Gauge boson

In particle physics, a gauge boson is a force carrier, a bosonic particle that carries any of the fundamental interactions of nature, commonly called forces.[1][2] Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge bosons—usually as virtual particles.

All known gauge bosons have a spin of 1. Therefore, all known gauge bosons are vector bosons.

Gauge bosons are different from the other kinds of bosons: first, fundamental scalar bosons (the Higgs boson); second, mesons, which are composite bosons, made of quarks; third, larger composite, non-force-carrying bosons, such as certain atoms.

Standard Model of Elementary Particles
The Standard Model of elementary particles, with the gauge bosons in the fourth column in red

Gauge bosons in the Standard Model

The Standard Model of particle physics recognizes four kinds of gauge bosons: photons, which carry the electromagnetic interaction; W and Z bosons, which carry the weak interaction; and gluons, which carry the strong interaction.[3]

Isolated gluons do not occur because they are color-charged and subject to color confinement.

Multiplicity of gauge bosons

In a quantized gauge theory, gauge bosons are quanta of the gauge fields. Consequently, there are as many gauge bosons as there are generators of the gauge field. In quantum electrodynamics, the gauge group is U(1); in this simple case, there is only one gauge boson. In quantum chromodynamics, the more complicated group SU(3) has eight generators, corresponding to the eight gluons. The three W and Z bosons correspond (roughly) to the three generators of SU(2) in GWS theory.

Massive gauge bosons

For technical reasons involving gauge invariance, gauge bosons are described mathematically by field equations for massless particles. Therefore, at a naïve theoretical level, all gauge bosons are required to be massless, and the forces that they describe are required to be long-ranged. The conflict between this idea and experimental evidence that the weak and strong interactions have a very short range requires further theoretical insight.

According to the Standard Model, the W and Z bosons gain mass via the Higgs mechanism. In the Higgs mechanism, the four gauge bosons (of SU(2)×U(1) symmetry) of the unified electroweak interaction couple to a Higgs field. This field undergoes spontaneous symmetry breaking due to the shape of its interaction potential. As a result, the universe is permeated by a nonzero Higgs vacuum expectation value (VEV). This VEV couples to three of the electroweak gauge bosons (the Ws and Z), giving them mass; the remaining gauge boson remains massless (the photon). This theory also predicts the existence of a scalar Higgs boson, which has been observed in experiments at the LHC.[4]

Beyond the Standard Model

Grand unification theories

The Georgi-Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons mediate interactions between quarks and leptons, hence violating conservation of baryon number and causing proton decay. Such bosons would be even more massive than W and Z bosons due to symmetry breaking. Analysis of data collected from such sources as the Super-Kamiokande neutrino detector has yielded no evidence of X and Y bosons.

Gravitons

The fourth fundamental interaction, gravity, may also be carried by a boson, called the graviton. In the absence of experimental evidence and a mathematically coherent theory of quantum gravity, it is unknown whether this would be a gauge boson or not. The role of gauge invariance in general relativity is played by a similar symmetry: diffeomorphism invariance.

W' and Z' bosons

W' and Z' bosons refer to hypothetical new gauge bosons (named in analogy with the Standard Model W and Z bosons).

See also

References

  1. ^ Gribbin, John (2000). Q is for Quantum – An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X.
  2. ^ Clark, John, E.O. (2004). The Essential Dictionary of Science. Barnes & Noble. ISBN 0-7607-4616-8.CS1 maint: Multiple names: authors list (link)
  3. ^ Veltman, Martinus (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific. ISBN 981-238-149-X.
  4. ^ "CERN and the Higgs boson". CERN. Retrieved 23 November 2016.

External links

Bilepton

A bilepton is a hypothetical particle predicted by the minimal 331 model. It is a spin one gauge boson which appears with single and double electric charge and with lepton number L=+2 and L=-2. It can mediate exotic processes such as V+A muon decay and muonium-antimuonium conversion. It can also give rise to exotic scattering processes such as electron plus electron goes to muon plus muon which is forbidden in the Standard Model. Detailed measurements made at the Paul Scherrer Institute (PSI) have searched for the exotic muon decay and placed a lower bound on the bilepton mass of about 1 TeV. Studies of muonium-antimuonium conversion, also at PSI, have imposed a similar bound. Higher statistics experiments are planned. The bilepton could alternatively be discovered in electron-electron collisions or in multi TeV proton-proton collisions at the Large Hadron Collider.

This particle was at first misnamed "dilepton", with a different pre-existing usage; the present name was introduced in 1996.

