W′ and Z′ bosons

In particle physics, W&prime; and Z&prime; bosons (or W-prime and Z-prime bosons) refer to hypothetical gauge bosons 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.

Types of W&prime; bosons
W&prime; bosons often arise in models with an extra SU(2) gauge group relative to the full Standard Model gauge group $SU(3) × SU(2) × U(1)$. The extended SU(2) × SU(2) symmetry spontaneously breaks into the diagonal subgroup SU(2)W which corresponds to the conventional SU(2) in electroweak theory.

More generally, there could be $n$ copies of SU(2), which are then broken down to a diagonal SU(2)W. This gives rise to $n$2 − 1 different W&prime;+, W&prime;−, and Z&prime; bosons. Such models might arise from a quiver diagram, for example.

In order for the W&prime; bosons to couple to weak 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&prime; 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&prime; comes from the breaking of an SU(2), it is generically accompanied by a Z&prime; boson of (almost) the same mass and with couplings related to the W&prime; couplings.

Another model with W&prime; bosons but without an additional SU(2) factor is the so-called 331 model with $$\; \beta = \pm \tfrac{1}{\sqrt{3\;} } ~.$$ The symmetry breaking chain SU(3)L × U(1)W → SU(2)W × U(1)Y leads to a pair of W&prime;± bosons and three Z&prime; bosons.

W&prime; bosons also arise in Kaluza–Klein theories with SU(2) in the bulk.

Types of Z&prime; bosons
Various models of physics beyond the Standard Model predict different kinds of Z&prime; bosons.


 * Models with a new U(1) gauge symmetry: The Z&prime; is the gauge boson of the (broken) U(1) symmetry.
 * E6 models: This type of model contains two Z&prime; bosons, which can mix in general.
 * Pati–Salam: In addition to a fourth leptonic "color", Pati–Salam includes a right handed weak interaction with W&prime; and Z&prime; bosons.
 * Topcolor and Top Seesaw Models of Dynamical Electroweak Symmetry Breaking: Both these models have Z&prime; bosons that select the formation of particular condensates.
 * Little Higgs models: These models typically include an enlarged gauge sector, which is broken down to the Standard Model gauge symmetry around the TeV scale. In addition to one or more Z&prime; bosons, these models often contain W&prime; bosons.
 * Kaluza–Klein models: The Z&prime; boson are the excited modes of a neutral bulk gauge symmetry.
 * Stueckelberg Extensions: The Z&prime; boson is sourced from couplings found in string theories with intersecting D-branes (see Stueckelberg action).

Direct searches for "wide resonance-width" models
The following statements pertain only to "wide resonance width" models.

A W&prime;-boson could be detected at hadron colliders through its decay to lepton plus neutrino or top quark plus bottom quark, after being produced in quark–antiquark annihilation. The LHC reach for W&prime; discovery is expected to be a few TeV.

Direct searches for Z&prime;-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&prime;-boson would be produced by quark–antiquark annihilation and decay to an electron–positron pair or a pair of opposite-charged muons. The most stringent current limits come from the Fermilab Tevatron, and depend on the couplings of the Z&prime;-boson (which control the production cross section); as of 2006, the Tevatron excludes Z&prime;-bosons up to masses of about 800 GeV for "typical" cross sections predicted in various models.

Direct searches for "narrow resonance-width" models
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&prime; 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 Stückelberg Z&prime; as well as a Z&prime; from a universal extra dimension (see  for links to these papers).

On 7 April 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&prime; boson.

On 2 June 2015, the ATLAS experiment at the LHC reported evidence for W&prime;-bosons at significance 3.4 $σ$, still too low to claim a formal discovery. Researchers at the CMS experiment also independently reported signals that corroborate ATLAS's findings.

In March 2021, there were some reports to hint at the possible existence of Z&prime; bosons as an unexpected difference in how beauty quarks decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1 $σ$, which is well below the 5 $σ$ level that is conventionally considered sufficient proof of a discovery.

Z&prime;–Y mixings
We might have gauge kinetic mixings between the U(1)&prime; of the Z&prime; boson and U(1)Y of hypercharge. This mixing leads to a tree level modification of the Peskin–Takeuchi parameters.