The Standard Model falls short of being a complete theory of fundamental interactions because it does not incorporate the physics of dark energy nor of the full theory of gravitation as described by general relativity. The theory does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly account for neutrino oscillations (and their nonzero masses). Although the Standard Model is believed to be theoretically selfconsistent, it has several apparently unnatural properties giving rise to puzzles like the strong CP problem and the hierarchy problem.
Nevertheless, the Standard Model is important to theoretical and experimental particle physicists alike. For theorists, the Standard Model is a paradigmatic example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, nonperturbative behavior, etc. It is used as a basis for building more exotic models that incorporate hypothetical particles, extra dimensions, and elaborate symmetries (such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model, such as the existence of dark matter and neutrino oscillations. In turn, experimenters have incorporated the Standard Model into simulators to help search for new physics beyond the Standard Model.
Recently, the Standard Model has found applications in fields besides particle physics, such as astrophysics, cosmology, and nuclear physics.
Contents[hide] 
[edit]Historical background
The first step towards the Standard Model was Sheldon Glashow's discovery in 1960 of a way to combine the electromagnetic and weak interactions.^{[1]} In 1967 Steven Weinberg^{[2]} and Abdus Salam^{[3]} incorporated the Higgs mechanism^{[4]}^{[5]}^{[6]} into Glashow's electroweak theory, giving it its modern form.
The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model. This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons.
After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973,^{[7]}^{[8]}^{[9]}^{[10]} the electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics for discovering it. The W and Z bosons were discovered experimentally in 1981, and their masses were found to be as the Standard Model predicted.
The theory of the strong interaction, to which many contributed, acquired its modern form around 1973–74, when experiments confirmed that the hadrons were composed of fractionally charged quarks.
[edit]Overview
At present, matter and energy are best understood in terms of the kinematics and interactions of elementary particles. To date, physics has reduced the laws governing the behavior and interaction of all known forms of matter and energy to a small set of fundamental laws and theories. A major goal of physics is to find the "common ground" that would unite all of these theories into one integrated theory of everything, of which all the other known laws would be special cases, and from which the behavior of all matter and energy could be derived (at least in principle).^{[11]}
The Standard Model groups two major extant theories—quantum electroweak and quantum chromodynamics—into an internally consistent theory that describes the interactions between all known particles in terms of quantum field theory. For a technical description of the fields and their interactions, see Standard Model (mathematical formulation).
[edit]Particle content
[edit]Fermions
Charge  First generation  Second generation  Third generation  

Quarks  +2/3  Up  u  Charm  c  Top  t 
−1/3  Down  d  Strange  s  Bottom  b  
Leptons  −1  Electron  e−  Muon  μ−  Tau  τ− 
0  Electron neutrino  ν e  Muon neutrino  ν μ  Tau neutrino  ν τ 
The Standard Model includes 12 elementary particles of spin known as fermions. According to the spinstatistics theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.
The fermions of the Standard Model are classified according to how they interact (or equivalently, by what charges they carry). There are six quarks (up, down, charm, strange,top, bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior (see table).
The defining property of the quarks is that they carry color charge, and hence, interact via the strong interaction. A phenomenon called color confinement results in quarks being perpetually (or at least since very soon after the start of the Big Bang) bound to one another, forming colorneutral composite particles (hadrons) containing either a quark and an antiquark (mesons) or three quarks (baryons). The familiar proton and the neutron are the two baryons having the smallest mass. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions both electromagnetically and via the weak interaction.
The remaining six fermions do not carry colour charge and are called leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriously difficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interact electromagnetically.
Each member of a generation has greater mass than the corresponding particles of lower generations. The first generation charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles. Specifically, all atoms consist of electrons orbiting atomic nuclei ultimately constituted of up and down quarks. Second and third generations charged particles, on the other hand, decay with very short half lives, and are observed only in very highenergy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter.
[edit]Gauge bosons
In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions.
