Neutralino
Encyclopedia
In particle physics
, the neutralino is a hypothetical particle predicted by supersymmetry
. There are four neutralinos that are fermion
s and are electrically neutral, the lightest of which is typically stable. They are typically labeled (the lightest), , and (the heaviest) although sometimes
is also used when 
is used to refer to chargino
s. These four states are mixtures of the Bino and the neutral Wino (which are the neutral electroweak Gaugino
s), and the neutral Higgsinos. As the neutralinos are Majorana fermion
s, each of them is identical with its antiparticle
. Because these particles only interact with the weak vector bosons, they are not directly produced at hadron colliders in copious numbers. They primarily appear as particles in cascade decays of heavier particles (decays that happen in multiple steps) usually originating from colored
supersymmetric particles such as squarks or gluino
s.
In R-parity
conserving models, the lightest neutralino is stable and all supersymmetric cascades 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:
The mass splittings between the different neutralinos will dictate which patterns of decays are allowed.
particles have partner particles with the same quantum number
s except for the quantum number spin
, which differs by 1/2 from its partner particle. Since the superpartners of the Z boson (zino
), the photon
(photino) and the neutral higgs
(higgsino
) have the same quantum numbers, they can mix
to form four eigenstates 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.
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 superpartners. In such cases supersymmetric processes at accelerators are characterized by 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.
This is an important signature to discriminate supersymmetry from Standard Model backgrounds.
. In many models the lightest neutralino can be produced thermally in the hot early universe
and leave approximately the right relic abundance to account for the observed dark matter
. A lightest neutralino of roughly 10–10000 GeV
is the leading weakly interacting massive particle (WIMP) dark matter candidate .
Neutralino dark matter could be observed experimentally in nature either indirectly or directly. In the former case, 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. In the latter case, 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.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, the neutralino is a hypothetical particle predicted by supersymmetry
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...
. There are four neutralinos that are fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s and are electrically neutral, the lightest of which is typically stable. They are typically labeled (the lightest), , and (the heaviest) although sometimes
is also used when is used to refer to charginoChargino
In particle physics, the chargino is a hypothetical particle which refers to the mass eigenstates of a charged superpartner, i.e. any new electrically charged fermion predicted by supersymmetry. They are linear combinations of the charged wino and charged higgsinos...
s. These four states are mixtures of the Bino and the neutral Wino (which are the neutral electroweak Gaugino
Gaugino
In particle physics, a gaugino is the hypothetical superpartner of a gauge field, as predicted by gauge theory combined with supersymmetry. They are fermions.In the minimal supersymmetric extension of the standard model the following gauginos exist:...
s), and the neutral Higgsinos. As the neutralinos are Majorana fermion
Majorana fermion
In physics, a Majorana fermion is a fermion which is its own anti-particle. The term is used in opposition to Dirac fermion, which describes particles that differ from their antiparticles...
s, each of them is identical with its antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
. Because these particles only interact with the weak vector bosons, they are not directly produced at hadron colliders in copious numbers. They primarily appear as particles in cascade decays of heavier particles (decays that happen in multiple steps) usually originating from colored
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
supersymmetric particles such as squarks or gluino
Gluino
A gluino is the hypothetical supersymmetric partner of a gluon. Gluinos are expected by supersymmetry theorists to be pair produced in particle accelerators such as the Large Hadron Collider if they exist....
s.
In R-parity
R-parity
R-parity is a concept in particle physics. In the supersymmetric extension of the Standard Model, baryon number and lepton number are no longer conserved by all of the renormalizable couplings...
conserving models, the lightest neutralino is stable and all supersymmetric cascades 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:
| → | + | → | Missing energy | + | + | |||||||||||
| → | + | → | + | + | → | Missing energy | + | + |
The mass splittings between the different neutralinos will dictate which patterns of decays are allowed.
Origins in supersymmetric theories
In supersymmetry models, all Standard ModelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
particles have partner particles with the same quantum number
Quantum number
Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
s except for the quantum number spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
, which differs by 1/2 from its partner particle. Since the superpartners of the Z boson (zino
Zino
Zino is a Greek social networking website. In 2009, it was ranked one of the top Greek social destinations.-History:Zino started as a continuation of a news-focused Greek network, chit-chat.gr, which was established in 2005, and eventually replaced it...
), the photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
(photino) and the neutral higgs
Higgs boson
The Higgs boson is a hypothetical massive elementary particle that is predicted to exist by the Standard Model of particle physics. Its existence is postulated as a means of resolving inconsistencies in the Standard Model...
(higgsino
Higgsino
In particle physics, a Higgsino, symbol , is the hypothetical superpartner of the Higgs boson, as predicted by supersymmetry. The Higgsino is a Dirac fermion and that is a weak isodoublet with hypercharge half under the Standard Model gauge symmetries...
) have the same quantum numbers, they can mix
Quantum superposition
Quantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...
to form four eigenstates of the mass operator called "neutralinos". In many models the lightest of the four neutralinos turns out to be the lightest supersymmetric particle
Lightest Supersymmetric Particle
In particle physics, the Lightest Supersymmetric Particle is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models. In models with R-parity conservation, the LSP is stable. There is extensive observational evidence for an additional...
(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 (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 the weak interaction. In this way they are phenomenologically similar to neutrinos, and so are not directly observable in particle detectors at accelerators.In models in which R-parity
R-parity
R-parity is a concept in particle physics. In the supersymmetric extension of the Standard Model, baryon number and lepton number are no longer conserved by all of the renormalizable couplings...
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 superpartners. In such cases supersymmetric processes at accelerators are characterized by 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.
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 comprise the universe's cold dark matterCold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...
. In many models the lightest neutralino can be produced thermally in the hot early universe
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
and leave approximately the right relic abundance to account for the observed dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
. A lightest neutralino of roughly 10–10000 GeV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
is the leading weakly interacting massive particle (WIMP) dark matter candidate .
Neutralino dark matter could be observed experimentally in nature either indirectly or directly. In the former case, 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. In the latter case, special purpose experiments such as the Cryogenic Dark Matter Search
Cryogenic Dark Matter Search
The Cryogenic Dark Matter Search is a series of experiments designed to directly detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with...
(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.