In particle physics, a W' boson (or W-prime boson) refers to a hypothetical new electrically charged gauge boson that couples to Standard Model fermions via their isospin. It is named in analogy to the Standard Model W boson.
Types of W' bosons
W' bosons often arise in models with an extra SU(2) gauge group. SU(2) × SU(2) 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 diagram, 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 are the so-called 331 models with }. The symmetry breaking chain leads to a pair of bosons and three Z' bosons.
W' bosons also arise in Kaluza-Klein theories with SU(2) in the bulk.
Searches for W' bosons
The most stringent limits on new W' bosons are set by their indirect effects on low-energy processes like muon decay, where they can substitute for the Standard Model W boson exchange.
A W' 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' discovery is expected to be a few TeV.
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