Interactions in physics are the ways that particles influence other particles. At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields, and gravitation allows particles with mass to attract one another in accordance with Einstein's theory ofgeneral relativity. The Standard Model explains such forces as resulting from matter particlesexchanging other particles, known as force mediating particles (strictly speaking, this is only so if interpreting literally what is actually an approximation method known as perturbation theory)^{[citation needed]}. When a force mediating particle is exchanged, at a macroscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to havemediated (i.e., been the agent of) that force. The Feynman diagram calculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze highenergy scattering experiments are in reasonable agreement with the data. However, perturbation theory (and with it the concept of a "forcemediating particle") fails in other situations. These include lowenergy quantum chromodynamics, bound states, and solitons.
The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons. As a result, they do not follow the Pauli exclusion principle that constrains fermions: thus bosons (e.g. photons) do not have a theoretical limit on their spatial density (number per volume). The different types of gauge bosons are described below.
 Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is welldescribed by the theory of quantum electrodynamics.
 The W+, W−, and Z gauge bosons mediate the weak interactions between particles of different flavors (all quarks and leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving the W± exclusively act on lefthanded particles and righthanded antiparticles only. Furthermore, the W± carries an electric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral Z boson interacts with both lefthanded particles and antiparticles. These three gauge bosons along with the photons are grouped together, as collectively mediating theelectroweak interaction.
 The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons are massless. The eightfold multiplicity of gluons is labeled by a combination of color and anticolor charge (e.g. red–antigreen).^{[nb 1]} Because the gluon has an effective color charge, they can also 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 by the diagrams on the right of this section.
[edit]Higgs boson
Main article: Higgs boson
The Higgs particle is a hypothetical massive scalar elementary particle theorized by Robert Brout, François Englert, Peter Higgs, Gerald Guralnik, C. R. Hagen, and Tom Kibble in 1964 (see 1964 PRL symmetry breaking papers) and is a key building block in the Standard Model.^{[12]}^{[13]}^{[14]}^{[15]} It has no intrinsic spin, and for that reason is classified as a boson (like the gauge bosons, which have integer spin). Because an exceptionally large amount of energy and beam luminosity are theoretically required to observe a Higgs boson in high energy colliders, it is the only fundamental particle predicted by the Standard Model that has yet to be observed.
The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except thephoton and gluon, are massive. In particular, the Higgs boson would explain why the photon has no mass, while the W and Z bosons are very heavy. Elementary particle masses, and the differences between electromagnetism (mediated by the photon) and the weak force (mediated by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory, the Higgs boson generates the masses of the leptons (electron, muon, and tau) and quarks.
As yet, no experiment has conclusively detected the existence of the Higgs boson. It is hoped that the Large Hadron Collider at CERN will confirm the existence of this particle. As of August 2011, a significant portion of the possible masses for the Higgs have been excluded at 95% confidence level: CMS has excluded the mass ranges 145–216 GeV,226–288 GeV and 310–400 GeV,^{[16]} while the ATLAS experiment has excluded 146–232 GeV, 256–282 GeV and 296–466 GeV.^{[17]} Note that these exclusions apply only to the Standard Model Higgs, and that more complex Higgs sectors which are possible in Beyond the Standard Model scenarios may be significantly more difficult to characterize. CERN director general Rolf Heuer has predicted that by the end of 2012 either the Standard Model Higgs boson will be observed, or excluded in all mass ranges, implying that the Standard Model is not the whole story.^{[18]}
On December 13, 2011 CERN announced that both ATLAS and CMS experiments had detected 'hints' of the Higgs boson in at approximately 124 GeV. These results were not sufficiently strong to announce that the Higgs boson had been found (ATLAS showed a 2.3 sigma level of certainty for an excess at 126 GeV, while CMS showed a 1.9 sigma level excess at 124 GeV) but the fact that two separate experiments show excesses in the same energy range has led to much excitement in the particle physics world.^{[19]}
[edit]Field content
The Standard Model has the following fields:
[edit]Spin 1
 A U(1) gauge field B_{μν} with coupling g′ (weak U(1), or weak hypercharge)
 An SU(2) gauge field W_{μν} with coupling g (weak SU(2), or weak isospin)
 An SU(3) gauge field G_{μν} with coupling g_{s} (strong SU(3), or color charge)
[edit]Spin ^{1}⁄_{2}
The spin particles are in representations of the gauge groups. For the U(1) group, we list the value of the weak hypercharge instead. The lefthanded fermionic fields are:
 An SU(3) triplet, SU(2) doublet, with U(1) weak hypercharge (lefthanded quarks)
 An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge (lefthanded downtype antiquark)
 An SU(3) singlet, SU(2) doublet with U(1) weak hypercharge −1 (lefthanded lepton)
 An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge (lefthanded uptype antiquark)
 An SU(3) singlet, SU(2) singlet with U(1) weak hypercharge 2 (lefthanded antilepton)
By CPT symmetry, there is a set of righthanded fermions with the opposite quantum numbers.