Charge (physics)

In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges correspond to the time-invariant generators of a symmetry group, and specifically, to the generators that commute with the Hamiltonian. Charges are often denoted by the letter Q, and so the invariance of the charge corresponds to the vanishing commutator , where H is the Hamiltonian. Thus, charges are associated with conserved quantum numbers; these are the eigenvalues q of the generator Q.

Danilo Zavrtanik

Danilo Zavrtanik (born 15 August 1953) is a Slovenian physicist and professor.

Born in Nova Gorica, he graduated in 1979 from the Faculty for Natural Sciences and Technology of the University of Ljubljana. In 1987, he obtained his PhD by defending a doctoral dissertation on "Angular distribution analysis of the reaction π−p -> π−π+n". From 2006 he is a full professor of physics at the University of Nova Gorica and the head of its Laboratory for Astroparticle Physics.

Before 1995 he was active in eksperimental particle physics through international collaborations CPLEAR and DELPHI at CERN, where he was involved in the development of particle detectors, studies of CP, T and CPT violation in the decays of neutral kaons K0 and studies of heavy quark decays, W gauge boson decays and Higgs boson searches. Since 1995 he is working in the field of astroparticle physics within the Pierre Auger Collaboration, focusing on studies of cosmic rays with extreme energies. He is a co-author of more than 350 scientific papers and more than 150 conference contributions. He has been a thesis adviser to a number of undergraduate and graduate students at the University of Ljubljana and the University of Nova Gorica.

From 1992 to 1996 he served as director-general of the Jožef Stefan Institute in Ljubljana. In 1995 he initiated the founding the School of Environmental Sciences in Nova Gorica which grew into Nova Gorica Polytechnic and finally into University of Nova Gorica. Since the beginning he has been serving as the president of this institution (from 2010 as a rector).

Among his many honours and awards the most distinguished are: "Order of Merit (Slovenia)" in 2005 "Ambassador of Science of the Republic of Slovenia" in 1997 and "Zois Award" (highest Slovenian award for scientific achievements) in 2004.

Dark photon

The dark photon (also hidden, heavy, para-, secluded photon, or phaeton) is a hypothetical hidden sector particle, proposed as a force carrier similar to the photon of electromagnetism but potentially connected to dark matter. In a minimal scenario, this new force can be introduced by extending the gauge group of the Standard Model of Particle Physics with a new abelian U(1) gauge symmetry. The corresponding new spin-1 gauge boson (i.e. the dark photon) can then couple very weakly to electrically charged particles through kinetic mixing with the ordinary photon and could thus be detected. Other types of couplings beyond kinetic mixing are also possible.

Dieter Zeppenfeld

Dieter Zeppenfeld from the University of Wisconsin, was awarded the status of Fellow in the American Physical Society, after they were nominated by their Division of Particles and Fields in 1999, for pioneering contributions to the theoretical formulation of effective electroweak gauge boson interactions in a model-independent way and in the linear-sigma model, which initiated phenomenological and experimental studies of gauge boson anomalous coup.

Gauge theory

In physics, a gauge theory is a type of field theory in which the Lagrangian is invariant under certain Lie groups of local transformations.

The term gauge refers to any specific mathematical formalism to regulate redundant degrees of freedom in the Lagrangian. The transformations between possible gauges, called gauge transformations, form a Lie group—referred to as the symmetry group or the gauge group of the theory. Associated with any Lie group is the Lie algebra of group generators. For each group generator there necessarily arises a corresponding field (usually a vector field) called the gauge field. Gauge fields are included in the Lagrangian to ensure its invariance under the local group transformations (called gauge invariance). When such a theory is quantized, the quanta of the gauge fields are called gauge bosons. If the symmetry group is non-commutative, then the gauge theory is referred to as non-abelian gauge theory, the usual example being the Yang–Mills theory.

Many powerful theories in physics are described by Lagrangians that are invariant under some symmetry transformation groups. When they are invariant under a transformation identically performed at every point in the spacetime in which the physical processes occur, they are said to have a global symmetry. Local symmetry, the cornerstone of gauge theories, is a stronger constraint. In fact, a global symmetry is just a local symmetry whose group's parameters are fixed in spacetime (the same way a constant value can be understood as a function of a certain parameter, the output of which is always the same).

Gauge theories are important as the successful field theories explaining the dynamics of elementary particles. Quantum electrodynamics is an abelian gauge theory with the symmetry group U(1) and has one gauge field, the electromagnetic four-potential, with the photon being the gauge boson. The Standard Model is a non-abelian gauge theory with the symmetry group U(1) × SU(2) × SU(3) and has a total of twelve gauge bosons: the photon, three weak bosons and eight gluons.

