Editing Neutralino

{{Short description|Neutral mass eigenstate formed from superpartners of gauge and Higgs bosons}}
{{Distinguish|neutrino}}
{{Infobox particle
| bgcolour =
| name = Neutralino
| image =
| caption =
| num_types = 4
| classification =
| composition =
| statistics =
| group =
| generation =
| interaction =
| antiparticle = self ([[truly neutral particle]])
| status = Hypothetical
| theorized =
| discovered =
| symbol = {{math|{{SubatomicParticle|Neutralino 1}}}}, {{math|{{SubatomicParticle|Neutralino 2}}}}, {{math|{{SubatomicParticle|Neutralino 3}}}}, {{math|{{SubatomicParticle|Neutralino 4}}}}
| mass =  > 300&nbsp;GeV
| mean_lifetime =
| decay_particle =
| electric_charge =0
| charge_radius =
| electric_dipole_moment =
| electric_polarizability =
| magnetic_moment =
| magnetic_polarizability =
| color_charge =
| spin = {{sfrac|2}}
| num_spin_states =
| lepton_number = 0
| baryon_number = 0
| strangeness =
| charm =
| bottomness =
| topness =
| isospin =
| weak_isospin =
| weak_isospin_3 =
| hypercharge =
| weak_hypercharge =
| chirality =
| B-L =
| X_charge =
| parity =
| g_parity =
| c_parity =
| r_parity = &minus;1
| condensed_symmetries =
}}

In [[supersymmetry]], the '''neutralino'''<ref name=Martin2010/>{{rp|71–74}} is a hypothetical particle. In the [[Minimal Supersymmetric Standard Model]] (MSSM), a popular model of realization of supersymmetry at a low energy, there are four neutralinos that are [[fermion]]s and are electrically neutral, the lightest of which is stable in an [[R-parity]] conserved scenario of MSSM. They are typically labeled {{math|{{SubatomicParticle|Neutralino 1}}}} (the lightest), {{math|{{SubatomicParticle|Neutralino 2}}}}, {{math|{{SubatomicParticle|Neutralino 3}}}} and {{math|{{SubatomicParticle|Neutralino 4}}}} (the heaviest) although sometimes <math> \tilde{\chi}_1^0, \ldots, \tilde{\chi}_4^0</math> is also used when <math> \tilde{\chi}_i^\pm</math> is used to refer to [[chargino]]s.
: {{small|(In this article, {{math| {{SubatomicParticle|Chargino 1+-}} }} is used for [[chargino]] #1, etc.) }}

These four states are composites of the [[Bino (particle)|bino]] and the neutral [[Wino (particle)|wino]] (which are the neutral electroweak [[gaugino]]s), and the neutral [[higgsinos]]. As the neutralinos are [[Majorana fermion]]s, each of them is identical to its [[antiparticle]].

== Expected behavior ==
If they exist, these particles would only interact with the [[W and Z bosons|weak vector bosons]], so they would not be directly produced at [[hadron collider]]s in copious numbers. They would primarily appear as particles in [[Decay chain|cascade decays]] (decays that happen in multiple steps) of heavier particles usually originating from [[Quantum chromodynamics|colored]] supersymmetric particles such as [[squark]]s or [[gluino]]s.

In [[R-parity]] conserving models, the lightest neutralino is stable and all supersymmetric cascade-decays end up decaying into this particle which leaves the detector unseen and its existence can only be inferred by looking for unbalanced momentum in a detector.

The heavier neutralinos typically decay through a neutral [[Z boson]] to a lighter neutralino or through a charged [[W boson]] to a light chargino:<ref>{{cite journal |author1=Nakamura |first=K. |display-authors=etal |date=2010 |others=Updated August 2009 by J.-F. Grivaz |title=Supersymmetry, Part&nbsp;II (Experiment) |url=http://pdg.lbl.gov/2010/reviews/rpp2010-rev-susy-2-experiment.pdf |journal=Journal of Physics G |volume=37 |issue=7 |pages=1309–1319 |collaboration=[[Particle Data Group]]}}</ref>
:{|
| {{math| {{SubatomicParticle|Neutralino 2}} }}
| &nbsp; {{math| →}} &nbsp;
| {{math| {{SubatomicParticle|Neutralino 1}} }}
| +
| {{math| {{SubatomicParticle|Z boson0}} }}
| colspan=6|
| &nbsp; {{math| → }} &nbsp;
| Missing energy
| +
| {{math| {{SubatomicParticle|Lepton+|link=yes}} }}
| +
| {{math| {{SubatomicParticle|Lepton-|link=yes}} }}
|-
| {{math| {{SubatomicParticle|Neutralino 2}} }}
| &nbsp; {{math| → }} &nbsp;
| {{math| {{SubatomicParticle|Chargino 1+-}} }}
| +
| {{math| {{SubatomicParticle|W boson-+}} }}
| &nbsp; {{math| → }} &nbsp;
| {{math| {{SubatomicParticle|Neutralino 1}} }}
| +
| {{math| {{SubatomicParticle|W boson+-}} }}
| +
| {{math| {{SubatomicParticle|W boson-+}} }}
| &nbsp; {{math| → }} &nbsp;
| Missing energy
| +
| {{math| {{SubatomicParticle|Lepton+}} }} + {{math| &nu;{{sub|ℓ}} }}
| +
| {{math| {{SubatomicParticle|Lepton-}} }} + {{math| {{overline|&nu;}}{{sub|ℓ}} }}
|}

The mass splittings between the different neutralinos will dictate which patterns of decays are allowed.

Up to present, neutralinos have never been observed or detected in an experiment.

