View source for W′ and Z′ bosons

{{Short description|Hypothetical particles in physics}}
{{Infobox Particle
| bgcolour =
| name = W&prime; and Z&prime; bosons
| image =
| caption =
| num_types =
| composition = [[Elementary particle]]
| statistics = [[Bosonic]]
| group = [[Gauge boson]]
| generation =
| interaction = [[Standard-Model Extension]]<ref name="pdg">
{{cite journal
 |author=J. Beringer et al. ([[Particle Data Group]])
 |year=2012
 |title=Review of Particle Physics
 |journal=[[Physical Review D]]
 |volume=86 |issue=1
 |pages=010001
 |doi=10.1103/PhysRevD.86.010001
 |url=http://pdg.lbl.gov/2012/reviews/contents_minireviews.html
|bibcode = 2012PhRvD..86a0001B |doi-access=free
 |hdl=10481/34377
 |hdl-access=free
 }}</ref>
| particle =
| antiparticle =
| status = Hypothetical
| theorized =
| discovered =
| symbol =
| mass = unknown
| mean_lifetime =
| decay_particle = similar to [[W and Z bosons]]
| electric_charge = W&prime;: ±1&nbsp;[[Elementary charge|''e'']]<br> Z&prime;: 0&nbsp;''e''
| color_charge =
| spin = 1<ref name="pdg" />
| num_spin_states = 2
}}

In [[particle physics]], '''W&prime; and Z&prime; bosons''' (or '''W-prime and Z-prime bosons''') refer to hypothetical [[gauge boson]]s that arise from extensions of the  [[electroweak interaction|electroweak symmetry]] of the [[Standard Model]]. They are named in analogy with the Standard Model [[W and Z bosons]].

==Types==
{{citation needed section|date=March 2021}}
===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 {{math|[[special unitary group|SU]](3) × SU(2) × [[U(1)]]}}. The extended {{nowrap|SU(2) × SU(2)}} symmetry spontaneously breaks into the [[diagonal subgroup]] SU(2)<sub>W</sub> which corresponds to the conventional SU(2) in electroweak theory.

More generally, there could be {{mvar|n}} copies of SU(2), which are then broken down to a diagonal SU(2)<sub>W</sub>. This gives rise to {{nowrap| {{mvar|n}}<sup>2</sup> − 1 }} different W&prime;<sup>+</sup>, W&prime;<sup>−</sup>, 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 <math>\; \beta = \pm \tfrac{1}{\sqrt{3\;} } ~.</math> The symmetry breaking chain {{nowrap|SU(3)<sub>L</sub> × U(1)<sub>W</sub> → SU(2)<sub>W</sub> × U(1)<sub>Y</sub>}} leads to a pair of W&prime;<sup>±</sup> bosons and three Z&prime; bosons.

W&prime; bosons also arise in [[Kaluza–Klein]] theories with SU(2) in the [[Brane cosmology|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{{which|date=March 2021}}: The Z&prime; is the gauge boson of the (broken) U(1) symmetry.
; [[E6 (mathematics)|E<sub>6</sub>]] models: This type of model contains two Z&prime; bosons, which can mix in general.
; [[Pati–Salam model|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 (physics)|string]] theories with intersecting [[D-brane]]s (see [[Stueckelberg action]]).

==Searches==


===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 [[Large Hadron Collider|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 [[resonance (particle physics)|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 [[muon]]s. 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 (physics)|cross section]]); as of 2006, the [[Tevatron]] excludes Z&prime;-bosons up to masses of about 800&nbsp;[[GeV]] for "typical" cross sections predicted in various models.<ref>
{{cite journal
 |author=A. Abulencia et al. ([[CDF collaboration]])
 |year=2006
 |title=Search for Z&prime; → e<sup>+</sup>e<sup>−</sup> using dielectron mass and angular distribution
 |journal=[[Physical Review Letters]]
 |volume=96
 |issue=21 |pages=211801
 |doi=10.1103/PhysRevLett.96.211801
 |pmid=16803227
 |arxiv=hep-ex/0602045
|bibcode=2006PhRvL..96u1801A
}}</ref>

===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%&nbsp;confidence level limits set by the Tevatron, and hence can produce detectable cross section signals for a Z&prime;&nbsp;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 ''{{cite web |url=http://sites.google.com/site/zprimeguide/ |title=The Z&prime; hunters' guide}}'' for links to these papers).