Gauge theories are also important in explaining gravitation in the theory of general relativity. Its case is somewhat unusual in that the gauge field is a tensor, the Lanczos tensor. Theories of quantum gravity, beginning with gauge gravitation theory, also postulate the existence of a gauge boson known as the graviton. Gauge symmetries can be viewed as analogues of the principle of general covariance of general relativity in which the coordinate system can be chosen freely under arbitrary diffeomorphisms of spacetime. Both gauge invariance and diffeomorphism invariance reflect a redundancy in the description of the system. An alternative theory of gravitation, gauge theory gravity, replaces the principle of general covariance with a true gauge principle with new gauge fields.

Historically, these ideas were first stated in the context of classical electromagnetism and later in general relativity. However, the modern importance of gauge symmetries appeared first in the relativistic quantum mechanics of electrons – quantum electrodynamics, elaborated on below. Today, gauge theories are useful in condensed matter, nuclear and high energy physics among other subfields.

Gaugino condensation

In particle physics, gaugino condensation is the nonzero vacuum expectation value in some models of a bilinear expression constructed in theories with supersymmetry from the superpartner of a gauge boson called the gaugino. The gaugino and the bosonic gauge field and the D-term are all components of a supersymmetric vector superfield in the Wess-Zumino gauge.

where represents the gaugino field (a spinor) and is an energy scale, a and b represent Lie algebra indices and α and β represent van der Waerden (two component spinor) indices. The mechanism is somewhat analogous to chiral symmetry breaking and is an example of a fermionic condensate.

In the superfield notation, is the gauge field strength and is a chiral superfield.

is also a chiral superfield and we see that what acquires a nonzero VEV is not the F-term of this chiral superfield. Because of this, gaugino condensation in and of itself does not lead to supersymmetry breaking. If we also have supersymmetry breaking, it is caused by something other than the gaugino condensate.

However, a gaugino condensate definitely breaks U(1)R symmetry as has an R-charge of 2.

Glossary of string theory

This page is a glossary of terms in string theory, including related areas such as supergravity, supersymmetry, and high energy physics.

Gluon

A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. In layman's terms, they "glue" quarks together, forming hadrons such as protons and neutrons.

In technical terms, gluons are vector gauge bosons that mediate strong interactions of quarks in quantum chromodynamics (QCD). Gluons themselves carry the color charge of the strong interaction. This is unlike the photon, which mediates the electromagnetic interaction but lacks an electric charge. Gluons therefore participate in the strong interaction in addition to mediating it, making QCD significantly harder to analyze than QED (quantum electrodynamics).

History of quantum field theory

In particle physics, the history of quantum field theory starts with its creation by Paul Dirac, when he attempted to quantize the electromagnetic field in the late 1920s. Major advances in the theory were made in the 1940s and 1950s, and led to the introduction of renormalized quantum electrodynamics (QED) developed by Richard Feynman. QED was so successful and accurately predictive that efforts were made to apply the same basic concepts for the other forces of nature. By the late 1970s, these efforts successfully utilized gauge theory in the strong nuclear force and weak nuclear force, producing the modern standard model of particle physics.

Efforts to describe gravity using the same techniques have, to date, failed. The study of quantum field theory is still flourishing, as are applications of its methods to many physical problems. It remains one of the most vital areas of theoretical physics today, providing a common language to several different branches of physics.

Lightest Supersymmetric Particle

In particle physics, the lightest supersymmetric particle (LSP) is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models. In models with R-parity conservation, the LSP is stable; in other words, the LSP cannot decay into any Standard Model particle, since all SM particles have the opposite R-parity. There is extensive observational evidence for an additional component of the matter density in the Universe that goes under the name dark matter. The LSP of supersymmetric models is a dark matter candidate and is a weakly interacting massive particle (WIMP).

Majoron

In particle physics, majorons (named after Ettore Majorana) are a hypothetical type of Goldstone boson that are theorized to mediate the neutrino mass violation of lepton number or B − L in certain high energy collisions such as

e− + e− → W− + W− + JWhere two electrons collide to form two W bosons and the majoron J. The U(1)B–L symmetry is assumed to be global so that the majoron is not "eaten up" by the gauge boson and spontaneously broken. Majorons were originally formulated in four dimensions by Y. Chikashige, R. N. Mohapatra and R. D. Peccei to understand neutrino masses by the seesaw mechanism and are being searched for in the neutrino-less double beta decay process. There are theoretical extensions of this idea into supersymmetric theories and theories involving extra compactified dimensions. By propagating through the extra spatial dimensions the detectable number of majoron creation events vary accordingly. Mathematically, majorons may be modeled by allowing them to propagate through a material while all other Standard Model forces are fixed to an orbifold point.