This describes one generation of leptons and quarks, and there are three generations, so there are three copies of each field. Note that there are twice as many lefthanded lepton field components as lefthanded antilepton field components in each generation, but an equal number of lefthanded quark and antiquark fields.
[edit]Spin 0
 An SU(2) doublet H with U(1) hypercharge +1 (Higgs field)
Note that , summed over the two SU(2) components, is invariant under both SU(2) and under U(1), and so it can appear as a renormalizable term in the Lagrangian, as can its square.^{[clarification needed]}
This field acquires a vacuum expectation value, leaving a combination of the weak isospin, , and weak hypercharge unbroken. This is the electromagnetic gauge group, and the photon remains massless. The standard formula for the electric charge (which defines the normalization of the weak hypercharge, , which would otherwise be somewhat arbitrary) is:^{[nb 2]}
[edit]Lagrangian
The Lagrangian for the spin 1 and spin ^{1}⁄_{2} fields is the most general renormalizable gauge field Lagrangian with no fine tunings:
 Spin 1:
where the traces are over the SU(2) and SU(3) indices hidden in W and G respectively. The twoindex objects are the field strengths derived from W and G the vector fields. There are also two extra hidden parameters: the theta angles for SU(2) and SU(3).
The spin^{1}⁄_{2} particles can have no mass terms because there is no right/left helicity pair with the same SU(2) and SU(3) representation and the same weak hypercharge. This means that if the gauge charges were conserved in the vacuum, none of the spin ^{1}⁄_{2} particles could ever swap helicity, and they would all be massless.
For a neutral fermion, for example a hypothetical righthanded lepton N (or N^{α} in relativistic twospinor notation), with no SU(3), SU(2) representation and zero charge, it is possible to add the term:^{[clarification needed]}
This term gives the neutral fermion a Majorana mass. Since the generic value for M will be of order 1, such a particle would generically be unacceptably heavy. The interactions are completely determined by the theory – the leptons introduce no extra parameters.
[edit]Higgs mechanism
Main article: Higgs mechanism
The Lagrangian for the Higgs includes the most general renormalizable self interaction:
The parameter v^{2} has dimensions of mass squared, and it gives the location where the classical Lagrangian is at a minimum. In order for the Higgs mechanism to work, v^{2} must be a positive number. v has units of mass, and it is the only parameter in the Standard Model which is not dimensionless. It is also much smaller than the Planck scale; it is approximately equal to the Higgs mass, and sets the scale for the mass of everything else. This is the only real finetuning to a small nonzero value in the Standard Model, and it is called the Hierarchy problem.
It is traditional to choose the SU(2) gauge so that the Higgs doublet in the vacuum has expectation value (v,0).
[edit]Masses and CKM matrix
Main article: Cabibbo–Kobayashi–Maskawa matrix
The rest of the interactions are the most general spin0 spin^{1}⁄_{2} Yukawa interactions, and there are many of these. These constitute most of the free parameters in the model. The Yukawa couplings generate the masses and mixings once the Higgs gets its vacuum expectation value.
The terms L^{*}HR^{[clarification needed]} generate a mass term for each of the three generations of leptons. There are 9 of these terms, but by relabeling L and R, the matrix can be diagonalized. Since only the upper component of H is nonzero, the upper SU(2) component of L mixes with R to make the electron, the muon, and the tau, leaving over a lower massless component, the neutrino. Note: Neutrino oscillations show neutrinos have mass.^{[20]} See also: Pontecorvo–Maki–Nakagawa–Sakata matrix.
