In physics, the combination of two theories of particle physics into a single framework to describe all interactions of subatomic particles except those due to gravity (see gravitation). The two theories, the electroweak theory and the theory of quantum chromodynamics, describe the interactions between particles in terms of the exchange of intermediary particles. The model has proved highly accurate in predicting certain interactions, but it does not explain all aspects of subatomic particles. For example, it cannot say how many particles there should be or what their masses are. The search goes on for a more complete theory, and in particular a unified field theory describing the strong, weak, and electromagnetic forces.
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The recent observation of neutrino oscillations will result in certain modifications of some of the parameters of the standard model.
The formulation of the unification of the electromagnetic and weak interactions in the Standard Model is due to Steven Weinberg, Abdus Salam and, subsequently, Sheldon Glashow. The unification model was initially proposed by Steven Weinberg in 1967, and completed integrating it with the proposals by P. Higgs, G. S. Guralnik, C. R. Hagen and T. W. B. Kibble, and F. Englert and R. Brout of spontaneous symmetry breaking which gives origin to the masses of all particles described in the model.
After the discovery, made at CERN of the existence of neutral weak currents, mediated by the boson, foreseen in the Standard Model, Glashow, Salam, and Weinberg received the Nobel Prize in Physics in 1979.
The standard model is a grouping of two major theories — quantum electroweak and quantum chromodynamics — which provides an internally consistent theory describing interactions between all experimentally observed particles. Technically, quantum field theory provides the mathematical framework for the standard model. The standard model describes each type of particle in terms of a mathematical field. For a technical description of the fields and their interactions, see standard model (mathematical formulation).
|Generation 1||Generation 2||Generation 3|
Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior (see table of fermions).
The defining property of the quarks is that they carry color charge, and hence, interact via the strong force. The infrared confining behavior of the strong force results in the quarks being perpetually bound to one another forming color-neutral composite particles (hadrons) of either two quarks (mesons) or three quarks (baryons). The familiar proton and the neutron are examples of the two lightest baryons. Quarks also carry electric charges and weak isospin. Hence they interact with other fermions electromagnetically and via the weak nuclear interactions.
The remaining six fermions that do not carry color charge are defined to be the leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by means of the weak nuclear force. For this reason neutrinos are notoriously difficult to detect in laboratories. However, the electron, muon and the tau lepton carry an electric charge so they interact electromagnetically, too.
Forces in physics are the ways that particles interact and influence each other. At a macro level, the electromagnetic force allows particles to interact with one another via electric and magnetic fields, and the force of gravitation allows two particles with mass to attract one another in accordance with Newton's Law of Gravitation. The standard model explains such forces as resulting from matter particles exchanging other particles, known as force mediating particles. When a force mediating particle is exchanged, at a macro level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force. Force mediating particles are believed to be the reason why the forces and interactions between particles observed in the laboratory and in the universe exist.
The known force mediating particles described by the Standard Model also all have spin (as do matter particles), but in their case, the value of the spin is 1, meaning that all force mediating particles are bosons. As a result, they do not follow the Pauli Exclusion Principle. The different types of force mediating particles are described below.
The interactions between all the particles described by the Standard Model are summarized in the illustration immediately above and to the right.
|Electromagnetic Force||Weak Nuclear Force||Strong Nuclear Force|
|Photon|| , , and |
The Higgs boson plays a unique role in the Standard Model, and a key role in explaining the origins of the mass of other elementary particles, in particular the difference between the massless photon and the very heavy W and Z bosons. Elementary particle masses, and the differences between electromagnetism (caused by the photon) and the weak force (caused by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory it generates the masses of the massive leptons (electron, muon and tau); and also of the quarks.
As of September 2008, no experiment has directly detected the existence of the Higgs boson, but there is some indirect evidence for it. It is hoped that upon the completion of the Large Hadron Collider, experiments conducted at CERN would bring experimental evidence confirming the existence of the particle.
Science, a journal of original scientific research, has reported: "...experimenters may have already overlooked a Higgs particle, argues theorist Chien-Peng Yuan of Michigan State University in East Lansing and his colleagues. They considered the simplest possible supersymmetric theory. Ordinarily, theorists assume that the lightest of theory's five Higgses is the one that drags on the W and Z. Those interactions then feed back on Higgs and push its mass above 121 times the mass of the proton, the highest mass searched for at CERN's Large Electron–Positron (LEP) collider, which ran from 1989 to 2000. But it's possible that the lightest Higgs weighs as little as 65 times the mass of a proton and has been missed, Yuan and colleagues argue in a paper to be published in Physical Review Letters`.
|Electron mass||511 keV|
|Muon mass||106 MeV|
|Tau lepton mass||1.78 GeV|
|Up quark mass||()||1.9 MeV|
|Down quark mass||()||4.4 MeV|
|Strange quark mass||()||87 MeV|
|Charm quark mass||()||1.32 GeV|
|Bottom quark mass||()||4.24 GeV|
|Top quark mass||(on-shell scheme)||172.7 GeV|
|CKM 12-mixing angle||0.229|
|CKM 23-mixing angle||0.042|
|CKM 13-mixing angle||0.004|
|CKM CP-Violating Phase||0.995|
|U(1) gauge coupling||()||0.357|
|SU(2) gauge coupling||()||0.652|
|SU(3) gauge coupling||()||1.221|
|QCD Vacuum Angle||~0|
|Higgs quadratic coupling||Unknown|
|Higgs self-coupling strength||Unknown|
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.