Massless particle

In particle physics, a massless particle is an elementary particle whose invariant mass is zero. The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons. Neutrinos were originally thought to be massless. However, because neutrinos change flavor as they travel, at least two of the types of neutrinos must have mass. The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita sharing the 2015 Nobel prize in physics.

Next-to-Minimal Supersymmetric Standard Model

In particle physics, NMSSM is an acronym for Next-to-Minimal Supersymmetric Standard Model. It is a supersymmetric extension to the Standard Model that adds an additional singlet chiral superfield to the MSSM and can be used to dynamically generate the term, solving the μ {\displaystyle \mu } -problem. Articles about the NMSSM are available for review.

The Minimal Supersymmetric Standard Model does not explain why the parameter in the superpotential term is at the electroweak scale. The idea behind the Next-to-Minimal Supersymmetric Standard Model is to promote the term to a gauge singlet, chiral superfield . Note that the scalar superpartner of the singlino is denoted by and the spin-1/2 singlino superpartner by in the following. The superpotential for the NMSSM is given by

where gives the Yukawa couplings for the Standard Model fermions. Since the superpotential has a mass dimension of 3, the couplings and are dimensionless; hence the -problem of the MSSM is solved in the NMSSM, the superpotential of the NMSSM being scale-invariant. The role of the term is to generate an effective term. This is done with the scalar component of the singlet getting a vacuum-expectation value of ; that is, we have

Without the term the superpotential would have a U(1)' symmetry, so-called Peccei–Quinn symmetry; see Peccei–Quinn theory. This additional symmetry would alter the phenomenology completely. The role of the term is to break this U(1)' symmetry. The term is introduced trilinearly such that is dimensionless. However, there remains a discrete symmetry, which is moreover broken spontaneously. In principle this leads to the domain wall problem. Introducing additional but suppressed terms, the symmetry can be broken without changing phenomenology at the electroweak scale. It is assumed that the domain wall problem is circumvented in this way without any modifications except far beyond the electroweak scale.

Other models have been proposed which solve the -problem of the MSSM. One idea is to keep the term in the superpotential and take the U(1)' symmetry into account. Assuming this symmetry to be local, an additional, gauge boson is predicted in this model, called the UMSSM.[citation needed]

Photon

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). Invariant mass of the photon is zero; it always moves at the speed of light within a vacuum.

Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of both waves and particles. For example, a single photon may be refracted by a lens and exhibit wave interference with itself, and it can behave as a particle with definite and finite measurable position or momentum, though not both at the same time as per Heisenberg's uncertainty principle. The photon's wave and quantum qualities are two observable aspects of a single phenomenon—they cannot be described by any mechanical model; a representation of this dual property of light that assumes certain points on the wavefront to be the seat of the energy is not possible. The quanta in a light wave are not spatially localized.

The modern concept of the photon was developed gradually by Albert Einstein in the early 20th century to explain experimental observations that did not fit the classical wave model of light. The benefit of the photon model is that it accounts for the frequency dependence of light's energy, and explains the ability of matter and electromagnetic radiation to be in thermal equilibrium. The photon model accounts for anomalous observations, including the properties of black-body radiation, that others (notably Max Planck) had tried to explain using semiclassical models. In that model, light is described by Maxwell's equations, but material objects emit and absorb light in quantized amounts (i.e., they change energy only by certain particular discrete amounts). Although these semiclassical models contributed to the development of quantum mechanics, many further experiments beginning with the phenomenon of Compton scattering of single photons by electrons, validated Einstein's hypothesis that light itself is quantized. In December 1926, American physical chemist Gilbert N. Lewis coined the widely-adopted name "photon" for these particles in a letter to Nature. After Arthur H. Compton won the Nobel Prize in 1927 for his scattering studies, most scientists accepted that light quanta have an independent existence, and the term "photon" was accepted.

In the Standard Model of particle physics, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass, and spin, are determined by this gauge symmetry. The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers, and for applications in optical imaging and optical communication such as quantum cryptography.

Supermultiplet

In theoretical physics, a supermultiplet is a representation of a supersymmetry algebra. It consists of a collection of particles, called superpartners, corresponding to operators in a quantum field theory which in superspace are represented by superfields.

Superfields were introduced by Abdus Salam and J. A. Strathdee in their 1974 article Supergauge Transformations. Operations on superfields and a partial classification were presented a few months later by Sergio Ferrara, Julius Wess and Bruno Zumino in Supergauge Multiplets and Superfields.