The terms QHU^{[clarification needed]} generate up masses, while QHD^{[clarification needed]} generate down masses. But since there is more than one righthanded singlet in each generation, it is not possible to diagonalize both with a good basis for the fields, and there is an extra CKM matrix.
[edit]Theoretical aspects
[edit]Construction of the Standard Model Lagrangian
Symbol  Description  Renormalization scheme (point)  Value 

m_{e}  Electron mass  511 keV  
m_{μ}  Muon mass  105.7 MeV  
m_{τ}  Tau mass  1.78 GeV  
m_{u}  Up quark mass  μ_{MS} = 2 GeV  1.9 MeV 
m_{d}  Down quark mass  μ_{MS} = 2 GeV  4.4 MeV 
m_{s}  Strange quark mass  μ_{MS} = 2 GeV  87 MeV 
m_{c}  Charm quark mass  μ_{MS} = m_{c}  1.32 GeV 
m_{b}  Bottom quark mass  μ_{MS} = m_{b}  4.24 GeV 
m_{t}  Top quark mass  Onshell scheme  172.7 GeV 
θ_{12}  CKM 12mixing angle  13.1°  
θ_{23}  CKM 23mixing angle  2.4°  
θ_{13}  CKM 13mixing angle  0.2°  
δ  CKM CPviolating Phase  0.995  
g_{1} or g'  U(1) gauge coupling  μ_{MS} = m_{Z}  0.357 
g_{2} or g  SU(2) gauge coupling  μ_{MS} = m_{Z}  0.652 
g_{3} or g_{s}  SU(3) gauge coupling  μ_{MS} = m_{Z}  1.221 
θ_{QCD}  QCD vacuum angle  ~0  
μ  Higgs quadratic coupling  Unknown  
λ  Higgs selfcoupling strength  Unknown 
Technically, quantum field theory provides the mathematical framework for the Standard Model, in which a Lagrangian controls the dynamics and kinematics of the theory. Each kind of particle is described in terms of a dynamical field that pervades spacetime. The construction of the Standard Model proceeds following the modern method of constructing most field theories: by first postulating a set of symmetries of the system, and then by writing down the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries.
The global Poincaré symmetry is postulated for all relativistic quantum field theories. It consists of the familiar translational symmetry, rotational symmetry and the inertial reference frame invariance central to the theory of special relativity. The local SU(3)×SU(2)×U(1) gauge symmetry is an internal symmetry that essentially defines the Standard Model. Roughly, the three factors of the gauge symmetry give rise to the three fundamental interactions. The fields fall into different representations of the various symmetry groups of the Standard Model (see table). Upon writing the most general Lagrangian, one finds that the dynamics depend on 19 parameters, whose numerical values are established by experiment. The parameters are summarized in the table at right.
[edit]Quantum chromodynamics sector
Main article: Quantum chromodynamics
The quantum chromodynamics (QCD) sector defines the interactions between quarks and gluons, with SU(3) symmetry, generated by T^{a}. Since leptons do not interact with gluons, they are not affected by this sector. The Dirac Lagrangian of the quarks coupled to the gluon fields is given by
is the SU(3) gauge field containing the gluons, are the Dirac matrices, D and U are the Dirac spinors associated with up and downtype quarks, and g_{s} is the strong coupling constant.
[edit]Electroweak sector
Main article: Electroweak interaction
The electroweak sector is a Yang–Mills gauge theory with the symmetry group U(1)×SU(2)_{L},
where B_{μ} is the U(1) gauge field; Y_{W} is the weak hypercharge—the generator of the U(1) group; is the threecomponent SU(2) gauge field; are the Pauli matrices—infinitesimal generators of the SU(2) group. The subscript L indicates that they only act on left fermions; g′ and g are coupling constants.
[edit]Higgs sector
where the indexes + and 0 indicate the electric charge (Q) of the components. The weak isospin (Y_{W}) of both components is 1.
