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# Z' boson

In particle physics, a Z' boson (or Z-prime boson) refers to a hypothetical new neutral gauge boson (named in analogy with the Standard Model Z boson).

## Types of Z' bosons

Various models of physics beyond the Standard Model 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.
• 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' 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 string theories with intersecting D-branes

## Z' boson searches

### Direct searches

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 muons. 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.[1] The LHC will extend this reach up to Z' masses as high as 5 TeV, due to its higher collision energy and higher luminosity.[citation needed]

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).

### Indirect searches

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.[citation needed] 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.[citation needed].

## 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 parameters.

1. ^ A. Abulencia et al (CDF collaboration), Search for Z' $\rightarrow$ e+e- using dielectron mass and angular distribution, Phys. Rev. Lett. 96, 211801 (2006), arXiv:hep-ex/0602045