standard model


  • The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetic, weak and strong interactions – excluding gravity)
    in the universe and classifying all known elementary particles.

  • Mathematical consistency of the Standard Model requires that any mechanism capable of generating the masses of elementary particles must become visible[clarification needed]
    at energies above 1.4 TeV;[40] therefore, the LHC (designed to collide two 7 TeV proton beams) was built to answer the question of whether the Higgs boson actually exists.

  • [59] There are also issues of quantum triviality, which suggests that it may not be possible to create a consistent quantum field theory involving elementary scalar particles.

  • Fundamental interactions The Standard Model describes three of the four fundamental interactions in nature; only gravity remains unexplained.

  • Though it addresses strong and electroweak interactions, the Standard Model does not consistently explain the canonical theory of gravitation, general relativity, in terms
    of quantum field theory.

  • If one insists on using only Standard Model particles, this can be achieved by adding a non-renormalizable interaction of leptons with the Higgs boson.

  • [50] The Standard Model also predicted the existence of the Higgs boson, which was found in 2012 at the Large Hadron Collider, the final fundamental particle predicted by
    the Standard Model to be experimentally confirmed.

  • The (fundamental) strong interaction is described by quantum chromodynamics, which is a component of the Standard Model.

  • The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model.

  • 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.

  • [46] 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.

  • quantum chromodynamics, QCD), to which many contributed, acquired its modern form in 1973–74 when asymptotic freedom was proposed[18][19] (a development which made QCD the
    main focus of theoretical research)[20] and experiments confirmed that the hadrons were composed of fractionally charged quarks.

  • The interactions between all the particles described by the Standard Model are summarized by the diagrams on the right of this section.

  • Inadequacies of the Standard Model that motivate such research include: • The model does not explain gravitation, although physical confirmation of a theoretical particle
    known as a graviton would account for it to a degree.

  • Contentions include the absence of an explanation in the Standard Model of particle physics for the observed amount of cold dark matter (CDM) and its contributions to dark
    energy, which are many orders of magnitude too large.

  • [47] Gravity[edit] See also: Quantum gravity and Gravity Despite being perhaps the most familiar fundamental interaction, gravity is not described by the Standard Model, due
    to contradictions that arise when combining general relativity, the modern theory of gravity, and quantum mechanics.

  • It is used as a basis for building more exotic models that incorporate hypothetical particles, extra dimensions, and elaborate symmetries (such as supersymmetry) to explain
    experimental results at variance with the Standard Model, such as the existence of dark matter and neutrino oscillations.

  • In the Standard Model, such an interaction is described as an exchange of bosons between the objects affected, such as a photon for the electromagnetic force and a gluon for
    the strong interaction.

  • [58] • The Higgs mechanism gives rise to the hierarchy problem if some new physics (coupled to the Higgs) is present at high energy scales.

  • The Standard Model is a paradigm of a quantum field theory for theorists, exhibiting a wide range of phenomena, including spontaneous symmetry breaking, anomalies, and non-perturbative

  • 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.

  • The reason for this is, among other things, that quantum field theories of gravity generally break down before reaching the Planck scale.

  • Although the Standard Model is believed to be theoretically self-consistent[note 1] and has demonstrated some success in providing experimental predictions, it leaves some
    physical phenomena unexplained and so falls short of being a complete theory of fundamental interactions.

  • The fields fall into different representations of the various symmetry groups of the Standard Model (see table).

  • Notice that the addition of fermion mass terms into the electroweak Lagrangian is forbidden, since terms of the form do not respect gauge invariance.

  • In the Standard Model, the weak force is understood in terms of the electroweak theory, which states that the weak and electromagnetic interactions become united into a single
    electroweak interaction at high energies.

  • The color confinement phenomenon results in quarks being strongly bound together such that they form color-neutral composite particles called hadrons; quarks cannot individually
    exist and must always bind with other quarks.

  • The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology.

  • [51] Challenges Physics beyond the Standard Model Unsolved problem in physics: What gives rise to the Standard Model of particle physics?

  • Weak nuclear force[edit] See also: Weak interaction and Electroweak interaction The weak interaction is responsible for various forms of particle decay, such as beta decay.

  • The Standard Model explains the four fundamental forces as arising from the interactions, with fermions exchanging virtual force carrier particles, thus mediating the forces.

  • 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.

  • Higgs sector[edit] Main article: Higgs mechanism In the Standard Model, the Higgs field is an doublet of complex scalar fields with four degrees of freedom: where the superscripts
    + and 0 indicate the electric charge of the components.

  • The scalar potential is given by where , so that acquires a non-zero Vacuum expectation value, which generates masses for the Electroweak gauge fields (the Higgs’ mechanism),
    and , so that the potential is bounded from below.

  • While regularized versions useful for approximate computations (for example lattice gauge theory) exist, it is not known whether they converge (in the sense of S-matrix elements)
    in the limit that the regulator is removed.

  • Tests and predictions The Standard Model predicted the existence of the W and Z bosons, gluon, top quark and charm quark, and predicted many of their properties before these
    particles were observed.

  • In these cases, in order for the weak scale to be much smaller than the Planck scale, severe fine tuning of the parameters is required; there are, however, other scenarios
    that include quantum gravity in which such fine tuning can be avoided.

  • Roughly, the three factors of the gauge symmetry give rise to the three fundamental interactions.

  • Electromagnetic interactions in the Standard Model are described by quantum electrodynamics.

  • [56] As long as new physics appears below or around, the neutrino masses can be of the right order of magnitude.

  • Fermions[edit] The Standard Model includes 12 elementary particles of spin half, known as fermions.

  • The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon, are massive.

  • Thus Z bosons are similar to the photon, aside from them being massive and interacting with the neutrino.

  • [34] The Standard Model includes 4 kinds of gauge bosons of spin 1, with bosons being quantum particles containing an integer spin.

  • Each kind of particle is described in terms of a dynamical field that pervades space-time.

  • The square of the covariant derivative leads to three and four point interactions between the electroweak gauge fields and and the scalar field .

  • The Dirac Lagrangian of the quarks coupled to the gluon fields is given by where is a three component column vector of Dirac Spinors, each element of which refers to a quark
    field with a specific color charge (i.e.

  • In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

  • It is possible to perform a gauge transformation on such that the ground state is transformed to a basis where and .

  • [3] For example, it does not fully explain baryon asymmetry, incorporate the full theory of gravitation[4] as described by general relativity, or account for the universe’s
    accelerating expansion as possibly described by dark energy.

  • W bosons have electric charge and mediate interactions that change the particle type (referred to as flavour) and charge.

  • The minimum of the potential is degenerate with an infinite number of equivalent ground state solutions, which occurs when .

  • [17] The theory of the strong interaction (i.e.

  • Quarks also carry electric charge and weak isospin, and thus interact with other fermions through electromagnetism and weak interaction.

  • Each member of a generation has a greater mass than the corresponding particle of generations prior.

  • [44][45] Theoretical aspects Construction of the Standard Model Lagrangian[edit] Technically, quantum field theory provides the mathematical framework for the Standard Model,
    in which a Lagrangian controls the dynamics and kinematics of the theory.


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