Before symmetry breaking, the Higgs Lagrangian is:
which can also be written as:
[edit]Additional symmetries of the Standard Model
From the theoretical point of view, the Standard Model exhibits four additional global symmetries, not postulated at the outset of its construction, collectively denotedaccidental symmetries, which are continuous U(1) global symmetries. The transformations leaving the Lagrangian invariant are:
The first transformation rule is shorthand meaning that all quark fields for all generations must be rotated by an identical phase simultaneously. The fields , and , are the 2nd (muon) and 3rd (tau) generation analogs of and fields.
By Noether's theorem, each symmetry above has an associated conservation law: the conservation of baryon number, electron number, muon number, and tau number. Each quark is assigned a baryon number of , while each antiquark is assigned a baryon number of . Conservation of baryon number implies that the number of quarks minus the number of antiquarks is a constant. Within experimental limits, no violation of this conservation law has been found.
Similarly, each electron and its associated neutrino is assigned an electron number of +1, while the antielectron and the associated antineutrino carry a −1 electron number. Similarly, the muons and their neutrinos are assigned a muon number of +1 and the tau leptons are assigned a tau lepton number of +1. The Standard Model predicts that each of these three numbers should be conserved separately in a manner similar to the way baryon number is conserved. These numbers are collectively known as lepton family numbers (LF).
Symmetry works differently for quarks than for leptons, mainly because the Standard Model predicts (incorrectly) that neutrinos are massless. However, in 2002 it was discovered that neutrinos have mass (now established to be not greater than 0.28 electron volts), and as neutrinos oscillate between flavors (muon neutrinos have been observed changing to tau neutrinos) the discovery of neutrino mass indicates that the conservation of lepton family number is violated.^{[21]}
In addition to the accidental (but exact) symmetries described above, the Standard Model exhibits several approximate symmetries. These are the "SU(2) custodial symmetry" and the "SU(2) or SU(3) quark flavor symmetry."
Symmetry  Lie Group  Symmetry Type  Conservation Law 

Poincaré  Translations×SO(3,1)  Global symmetry  Energy, Momentum, Angular momentum 
Gauge  SU(3)×SU(2)×U(1)  Local symmetry  Color charge, Weak isospin, Electric charge, Weak hypercharge 
Baryon phase  U(1)  Accidental Global symmetry  Baryon number 
Electron phase  U(1)  Accidental Global symmetry  Electron number 
Muon phase  U(1)  Accidental Global symmetry  Muon number 
Tau phase  U(1)  Accidental Global symmetry  Tau number 
Field (1st generation)  Spin  Gauge group Representation  Baryon Number  Electron Number  

Lefthanded quark  (3, 2, +^{1}⁄_{3})  
Lefthanded up antiquark  
Lefthanded down antiquark  
Lefthanded lepton  (, , )  
Lefthanded antielectron  (, , )  
Hypercharge gauge field  (, , )  
Isospin gauge field  (, , )  
Gluon field  (, , )  
Higgs field  (, , ) 
[edit]List of Standard Model fermions
This table is based in part on data gathered by the Particle Data Group.^{[22]}
Generation 1  

Fermion (lefthanded)  Symbol  Electric charge  Weak isospin  Weak hypercharge  Color charge ^{[lhf 1]}  Mass^{[lhf 2]}  
Electron  511 keV  
Positron  511 keV  
Electron neutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Electron antineutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Up quark  ~ 3 MeV^{[lhf 5]}  
Up antiquark  ~ 3 MeV^{[lhf 5]}  
Down quark  ~ 6 MeV^{[lhf 5]}  
Down antiquark  ~ 6 MeV^{[lhf 5]}  
Generation 2  
Fermion (lefthanded)  Symbol  Electric charge  Weak isospin  Weak hypercharge  Color charge ^{[lhf 1]}  Mass ^{[lhf 2]}  
Muon  106 MeV  
Antimuon  106 MeV  
Muon neutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Muon antineutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Charm quark  ~ 1.337 GeV  
Charm antiquark  ~ 1.3 GeV  
Strange quark  ~ 100 MeV  
Strange antiquark  ~ 100 MeV  
Generation 3  
Fermion (lefthanded)  Symbol  Electric charge  Weak isospin  Weak hypercharge  Color charge^{[lhf 1]}  Mass^{[lhf 2]}  
Tau  1.78 GeV  
Antitau  1.78 GeV  
Tau neutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Tau antineutrino  < 0.28 eV^{[lhf 3]}^{[lhf 4]}  
Top quark  171 GeV  
Top antiquark  171 GeV  
Bottom quark  ~ 4.2 GeV  
Bottom antiquark  ~ 4.2 GeV  

[edit]Tests and predictions
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2008) 
