Lightest Supersymmetric Particle
Encyclopedia
In particle physics
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 Lightest Supersymmetric Particle (LSP) is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...

. In models with 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...

 conservation, the LSP is stable. There is extensive observational evidence for an additional component of the matter density in the Universe that goes under the name 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...

. The LSP of supersymmetric models is a dark matter candidate and is a Weakly interacting massive particle (WIMP).

Dark matter particles must be electrically neutral; otherwise they would scatter light and thus not be "dark". They must also almost certainly be non-colored
.
With these constraints, the LSP could be the lightest neutralino
Neutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

, the gravitino
Gravitino
The gravitino is the supersymmetric partner of the graviton, as predicted by theories combining general relativity and supersymmetry; i.e. supergravity theories...

, or the lightest sneutrino.

Sneutrino dark matter is ruled out in the Minimal Supersymmetric Standard Model
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...


(MSSM) because of the current limits on the interaction cross section of dark matter particles with ordinary matter as measured by direct detection experiments—the sneutrino interacts via Z boson exchange and would have been detected by now if it makes up the dark matter. Extended models with right-handed or sterile sneutrinos reopen the possibility of sneutrino dark matter by lowering the interaction cross section
.

Neutralino dark matter is the favored possibility. In most models the lightest neutralino is mostly bino (superpartner of the hypercharge
Weak hypercharge
The weak hypercharge in particle physics is a conserved quantum number relating the electrical charge and the third component of weak isospin, and is similar to the Gell-Mann–Nishijima formula for the hypercharge of strong interactions...

 gauge boson
Gauge boson
In particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental forces of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.-...

 field B), with some admixture of neutral wino
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:...

 (superpartner of the weak isospin
Weak isospin
In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I with the third component written as Tz, T3, Iz or I3...

 gauge boson field W0) and/or neutral 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...

.

Gravitino dark matter is a possibility in supersymmetric models in which the scale of supersymmetry breaking is low, around 100 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....

. In such models the gravitino is very light, of order an eV
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...

. As dark matter, the gravitino is sometimes called a super-WIMP because its interaction strength is much weaker than that of other supersymmetric dark matter candidates. For the same reason, its direct thermal production in the early universe is too inefficient to account for the observed dark matter abundance. Rather, gravitinos would have to be produced through the decay of the next-to-lightest supersymmetric particle (NLSP).

In extra dimensional theories, there are analogous particles called LKP's or Lightest Kaluza-Klein Particle. These are the stable particles of extra-dimensional theories .

See also

  • 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...

  • 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...

  • Neutralino
    Neutralino
    In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

  • Gravitino
    Gravitino
    The gravitino is the supersymmetric partner of the graviton, as predicted by theories combining general relativity and supersymmetry; i.e. supergravity theories...

  • Sneutrino
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