On 7&nbsp;April 2011, the [[Collider Detector at Fermilab|CDF collaboration]] at the Tevatron reported an excess in proton–[[antiproton]] collision [[event (particle physics)|events]] that produce a W&nbsp;boson accompanied by two hadronic [[jet (particle physics)|jets]]. This could possibly be interpreted in terms of a Z&prime;&nbsp;boson.<ref>
{{cite web
 |first=Emma |last=Woollacott
 |date=2011-04-07
 |title=Tevatron data indicates unknown new particle
 |website=TG Daily
 |url=http://www.tgdaily.com/general-sciences-features/55233-tevatron-data-indicates-unknown-new-particle
}}
</ref><ref>
{{cite web
 |title=Fermilab's data peak that causes excitement
 |publisher=Fermilab/SLAC
 |date=2011-04-07
 |website=Symmetry Magazine
 |url=http://www.symmetrymagazine.org/breaking/2011/04/07/fermilabs-data-peak-that-causes-excitement
}}
</ref>

On 2&nbsp;June 2015, the [[ATLAS experiment]] at the LHC reported evidence for W&prime;-bosons at significance 3.4&nbsp;[[standard deviation|{{mvar|σ}}]], still too low to claim a formal discovery.<ref name="W'-NewScientist">
{{cite news
 |first=Michael |last=Slezak
 |date=22 August 2015
 |title=Possible new particle hints that universe may not be left-handed
 |magazine=[[New Scientist]]
 |url=https://www.newscientist.com/article/mg22730354-600-possible-new-particle-hints-that-universe-may-not-be-left-handed/
}}
</ref> Researchers at the [[CMS experiment]] also independently reported signals that corroborate ATLAS's findings.

In March&nbsp;2021, there were some reports to hint at the possible existence of Z&prime;&nbsp;bosons as an unexpected difference in how [[Bottom quark|beauty quarks]] decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1&nbsp;[[Standard deviation|{{mvar|σ}}]], which is well below the 5&nbsp;{{mvar|σ}} level that is conventionally considered sufficient proof of a discovery.<ref>
{{cite magazine
 | first=Hamish |last=Johnston
 | date= 23 March 2021
 | title=Has a new particle called a 'leptoquark' been spotted at CERN?
 | magazine=Physics World
 | url= https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/
 | archive-url = https://web.archive.org/web/20210324054334/https://physicsworld.com/a/has-a-new-particle-called-a-leptoquark-been-spotted-at-cern/
 | archive-date = 24 March 2021
}}
</ref>

==Z&prime;–Y mixings==
We might have gauge [[kinetic mixing]]s between the U(1)&prime; of the Z&prime; boson and U(1)<sub>Y</sub> of [[hypercharge]]. This mixing leads to a [[Feynman diagram|tree level]] modification of the [[Peskin–Takeuchi parameter]]s.

==See also==
* {{annotated link|X and Y bosons}}
* [[List of hypothetical particles]]

