| Status | Hypothetical |
|---|---|
| Symbol | N͂0 1,N͂0 2,N͂0 3,N͂0 4 |
| Antiparticle | self (truly neutral particle) |
| Types | 4 |
| Mass | > 300 GeV |
| Electric charge | 0 |
| Spin | 1/2 |
| Lepton number | 0 |
| Baryon number | 0 |
| R parity | −1 |
Insupersymmetry, theneutralino[1]: 71–74 is a hypothetical particle. In theMinimal Supersymmetric Standard Model (MSSM), a popular model of realization of supersymmetry at a low energy, there are four neutralinos that arefermions and are electrically neutral, the lightest of which is stable in anR-parity conserved scenario of MSSM. They are typically labeledN͂0
1 (the lightest),N͂0
2,N͂0
3 andN͂0
4 (the heaviest) although sometimes is also used when is used to refer tocharginos.
These four states are composites of thebino and the neutralwino (which are the neutral electroweakgauginos), and the neutralhiggsinos. As the neutralinos areMajorana fermions, each of them is identical to itsantiparticle.
If they exist, these particles would only interact with theweak vector bosons, so they would not be directly produced athadron colliders in copious numbers. They would primarily appear as particles incascade decays (decays that happen in multiple steps) of heavier particles usually originating fromcolored supersymmetric particles such assquarks orgluinos.
InR-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 neutralZ boson to a lighter neutralino or through a chargedW boson to a light chargino:[2]
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.
In supersymmetry models, allStandard Model particles have partner particles with the samequantum numbers except for the quantum numberspin, which differs by1⁄2 from its partner particle. Since the superpartners of theZ boson (zino), thephoton (photino) and theneutral higgs (higgsino) have the same quantum numbers, they canmix to form foureigenstates of the mass operator called "neutralinos". In many models the lightest of the four neutralinos turns out to be thelightest supersymmetric particle (LSP), though other particles may also take on this role.
The exact properties of each neutralino will depend on the details of the mixing[1]: 71–74 (e.g. whether they are more higgsino-like or gaugino-like), but they tend to have masses at the weak scale (100 GeV ~ 1 TeV) and couple to other particles with strengths characteristic of theweak interaction. In this way, except for mass, they are phenomenologically similar toneutrinos, 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 othersuperpartners.[1]: 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.[4][6] This is an important signature to discriminate supersymmetry from Standard Model backgrounds.
As a heavy, stable particle, the lightest neutralino is an excellent candidate to form the universe'scold dark matter.[1]: 99 [5]: 8 [7] In many models[which?] the lightest neutralino can be produced thermally in thehot early universe and leave approximately the right relic abundance to account for the observeddark matter. A lightest neutralino of roughly10–10000 GeV is the leadingweakly interacting massive particle (WIMP) dark matter candidate.[1]: 124
Neutralino dark matter could be observed experimentally in nature either indirectly or directly. For indirect observation,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.[4] For direct observation, special purpose experiments such as theCryogenic 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.
| Elementary |
| ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Composite |
| ||||||||||||||||||||||||
| Quasiparticles | |||||||||||||||||||||||||
| Lists | |||||||||||||||||||||||||
| Related | |||||||||||||||||||||||||
| Forms of dark matter | |||||||
|---|---|---|---|---|---|---|---|
| Hypothetical particles | |||||||
| Theories and objects | |||||||
| Search experiments |
| ||||||
| Potentialdark galaxies | |||||||
| Related | |||||||
