W′ bosons often arise in models with an extraSU(2)gauge group relative to the fullStandard Model gauge groupSU(3) × SU(2) × U(1). The extendedSU(2) × SU(2) symmetry spontaneously breaks into thediagonal subgroup SU(2)W which corresponds to the conventional SU(2) in electroweak theory.
More generally, there could ben copies of SU(2), which are then broken down to a diagonal SU(2)W. This gives rise ton2 − 1 different W′+, W′−, and Z′ bosons.Such models might arise from aquiver diagram, for example.
In order for the W′ bosons to couple toweak isospin, the extra SU(2) and the Standard Model SU(2) must mix; one copy of SU(2) must break around theTeV scale (to get W′ bosons with a TeV mass) leaving a second SU(2) for the Standard Model. This happens inLittle 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-called331 model with The symmetry breaking chainSU(3)L × U(1)W → SU(2)W × U(1)Y leads to a pair of W′± bosons and three Z′ bosons.
W′ bosons also arise inKaluza–Klein theories with SU(2) in thebulk.
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′ bosons, these models often contain W′ bosons.
The following statements pertain only to "wideresonance width" models.
A W′-boson could be detected at hadron colliders through its decay tolepton plusneutrino ortop quark plusbottom quark, after being produced in quark–antiquarkannihilation. TheLHC reach for W′ discovery is expected to be a fewTeV.
Direct searches for Z′-bosons are carried out athadron colliders, since these give access to the highest energies available. The search looks for high-mass dileptonresonances: the Z′-boson would be produced by quark–antiquark annihilation and decay to anelectron–positron pair or a pair of opposite-chargedmuons. The most stringent current limits come from theFermilabTevatron, and depend on the couplings of the Z′-boson (which control the productioncross section); as of 2006, theTevatron excludes Z′-bosons up to masses of about 800 GeV for "typical" cross sections predicted in various models.[2]
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′ 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′ as well as a Z′ from a universal extra dimension (see"The Z′ hunters' guide". for links to these papers).
On 7 April 2011, theCDF collaboration at the Tevatron reported an excess in proton–antiproton collisionevents that produce a W boson accompanied by two hadronicjets. This could possibly be interpreted in terms of a Z′ boson.[3][4]
On 2 June 2015, theATLAS experiment at the LHC reported evidence for W′-bosons at significance 3.4 σ, still too low to claim a formal discovery.[5] Researchers at theCMS 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′ bosons as an unexpected difference in howbeauty 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.[6]