==References==
{{reflist|25em}}

==Further reading==
* {{cite arXiv
 |author=T.G. Rizzo
 |title=Z&prime; Phenomenology and the LHC
 |eprint=hep-ph/0610104
 |year=2006
}}, a pedagogical overview of Z&prime; phenomenology ([[Theoretical Advanced Study Institute|TASI 2006]] lectures)
*{{cite news
 |author=P. Rincon
 |url=http://news.bbc.co.uk/1/hi/sci/tech/8685541.stm
 |title=LHC particle search 'nearing', says physicist
 |publisher=[[BBC News]]
| date=17 May 2010
}}
More advanced:
{{refbegin}}
*{{cite journal
|arxiv = hep-ex/0602045
|title = Search for Z&prime; → e<sup>+</sup>e<sup>−</sup> using dielectron mass and angular distribution
|author = Abulencia, A.
|author2 = CDF Collaboration
|journal = Physical Review Letters
|volume = 96
|issue = 211801
|pages = 211801
|year = 2006
|bibcode = 2006PhRvL..96u1801A |doi = 10.1103/PhysRevLett.96.211801 |pmid = 16803227
|display-authors=etal}}
*{{cite journal
|arxiv = hep-ph/0210086
|title = Radiative corrections to Higgs masses in Z&prime; models
|author = Amini, Hassib
|journal = New Journal of Physics
|volume = 5
|issue = 49
|pages = 49
|year = 2003
|bibcode = 2003NJPh....5...49A |doi = 10.1088/1367-2630/5/1/349 |s2cid = 15264222
}}
*{{cite journal
|arxiv = hep-ph/0003286
|title = Supersymmetric extension of the standard model with naturally stable proton
|author = Aoki, Mayumi
|author2=Oshimo, Noriyuki
 |journal = Physical Review D
|volume = 62
|issue = 55013
|pages = 55013
|year = 2000
|doi = 10.1103/PhysRevD.62.055013
|bibcode = 2000PhRvD..62e5013A
|s2cid = 14168936
}}
*{{cite journal
|doi = 10.1103/PhysRevLett.84.5269
|arxiv = hep-ph/9907481
|title =  A supersymmetric model with an extra U(1) gauge symmetry
|author = Aoki, Mayumi
|author2=Oshimo, Noriyuki
 |journal = Physical Review Letters
|volume = 84
|issue = 23
|pages = 5269–5272
|year = 2000
|pmid = 10990921
|bibcode=2000PhRvL..84.5269A
|s2cid = 15033987
}}
*{{cite journal
|arxiv = hep-ph/0212073
|title = Nonexotic neutral gauge bosons
|author = Appelquist, Thomas |author2=Dobrescu, Bogdan A. |author3=Hopper, Adam R.
|journal = Physical Review D
|volume = 68
|issue = 35012
|pages = 35012
|year = 2003
|doi = 10.1103/PhysRevD.68.035012
|bibcode = 2003PhRvD..68c5012A
|s2cid = 119091245
}}
*{{cite journal
|arxiv = hep-ph/9603212
|title = Leptophobic U(1)s and the R<sub>b</sub>–R<sub>c</sub> crisis
|author = Babu, K. S. |author2=Kolda, Christopher F. |author3=March-Russell, John
|volume = 54
|issue = 7
|pages = 4635–4647
|year = 1996
|bibcode = 1996PhRvD..54.4635B |doi = 10.1103/PhysRevD.54.4635 |pmid = 10021145
|journal=Physical Review D|s2cid = 38299279
}}
*{{cite journal
|title = Use of Z lepton asymmetry to determine mixing between Z boson and Z&prime; boson of E<sub>6</sub> superstrings
|author = Barger, Vernon D.
|author2=Whisnant, K.
 |journal = Physical Review D
|volume = 36
|issue = 3 | pages=979–82
|year = 1987
|doi=10.1103/PhysRevD.36.979
|pmid = 9958259
|bibcode = 1987PhRvD..36..979B }}
*{{cite journal
|arxiv = hep-ph/0503186
|title = The origin of a peculiar extra U(1)
|author = Barr, S.M.
|author2=Dorsner, I.
 |journal = Physical Review D
|volume = 72
|issue = 15011
|pages = 015011
|year = 2005
|bibcode = 2005PhRvD..72a5011B |doi = 10.1103/PhysRevD.72.015011 |s2cid = 119492913
}}
*{{cite journal
|arxiv = hep-ph/0510181
|title = Anomaly-free sets of fermions
|author = Batra, Puneet |author2=Dobrescu, Bogdan A. |author3=Spivak, David
|journal = Journal of Mathematical Physics
|volume = 47
|issue = 82301
|pages = 2301
|year = 2006
|doi = 10.1063/1.2222081
|bibcode = 2006JMP....47h2301B
|s2cid = 9830964
}}
*{{cite journal
|arxiv = hep-ph/0408098
|title = Z&prime; gauge bosons at the Tevatron
|author = Carena, Marcela S.
|author2 = Daleo, Alejandro|author3 = Dobrescu, Bogdan A.|author4 = Tait, Tim M. P.
|journal = Physical Review D
|volume = 70
|issue = 93009
|pages = 093009
|year = 2004
|bibcode = 2004PhRvD..70i3009C |doi = 10.1103/PhysRevD.70.093009 |s2cid = 118616267