==Origins in supersymmetric theories==
In supersymmetry models, all [[Standard Model]] particles have partner particles with the same [[quantum number]]s except for the quantum number [[Spin (physics)|spin]], which differs by {{frac|1|2}} from its partner particle. Since the superpartners of the [[Z boson]] ([[Gaugino|zino]]), the [[photon]] ([[photino]]) and the [[higgs boson|neutral higgs]] ([[higgsino]]) have the same quantum numbers, they can [[quantum superposition|mix]] to form four [[eigenstate]]s of the mass operator called "neutralinos". In many models the lightest of the four neutralinos turns out to be the [[lightest supersymmetric particle]] (LSP), though other particles may also take on this role.

==Phenomenology==
The exact properties of each neutralino will depend on the details of the mixing{{refn|name=Martin2010|{{cite arXiv |last=Martin |first=Stephen P. |eprint=hep-ph/9709356v5 |title=A Supersymmetry Primer |date=2008}} Also published in Kane (2010).<ref>{{cite book |last=Martin |first=Stephen P. |chapter=Chapter 1: A Supersymmetry Primer |title=Perspectives on Supersymmetry |volume=II |date=2010 |editor-last=Kane |editor-first=Gordon L. |publisher=[[World Scientific]] |isbn=978-981-4307-48-2}}</ref>}}{{rp|71–74}} (e.g. whether they are more higgsino-like or gaugino-like), but they tend to have masses at the weak scale (100&nbsp;GeV ~ 1&nbsp;TeV) and couple to other particles with strengths characteristic of the [[weak nuclear force|weak interaction]]. In this way, except for mass, they are phenomenologically similar to [[neutrinos]], and so are not directly observable in particle detectors at accelerators.

In models in which R-parity is conserved and the lightest of the four neutralinos is the LSP, the lightest neutralino is stable and is eventually produced in the decay chain of all other [[Superpartner|superpartners]].<ref name=Martin2010/>{{rp|83}} In such cases supersymmetric processes at accelerators are characterized by the expectation of a large discrepancy in energy and momentum between the visible initial and final state particles, with this energy being carried off by a neutralino which departs the detector unnoticed.<ref name="Feng">{{cite journal |doi=10.1146/annurev-astro-082708-101659 |last=Feng |first=Jonathan L. |journal=Annual Review of Astronomy and Astrophysics |volume=48 |date=2010 |pages=495–545 |arxiv=1003.0904 |title=Dark Matter Candidates from Particle Physics and Methods of Detection |bibcode=2010ARA&A..48..495F|s2cid=11972078 }}</ref>{{refn|{{cite book |last1=Ellis |first1=John |authorlink1=John Ellis (physicist, born 1946) |last2=Olive |first2=Keith A. |arxiv=1001.3651  |title=Supersymmetric Dark Matter Candidates |url=https://archive.org/details/arxiv-1001.3651 |date=2010|bibcode=2010pdmo.book..142E }} Also published as Chapter&nbsp;8 in Bertone (2010)<ref name=Bertone2010/>}} This is an important signature to discriminate supersymmetry from Standard Model backgrounds.

==Relationship to dark matter==
As a heavy, stable particle, the lightest neutralino is an excellent candidate to form the universe's [[cold dark matter]].<ref name=Martin2010/>{{rp|page=99}}<ref name=Bertone2010>{{cite book |editor-last=Bertone |editor-first=Gianfranco |title=Particle Dark Matter: Observations, Models and Searches |publisher=[[Cambridge University Press]] |date=2010 |isbn=978-0-521-76368-4|title-link=Particle Dark Matter }}</ref>{{rp|page=8}}<ref>{{cite journal |author=Nakamura |first=K. |display-authors=etal |date=2010 |others=Revised September 2009 by M. Drees & G. Gerbier |title=Dark Matter |url=http://pdg.lbl.gov/2010/reviews/rpp2010-rev-dark-matter.pdf |journal=Journal of Physics G |volume=37 |issue=7A |pages=255–260 |collaboration=[[Particle Data Group]]}}</ref> In many models{{which|date=May 2015}} the lightest neutralino can be produced thermally in the [[Big Bang|hot early universe]] and leave approximately the right relic abundance to account for the observed [[dark matter]]. A lightest neutralino of roughly {{val|10|-|10000|u=[[electronvolt|GeV]]}} is the leading [[weakly interacting massive particle]] (WIMP) dark matter candidate.<ref name=Martin2010/>{{rp|page=124}}

Neutralino dark matter could be observed experimentally in nature either indirectly or directly. For indirect observation, [[Gamma-ray astronomy|gamma ray]] and neutrino telescopes look for evidence of neutralino annihilation in regions of high dark matter density such as the galactic or solar centre.<ref name="Feng"/> For direct observation, special purpose experiments such as the [[Cryogenic Dark Matter Search]] (CDMS) seek to detect the rare impacts of WIMPs in terrestrial detectors. These experiments have begun to probe interesting supersymmetric parameter space, excluding some models for neutralino dark matter, and upgraded experiments with greater sensitivity are under development.

==See also==
* [[List of particles#Hypothetical particle anchor|List of hypothetical particles]] 
* {{annotated link|Lightest supersymmetric particle}}
* {{annotated link|Truly neutral particle}}
* [[WISP (quantum mechanics)|Weakly interacting slender particle]]

==References==
{{Reflist|25em}}

{{Particles}}
{{Dark matter}}
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[[Category:Dark matter]]
[[Category:Fermions]]
[[Category:Supersymmetric quantum field theory]]
[[Category:Hypothetical elementary particles]]