The Standard Model (SM) predicted the existence of the W and Z bosons, gluon, and the top andcharm quarks before these particles were observed. Their predicted properties were experimentally confirmed with good precision. To give an idea of the success of the SM, the following table compares the measured masses of the W and Z bosons with the masses predicted by the SM:
Quantity  Measured (GeV)  SM prediction (GeV) 

Mass of W boson  80.387 ± 0.019  80.390 ± 0.018 
Mass of Z boson  91.1876 ± 0.0021  91.1874 ± 0.0021 
The SM also makes several predictions about the decay of Z bosons, which have been experimentally confirmed by the Large ElectronPositron Collider at CERN.
In May 2012 BaBar Collaboration reported that their recently analyzed data may suggest possible flaws in the Standard Model of particle physics.^{[23]}^{[24]} These data show that a particular type of particle decay called "B to Dstartaunu" happens more often than the Standard Model says it should. In this type of decay, a particle called the Bbar meson decays into a D meson, an antineutrino and a taulepton. While the level of certainty of the excess (3.4 sigma) is not enough to claim a break from the Standard Model, the results are a potential sign of something amiss and are likely to impact existing theories, including those attempting to deduce the properties of Higgs bosons.^{[25]}
[edit]Challenges
See also: Beyond the Standard Model
This unreferenced section requires citations to ensureverifiability. 

Selfconsistency of the Standard Model has not been mathematically proven. While computational approximations (for example using lattice gauge theory) exist, it is not known whether they converge in the limit. A key question related to the consistency is the Yang–Mills existence and mass gap problem.
There is some experimental evidence consistent with neutrinos having mass, which the Standard Model does not allow.^{[26]} To accommodate such findings, the Standard Model can be modified by adding a nonrenormalizable interaction of lepton fields with the square of the Higgs field. This is natural in certain grand unified theories, and if new physics appears at about 10^{16} GeV, the neutrino masses are of the right order of magnitude.
Currently, there is one elementary particle predicted by the Standard Model that has yet to be observed: the Higgs boson. A major reason for building the Large Hadron Collider is that the high energies of which it is capable are expected to make the Higgs boson observable. However, as of January 2012, there is only indirect empirical evidence for the existence of the Higgs boson, so that its discovery cannot be claimed. Moreover, some theoretical concerns have been raised positing that elementary scalar Higgs particles cannot exist (see Quantum triviality).
Theoretical and experimental research has attempted to extend the Standard Model into a Unified Field Theory or a Theory of everything, a complete theory explaining all physical phenomena including constants. Inadequacies of the Standard Model that motivate such research include:
 It does not attempt to explain gravitation, although a theoretical particle known as a graviton would help explain it, and unlike for the strong and electroweak interactions of the Standard Model, there is no known way of describing general relativity, the canonical theory of gravitation, consistently in terms of quantum field theory. The reason for this is, among other things, that quantum field theories of gravity generally break down before reaching the Planck scale. As a consequence, we have no reliable theory for the very early universe;
 Some consider it to be adhoc and inelegant, requiring 19 numerical constants whose values are unrelated and arbitrary. Although the Standard Model, as it now stands, can explain why neutrinos have masses, the specifics of neutrino mass are still unclear. It is believed that explaining neutrino mass will require an additional 7 or 8 constants, which are also arbitrary parameters;
 The Higgs mechanism gives rise to the hierarchy problem if any new physics (such as quantum gravity) is present at high energy scales. In order for the weak scale to be much smaller than the Planck scale, severe fine tuning of Standard Model parameters is required;
 It should be modified so as to be consistent with the emerging "Standard Model of cosmology." In particular, the Standard Model cannot explain the observed amount ofcold dark matter (CDM) and gives contributions to dark energy which are far too large. It is also difficult to accommodate the observed predominance of matter over antimatter (matter/antimatter asymmetry). The isotropy and homogeneity of the visible universe over large distances seems to require a mechanism like cosmic inflation, which would also constitute an extension of the Standard Model.