The most commonly used supermultiplets are vector multiplets, chiral multiplets (in 4d N=1 supersymmetry for example), hypermultiplets (in 4d N=2 supersymmetry for example), tensor multiplets and gravity multiplets. The highest component of a vector multiplet is a gauge boson, the highest component of a chiral or hypermultiplet is a spinor, the highest component of a gravity multiplet is a graviton. The names are defined so as to be invariant under dimensional reduction, although the organization of the fields as representations of the Lorentz group changes.

Note, however, that the use of these names for the different multiplets can vary in literature. Sometimes a chiral multiplet (whose highest component is a spinor) can be referred to as a scalar multiplet. Also, in N=2 SUSY, a vector multiplet (whose highest component is a vector) can sometimes be referred to as a chiral multiplet.

Especially in theories with extended supersymmetry, supermultiplets can be divided to short supermultiplets and long supermultiplets, essentially according to the dimensionality. The short supermultiplets coincide with the BPS states.

A scalar is never the highest component of a superfield, whether it appears in a superfield at all depends on the dimension of the spacetime. For example, in a 10-dimensional N=1 theory the vector multiplet contains only a vector and a Majorana-Weyl spinor, while its dimensional reduction on a d-dimensional torus is a vector multiplet containing d real scalars. Similarly, in an 11-dimensional theory there is only one supermultiplet with a finite number of fields, the gravity multiplet, and it contains no scalars. However again its dimensional reduction on a d-torus to a maximal gravity multiplet does contain scalars.

Vacuum polarization

In quantum field theory, and specifically quantum electrodynamics, vacuum polarization describes a process in which a background electromagnetic field produces virtual electron–positron pairs that change the distribution of charges and currents that generated the original electromagnetic field. It is also sometimes referred to as the self-energy of the gauge boson (photon).

After developments in radar equipment for World War II resulted in higher accuracy for measuring the energy levels of the hydrogen atom, I.I. Rabi made measurements of the Lamb shift and the anomalous magnetic dipole moment of the electron. These effects corresponded to the deviation from the Dirac equation predicted value of two of the spectroscopic electron g-factor values. Later, Hans Bethe theoretically calculated those shifts in the hydrogen energy levels due to vacuum polarization on his return train ride from the Shelter Island Conference to Cornell.

The effects of vacuum polarization have been routinely observed experimentally since then as very well understood background effects. Vacuum polarization referred to below as the one loop contribution occurs with leptons (electron-positron pairs) or quarks, the former (leptons) first observed in 1940s but also more recently observed in 1997 using the TRISTAN particle accelerator in Japan, the latter (quarks) along with multiple quark-gluon loop contributions from the early 1970s to mid-1990s using the VEPP-2M particle accelerator at the Budker Institute of Nuclear Physics in Siberia in Russia and many other accelerator laboratories worldwide.

W and Z bosons

The W and Z bosons are together known as the weak or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are W+, W−, and Z. The W bosons have either a positive or negative electric charge of 1 elementary charge and are each other's antiparticles. The Z boson is electrically neutral and is its own antiparticle. The three particles have a spin of 1. The W bosons have a magnetic moment, but the Z has none. All three of these particles are very short-lived, with a half-life of about 3×10−25 s. Their experimental discovery was a triumph for what is now known as the Standard Model of particle physics.

The W bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the "Z particle", and later gave the explanation that it was the last additional particle needed by the model. The W bosons had already been named, and the Z bosons were named for having zero electric charge.The two W bosons are verified mediators of neutrino absorption and emission. During these processes, the W boson charge induces electron or positron emission or absorption, thus causing nuclear transmutation. The Z boson is not involved in the absorption or emission of electrons and positrons.

The Z boson mediates the transfer of momentum, spin and energy when neutrinos scatter elastically from matter (a process which conserves charge). Such behavior is almost as common as inelastic neutrino interactions and may be observed in bubble chambers upon irradiation with neutrino beams. Whenever an electron is observed as a new free particle suddenly moving with kinetic energy, it is inferred to be a result of a neutrino interacting directly with the electron, since this behavior happens more often when the neutrino beam is present. In this process, the neutrino simply strikes the electron and then scatters away from it, transferring some of the neutrino's momentum to the electron.

Because neutrinos are neither affected by the strong force nor the electromagnetic force, and because the gravitational force between subatomic particles is negligible, such an interaction can only happen via the weak force. Since such an electron is not created from a nucleon, and is unchanged except for the new force impulse imparted by the neutrino, this weak force interaction between the neutrino and the electron must be mediated by an electromagnetically neutral, weak-force boson particle. Thus, this interaction requires a Z boson.

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