}}
*{{cite journal
|arxiv = hep-ph/0503290
|title = The Minimal U(1)′ extension of the MSSM
|author = Demir, Durmus A. |author2=Kane, Gordon L. |author3=Wang, Ting T.
|journal = Physical Review D
|volume = 72
|issue = 15012
|pages = 015012
|year = 2005
|bibcode = 2005PhRvD..72a5012D |doi = 10.1103/PhysRevD.72.015012 |s2cid = 17656689
}}
*{{cite journal
|doi = 10.1016/j.physletb.2003.09.103
|arxiv = hep-ph/0307020
|title = Z&prime; studies at the LHC: an update
|author = Dittmar, Michael |author2=Nicollerat, Anne-Sylvie |author3=Djouadi, Abdelhak
|journal = Physics Letters B
|volume = 583
|issue = 1–2
|pages = 111–120
|year = 2004
|bibcode = 2004PhLB..583..111D |s2cid = 15749848
}}
*{{cite journal
|arxiv = 0704.1395
|title = Higgs and Z&prime; phenomenology in ''B''−''L'' extension of the standard model at LHC
|author = Emam, W.
|author2=Khalil, S.
 |journal = European Physical Journal C
|volume = 522
|issue = 3
|pages = 625–633
|year = 2007
|bibcode = 2007EPJC...52..625E |doi = 10.1140/epjc/s10052-007-0411-7 |s2cid = 18985438
}}
*{{cite journal
|doi = 10.1016/S0550-3213(00)00427-2
|arxiv = hep-ph/0006051
|title = Chiral models of weak scale supersymmetry
|author = Erler, Jens
|journal = Nuclear Physics B
|volume = 586
|issue = 1
|pages = 73–91
|year = 2000
|bibcode = 2000NuPhB.586...73E |s2cid = 16728305
}}
*{{cite journal
|doi = 10.1016/S0370-2693(00)00187-8
|arxiv = hep-ph/0001073
|title = Alternative supersymmetric spectra
|author = Everett, Lisa L. |author2=Langacker, Paul |author3=Plumacher, Michael |author4=Wang, Jing
|journal = Physics Letters B
|volume = 477
|issue = 1–3
|pages = 233–241
|year = 2000
|bibcode = 2000PhLB..477..233E |s2cid = 16294141
}}
*{{cite journal
|arxiv = hep-ph/0110233
|title = Constraints on heavy Z&prime; couplings from Δ''S'' = 2 B<sup>−</sup> → K<sup>−</sup>K<sup>−</sup>π<sup>+</sup> decay
|author = Fajfer, S.
|author2=Singer, P.
 |journal = Physical Review D
|volume = 65
|issue = 17301
|pages = 017301
|year = 2002
|bibcode = 2001PhRvD..65a7301F |doi = 10.1103/PhysRevD.65.017301 |s2cid = 117080415}}
*{{cite journal
|arxiv = hep-ph/0611174
|title = The Z&prime; reconsidered
|author = Ferroglia, A.
|author2 = Lorca, A.|author3 = van der Bij, J. J.
|journal = Annalen der Physik
|volume = 16
|issue = 7–8
|pages = 563–578
|year = 2007
|bibcode = 2007AnP...519..563F |doi = 10.1002/andp.200710249 |s2cid = 17347199
}}
*{{cite journal
|doi = 10.1016/j.physletb.2007.03.049
|arxiv = hep-ph/0703269
|title = Dilepton signatures of family non-universal U(1)′
|author = Hayreter, Alper
|journal = Physics Letters B
|volume = 649
|issue = 2–3
|pages = 191–196
|year = 2007
|bibcode = 2007PhLB..649..191H |s2cid = 16059648
}}
*{{cite journal
|arxiv = hep-ph/0412190
|title = Z&prime; discovery limits for supersymmetric E<sub>6</sub> models
|author = Kang, Junhai
|author2=Langacker, Paul
 |journal = Physical Review D
|volume = 71
|issue = 35014
|pages = 035014
|year = 2005
|bibcode = 2005PhRvD..71c5014K |doi = 10.1103/PhysRevD.71.035014 |s2cid = 9649745
}}
*{{cite journal
|arxiv = hep-ph/0512019
|title = The tension between gauge coupling unification, the Higgs boson mass, and a gauge-breaking origin of the supersymmetric μ-term
|author = Morrissey, David E.
|author2=Wells, James D.
 |journal = Physical Review D
|volume = 74
|issue = 15008
|pages = 15008
|year = 2006
|doi = 10.1103/PhysRevD.74.015008
|bibcode = 2006PhRvD..74a5008M
|s2cid = 119467594
}}
{{refend}}

==External links==
* [http://sites.google.com/site/zprimeguide/ The Z&prime; Hunter's Guide], a collection of papers and talks regarding Z&prime; physics
* [http://xstructure.inr.ac.ru/x-bin/theme3.py?level=1&index1=236694 Z&prime; physics on arxiv.org]

{{Particles}}

{{DEFAULTSORT:W' and Z' bosons}}
[[Category:Gauge bosons]]
[[Category:Hypothetical elementary particles]]
[[Category:Force carriers]]
[[Category:Subatomic particles with spin 1]]