Currently no proposed Theory of everything has been conclusively verified.
[edit]See also
Book: Particles of the Standard Model  
Wikipedia books are collections of articles that can be downloaded or ordered in print. 
 1964 PRL symmetry breaking papers
 C. R. Hagen
 Diagrams: Feynman diagram  Penguin diagram
 Elementary particle: Boson, Fermion
 Flavour
 Fundamental interaction:
 Gauge theory: Nontechnical introduction to gauge theory
 Generation
 Higgs mechanism: Higgs boson, Higgsless model
 J. C. Ward
 J. J. Sakurai Prize for Theoretical Particle Physics
 Lagrangian
 Noncommutative Standard Model
 Open questions: BTeV experiment, CP violation, Neutrino masses, Quark matter
 Quantum field theory
 Standard Model: Mathematical formulation of, Physics beyond the Standard Model
 Unparticle physics
[edit]Notes and references
[edit]Notes
[edit]References
 ^ S.L. Glashow (1961). "Partialsymmetries of weak interactions". Nuclear Physics 22(4): 579–588. Bibcode 1961NucPh..22..579G. DOI:10.1016/00295582(61)904692.
 ^ S. Weinberg (1967). "A Model of Leptons". Physical Review Letters 19 (21): 1264–1266. Bibcode 1967PhRvL..19.1264W. DOI:10.1103/PhysRevLett.19.1264.
 ^ A. Salam (1968). N. Svartholm. ed. Elementary Particle Physics: Relativistic Groups and Analyticity. Eighth Nobel Symposium. Stockholm: Almquvist and Wiksell. pp. 367.
 ^ F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–323. Bibcode 1964PhRvL..13..321E.DOI:10.1103/PhysRevLett.13.321.
 ^ P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons".Physical Review Letters 13 (16): 508–509. Bibcode 1964PhRvL..13..508H.DOI:10.1103/PhysRevLett.13.508.
 ^ G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585–587. Bibcode1964PhRvL..13..585G. DOI:10.1103/PhysRevLett.13.585.
 ^ F.J. Hasert et al. (1973). "Search for elastic muonneutrino electron scattering".Physics Letters B 46: 121. Bibcode 1973PhLB...46..121H. DOI:10.1016/03702693(73)904942.
 ^ F.J. Hasert et al. (1973). "Observation of neutrinolike interactions without muon or electron in the gargamelle neutrino experiment". Physics Letters B 46: 138. Bibcode1973PhLB...46..138H. DOI:10.1016/03702693(73)904991.
 ^ F.J. Hasert et al. (1974). "Observation of neutrinolike interactions without muon or electron in the Gargamelle neutrino experiment". Nuclear Physics B 73: 1. Bibcode1974NuPhB..73....1H. DOI:10.1016/05503213(74)900388.
 ^ D. Haidt (4 October 2004). "The discovery of the weak neutral currents". CERN Courier. Retrieved 8 May 2008.
 ^ "Details can be worked out if the situation is simple enough for us to make an approximation, which is almost never, but often we can understand more or less what is happening." from The Feynman Lectures on Physics, Vol 1. pp. 2–7
 ^ F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–323. Bibcode 1964PhRvL..13..321E.DOI:10.1103/PhysRevLett.13.321.
 ^ P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons".Physical Review Letters 13 (16): 508–509. Bibcode 1964PhRvL..13..508H.DOI:10.1103/PhysRevLett.13.508.
 ^ G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585–587. Bibcode1964PhRvL..13..585G. DOI:10.1103/PhysRevLett.13.585.
