W' and Z' bosons
Encyclopedia
In particle physics
, W' and Z' bosons (or W-prime and Z-prime bosons) refer to hypothetical new gauge boson
s that arise from extensions of the electroweak symmetry
of the Standard Model
. They are named in analogy with the Standard Model W and Z bosons
.
, for example. In order for the W' bosons to couple to isospin, the extra SU(2) and the Standard Model SU(2) must mix; one copy of SU(2) must break around the TeV
scale (to get W' bosons with a TeV mass) leaving a second SU(2) for the Standard Model. This happens in Little Higgs
models that contain more than one copy of SU(2). Because the W' comes from the breaking of an SU(2), it is generically accompanied by a Z' boson of (almost) the same mass and with couplings related to the W' couplings.
Another model with W' bosons but without an additional SU(2) factor is the so-called 331 model
with . The symmetry breaking chain leads to a pair of W'± bosons and three Z' bosons.
W' bosons also arise in Kaluza-Klein theories with SU(2) in the bulk
.
predict different kinds of Z' bosons.
plus neutrino
or top quark
plus bottom quark
, after being produced in quark
-antiquark annihilation. The LHC
reach for W' discovery is expected to be a few TeV
.
Direct searches for Z' bosons are carried out at hadron
colliders, since these give access to the highest energies available. The search looks for high-mass dilepton resonances: the Z' boson would be produced by quark
-antiquark annihilation and decay to an electron
-positron
pair or a pair of opposite-charged muon
s. The most stringent current limits come from the Fermilab
Tevatron
, and depend on the couplings of the Z' boson (which control the production cross section); as of 2006, the Tevatron
excludes Z' bosons up to masses of about 800 GeV
for "typical" cross sections predicted in various models. The LHC
will extend this reach up to Z' masses as high as 5 TeV
, due to its higher collision energy and higher luminosity
.
The above statements apply to "wide width" models. A recent classes of models have emerged that naturally provide cross section signatures that fall on the edge, or slightly below the 95 confidence level limits set by the Tevatron, and hence can produce detectable cross section signals for a Z' boson in a mass range much closer to the Z pole mass than the "wide width" models discussed above.
These "narrow width" models which fall into this category are those that predict a Stueckelberg Z' as well as a Z' from a universal extra dimension (see the Z' Hunter's Guide for links to these papers).
On April 7, 2011, the CDF collaboration
at the Tevatron reported an excess in proton-antiproton collision events that produce a W boson accompanied by two hadronic jets
. This could possibly be interpreted in terms of a Z' boson.
decay, where they can substitute for the Standard Model W boson exchange.
Indirect searches for Z' bosons are carried out at electron-positron colliders, since these give access to high-precision measurements of the properties of the Standard Model Z boson. The constraints come from mixing between the Z' and the Z, and are model dependent because they depend not only on the Z' mass but also its mixing with the Z. The current most stringent limits are from the CERN
LEP collider, which constrains Z' bosons to be heavier than a few hundred GeV, for typical model parameters. The ILC
will extend this reach up to 5 to 10 TeV depending on the model under consideration, providing complementarity with the LHC
because it will offer measurements of additional properties of the Z' boson..
s.
lectures)
More advanced:
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, W' and Z' bosons (or W-prime and Z-prime bosons) refer to hypothetical new gauge boson
Gauge boson
In particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental forces of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.-...
s that arise from extensions of the electroweak symmetry
Electroweak interaction
In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different...
of the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
. They are named in analogy with the Standard Model W and Z bosons
W and Z bosons
The W and Z bosons are the elementary particles that mediate the weak interaction; their symbols are , and . The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own...
.
Types of W' bosons
W' bosons often arise in models with an extra SU(2) gauge group. is spontaneously broken to the diagonal subgroup SU(2)W which corresponds to the electroweak SU(2). More generally, we might have n copies of SU(2), which are then broken down to a diagonal SU(2)W. This gives rise to n−1 W+', W−' and Z' bosons. Such models might arise from quiver diagramQuiver diagram
In physics, a quiver diagram is a graph representing the matter content of a gauge theory that describes D-branes on orbifolds.Each node of the graph corresponds to a factor U of the gauge group, and each link represents a field in the bifundamental representation.The relevance of quiver diagrams...
, for example. In order for the W' bosons to couple to isospin, the extra SU(2) and the Standard Model SU(2) must mix; one copy of SU(2) must break around the TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
scale (to get W' bosons with a TeV mass) leaving a second SU(2) for the Standard Model. This happens in Little Higgs
Little Higgs
In particle physics, little Higgs models are based on the idea that the Higgs boson is a pseudo-Goldstone boson arising from some global symmetry breaking at a TeV energy scale...
models that contain more than one copy of SU(2). Because the W' comes from the breaking of an SU(2), it is generically accompanied by a Z' boson of (almost) the same mass and with couplings related to the W' couplings.
Another model with W' bosons but without an additional SU(2) factor is the so-called 331 model
331 model
The 331 model in particle physics offers an explanation of why there must exist three families of quarks and leptons. One curious feature of the Standard Model is that the anomaly cancels exactly, for each quark-lepton family, of which we know three...
with . The symmetry breaking chain leads to a pair of W'± bosons and three Z' bosons.
W' bosons also arise in Kaluza-Klein theories with SU(2) in the bulk
Brane cosmology
Brane cosmology refers to several theories in particle physics and cosmology motivated by, but not exclusively derived from, superstring theory and M-theory.-Brane and bulk:...