In the standard model the Higgs field is a complex spinor of the group ,
Before the symmetry breaking the Higgs Lagrangian is given as
By Noether's theorem, each of these symmetries yields an associated conservation law. They are the conservation of baryon number, electron number, muon number, and tau number. Each quark carries 1/3 of a baryon number, while each antiquark carries -1/3 of a baryon number. The conservation law implies that the total number of quarks minus number of antiquarks stays constant throughout time. Within experimental limits, no violation of this conservation law has been found.
Similarly, each electron and its associated neutrino carries +1 electron number, while the antielectron and the associated antineutrino carry -1 electron number, the muons carry +1 muon number and the tau leptons carry +1 tau number. The standard model predicts that each of these three numbers should be conserved separately in a manner similar to the baryon number. These numbers are collectively known as lepton family numbers (LF). The difference in the symmetry structures between the quark and the lepton sectors is due to the masslessness of neutrinos in the standard model. However, it was recently found that neutrinos have small mass, and oscillate between flavors, signaling the violation of these three quantum numbers.
In addition to the accidental (but exact) symmetries described above, the standard model exhibits a set of 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||TranslationsSO(3,1)||Global symmetry||Energy, Momentum, Angular momentum|
|Gauge||SU(3)SU(2)U(1)||Local symmetry||Electric charge, Weak isospin, Color charge|
|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-lepton number|
|Left-handed quark||(, , )|
|Right-handed up quark||(, , )|
|Right-handed down quark||(, , )|
|Left-handed lepton||(, , )|
|Right-handed electron||(, , )|
|Hypercharge gauge field||(, , )|
|Isospin gauge field||(, , )|
|Gluon field||(, , )|
|Higgs field||(, , )|
|Electron-neutrino||< 2 eV ****|
|Up quark||~ 3 MeV ***|
|Up antiquark||~ 3 MeV ***|
|Down quark||~ 6 MeV ***|
|Down antiquark||~ 6 MeV ***|
|Muon-neutrino||< 2 eV ****|
|Charm quark||~ 1.337 GeV|
|Charm antiquark||~ 1.3 GeV|
|Strange quark||~ 100 MeV|
|Strange antiquark||~ 100 MeV|
|Tau lepton||1.78 GeV|
|Anti-tau lepton||1.78 GeV|
|Tau-neutrino||< 2 eV ****|
|Top quark||171 GeV|
|Top antiquark||171 GeV|
|Bottom quark||~ 4.2 GeV|
|Bottom antiquark||~ 4.2 GeV|
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:
|Quantity||Measured (GeV)||SM prediction (GeV)|
|Mass of W boson||80.398±0.025||80.3900±0.0180|
|Mass of Z boson||91.1876±0.0021||91.1874±0.0021|
The Standard Model of particle physics has been empirically determined through experiments over the past fifty years. Currently the Standard Model predicts that there is one more particle to be discovered, the Higgs boson. One of the reasons for building the Large Hadron Collider is that the increase in energy is expected to make the Higgs observable. However, as of August 2008, there are only indirect experimental indications for the existence of the Higgs boson and it can not be claimed to be found.
There has been a great deal of both theoretical and experimental research exploring whether the Standard Model could be extended into a complete theory of everything. This area of research is often described by the term 'Beyond the Standard Model'. There are several motivations for this research. First, the Standard Model does not attempt to explain gravity, and it is unknown how to combine quantum field theory which is used for the Standard Model with general relativity which is the best physical model of gravity. This means that there is not a good theoretical model for phenomena such as the early universe.
Another avenue of research is related to the fact that the standard model seems very ad-hoc and inelegant. For example, the theory contains many seemingly unrelated parameters of the theory — 21 in all (18 parameters in the core theory, plus G, c and h; there are believed to be an additional 7 or 8 parameters required for the neutrino masses, although neutrino masses are outside the standard model and the details are unclear). Research also focuses on the Hierarchy problem (why the weak scale and Planck scale are so disparate), and attempts to reconcile the emerging Standard Model of Cosmology with the Standard Model of particle physics. Many questions relate to the initial conditions that led to the presently observed Universe. Examples include: Why is there a matter/antimatter asymmetry? Why is the Universe isotropic and homogeneous at large distances?