 ^ G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode2009IJMPA..24.2601G. DOI:10.1142/S0217751X09045431.
 ^ http://cms.web.cern.ch/cms/News/2011/LP11/
 ^ http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLASCONF2011135/
 ^ http://www.zdnet.co.uk/news/emergingtech/2011/07/25/cernhiggsbosonanswertocomebyendof201240093510/
 ^ http://news.cnet.com/830130685_357342044264/cernphysicistsfindhintofhiggsboson/
 ^ http://operaweb.lngs.infn.it/spip.php?rubrique14 31May2010 Press Release.
 ^ "Neutrino 'ghost particle' sized up by astronomers". BBC News. 22 June 2010.
 ^ W.M. Yao et al. (Particle Data Group) (2006). "Review of Particle Physics: Quarks". Journal of Physics G 33: 1. arXiv:astroph/0601168. Bibcode2006JPhG...33....1Y. DOI:10.1088/09543899/33/1/001.
 ^ BABAR Data in Tension with the Standard Model (SLAC pressrelease).
 ^ BaBar Collaboration, Evidence for an excess of B > D(*) Tau Nu decays,arXiv:1205.5442.
 ^ BaBar data hint at cracks in the Standard Model (EScienceNews.com).
 ^ CERN Press Release
[edit]Further reading
 R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume.
 B.A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Press. ISBN 080187971X.
 Introductory textbooks
 I. Aitchison, A. Hey (2003). Gauge Theories in Particle Physics: A Practical Introduction.. Institute of Physics. ISBN 9780585445502.
 W. Greiner, B. Müller (2000). Gauge Theory of Weak Interactions. Springer. ISBN 3540676724.
 G.D. Coughlan, J.E. Dodd, B.M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists. Cambridge University Press.
 D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0471603864.
 G.L. Kane (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0201117495.
 Advanced textbooks
 T.P. Cheng, L.F. Li (2006). Gauge theory of elementary particle physics. Oxford University Press. ISBN 0198519613. Highlights the gauge theory aspects of the Standard Model.
 J.F. Donoghue, E. Golowich, B.R. Holstein (1994). Dynamics of the Standard Model. Cambridge University Press. ISBN 9780521476522. Highlights dynamical andphenomenological aspects of the Standard Model.
 L. O'Raifeartaigh (1988). Group structure of gauge theories. Cambridge University Press. ISBN 0521347858. Highlights grouptheoretical aspects of the Standard Model.
 Journal articles
 E.S. Abers, B.W. Lee (1973). "Gauge theories". Physics Reports 9: 1–141. Bibcode 1973PhR.....9....1A. DOI:10.1016/03701573(73)900276.
 Y. Hayato et al. (1999). "Search for Proton Decay through p → νK^{+} in a Large Water Cherenkov Detector". Physical Review Letters 83 (8): 1529. arXiv:hepex/9904020.Bibcode 1999PhRvL..83.1529H. DOI:10.1103/PhysRevLett.83.1529.
 S.F. Novaes (2000). "Standard Model: An Introduction". arXiv:hepph/0001283 [hepph].
 D.P. Roy (1999). "Basic Constituents of Matter and their Interactions — A Progress Report.". arXiv:hepph/9912523 [hepph].
 F. Wilczek (2004). "The Universe Is A Strange Place". Nuclear Physics B  Proceedings Supplements 134: 3. arXiv:astroph/0401347. Bibcode 2004NuPhS.134....3W.DOI:10.1016/j.nuclphysbps.2004.08.001.
[edit]External links
 "LHC sees hint of lightweight Higgs boson" "New Scientist".
 "Standard Model may be found incomplete," New Scientist.
 "Observation of the Top Quark" at Fermilab.
 "The Standard Model Lagrangian." After electroweak symmetry breaking, with no explicit Higgs boson.
 "Standard Model Lagrangian" with explicit Higgs terms. PDF, PostScript, and LaTeX versions.
 "The particle adventure." Web tutorial.
 Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, Part 2, Part 3a, Part 3b.


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