.
Types of Z' bosons
Various models of physics beyond the Standard ModelBeyond the Standard Model
Physics beyond the Standard Model refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy...
predict different kinds of Z' bosons.
- Models with a new U(1) gauge symmetry. The Z' is the gauge boson of the (broken) U(1) symmetry.
- E6 models. This type of model contains two Z' bosons, which can mix in general.
- TopcolorTopcolorIn theoretical physics, Topcolor is a model of dynamical electroweak symmetry breaking in which the top quark and anti-top quark form a top quark condensate and act effectively like the Higgs boson. This is analogous to the phenomenon of superconductivity....
and Top Seesaw Models of Dynamical Electroweak Symmetry Breaking have Z' bosons to select the formation of particular condensates. - Little HiggsLittle HiggsIn particle physics, little Higgs models are based on the idea that the Higgs boson is a pseudo-Goldstone boson arising from some global symmetry breaking at a TeV energy scale...
models. These models typically include an enlarged gauge sector, which is broken down to the Standard Model gauge symmetry around the TeVTEVTEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
scale. In addition to one or more Z' bosons, these models often contain W' bosons. - Kaluza-Klein models. The Z' boson are the excited modes of a neutral bulk gauge symmetry.
- Stueckelberg Extensions (see Stueckelberg action). The Z' boson is sourced from couplings found in stringString (physics)A string is a hypothetical vibrating one-dimensional sub-atomic structure and one of the main objects of study in string theory, a branch of theoretical physics. There are different string theories, many of which are unified by M-theory. A string is an object with a one-dimensional spatial extent,...
theories with intersecting D-braneD-braneIn string theory, D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named. D-branes were discovered by Dai, Leigh and Polchinski, and independently by Hořava in 1989...
s
Direct searches
A W' boson could be detected at hadron colliders through its decay to leptonLepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
plus neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
or top quark
Top quark
The top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...
plus bottom quark
Bottom quark
The bottom quark, also known as the beauty quark, is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
, after being produced in quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
-antiquark annihilation. The LHC
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
reach for W' discovery is expected to be a few TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
.
Direct searches for Z' bosons are carried out at hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
colliders, since these give access to the highest energies available. The search looks for high-mass dilepton resonances: the Z' boson would be produced by quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
-antiquark annihilation and decay to an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
-positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
pair or a pair of opposite-charged muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s. The most stringent current limits come from the Fermilab
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
, and depend on the couplings of the Z' boson (which control the production cross section); as of 2006, the Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
excludes Z' bosons up to masses of about 800 GeV
GEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
for "typical" cross sections predicted in various models. The LHC
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
will extend this reach up to Z' masses as high as 5 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
, due to its higher collision energy and higher luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
.
The above statements apply to "wide width" models. A recent classes of models have emerged that naturally provide cross section signatures that fall on the edge, or slightly below the 95 confidence level limits set by the Tevatron, and hence can produce detectable cross section signals for a Z' boson in a mass range much closer to the Z pole mass than the "wide width" models discussed above.
These "narrow width" models which fall into this category are those that predict a Stueckelberg Z' as well as a Z' from a universal extra dimension (see the Z' Hunter's Guide for links to these papers).
On April 7, 2011, the CDF collaboration
Collider Detector at Fermilab
The Collider Detector at Fermilab experimental collaboration studies high energy particle collisions at the Tevatron,the world's former highest-energy particle accelerator...
at the Tevatron reported an excess in proton-antiproton collision events that produce a W boson accompanied by two hadronic jets
Jet (particle physics)
A jet is a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics or heavy ion experiment. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons...
. This could possibly be interpreted in terms of a Z' boson.
Indirect searches
The most stringent limits on new W' bosons are set by their indirect effects on low-energy processes like muonMuon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
decay, where they can substitute for the Standard Model W boson exchange.
Indirect searches for Z' bosons are carried out at electron-positron colliders, since these give access to high-precision measurements of the properties of the Standard Model Z boson. The constraints come from mixing between the Z' and the Z, and are model dependent because they depend not only on the Z' mass but also its mixing with the Z. The current most stringent limits are from the CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
LEP collider, which constrains Z' bosons to be heavier than a few hundred GeV, for typical model parameters. The ILC
International Linear Collider
The International Linear Collider is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, and, if approved after the project has published its Technical Design Report, planned for 2012, could be completed in the late 2010s. A later upgrade to 1000...
will extend this reach up to 5 to 10 TeV depending on the model under consideration, providing complementarity with the LHC
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
because it will offer measurements of additional properties of the Z' boson..
Z'-Y mixings
We might have gauge kinetic mixings between the U(1)' of the Z' boson and U(1)Y of hypercharge. This mixing leads to a tree level modification of the Peskin-Takeuchi parameterPeskin-Takeuchi parameter
In particle physics, the Peskin–Takeuchi parameters are a set of three measurable quantities, called S, T, and U, that parameterize potential new physics contributions to electroweak radiative corrections...
s.
Further reading
, a pedagogical overview of Z' phenomenology (TASI 2006Theoretical Advanced Study Institute
The Theoretical Advanced Study Institute or TASI is a four-week summer school in high-energy physics or astrophysics held yearly at the University of Colorado at Boulder. The school is meant primarily for advanced graduate students and consists of a series of pedagogical lectures on selected...
lectures)
More advanced:
External links
- The Z' Hunter's Guide, a collection of papers and talks regarding Z' physics
- Z' physics on arxiv.org