically particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics. ... The neutrino is an elementary particle. ... This article or section seems not to be written in the formal tone expected of an encyclopedia entry. ... This article or section is not written in the formal tone expected of an encyclopedia article. ... Results from FactBites:

 Theory: The Standard Model (SLAC VVC) (331 words) The Standard Model is the name given to the current theory of fundamental particles and how they interact. The Standard Model was the triumph of particle physics of the 1970's. Thus, this one aspect of the Standard Model does not yet have the status of "theory" but still remains in the realm of hypothesis or model.
 Particle Physics Today: the Standard Model (0 words) The Standard Model is by now a well-tested physics theory, used to explain and exactly predict a vast variety of phenomena. The standard model includes three types of forces acting among particles: strong, weak and electromagnetic. A big success of the Standard Model is the unification of the electromagnetic and the weak forces into the so-called electroweak force.
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