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particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
, the lightest supersymmetric particle (LSP) 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; in other words, it cannot decay into any
Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...
particle, since all SM particles have the
opposite In lexical semantics, opposites are words lying in an inherently incompatible binary relationship. For example, something that is ''even'' entails that it is not ''odd''. It is referred to as a 'binary' relationship because there are two members i ...
R-parity. There is extensive observational evidence for an additional component of the matter density in the universe, which goes under the name
dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
. The LSP of supersymmetric models is a dark matter candidate and is a
weakly interacting massive particle Weakly interacting massive particles (WIMPs) are hypothetical particles that are one of the proposed candidates for dark matter. There exists no formal definition of a WIMP, but broadly, it is an elementary particle which interacts via gravity an ...
(WIMP).


Constraints on LSP from cosmology

The LSP is unlikely to be a charged wino, charged higgsino, slepton, sneutrino, gluino, squark, or
gravitino In supergravity theories combining general relativity and supersymmetry, the gravitino () is the gauge fermion supersymmetric partner of the hypothesized graviton. It has been suggested as a candidate for dark matter. If it exists, it is a f ...
but is most likely a mixture of neutral higgsinos, the bino and the neutral winos, i.e. a
neutralino In supersymmetry, the neutralino 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 fermions and are electrically ...
. In particular, if the LSP were charged (and is abundant in our galaxy) such particles would have been captured by the Earth's magnetic field and form heavy hydrogen-like atoms. Searches for anomalous hydrogen in natural water however have been without any evidence for such particles and thus put severe constraints on the existence of a charged LSP.


As a dark matter candidate

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 ''Colored'' (or ''coloured'') is a racial descriptor historically used in the United States during the Jim Crow era to refer to an African American. In many places, it may be considered a slur. Dictionary definitions The word ''colored'' wa ...
. With these constraints, the LSP could be the lightest neutralino, the gravitino, or the lightest sneutrino. *''Sneutrino dark matter'' is ruled out in the
Minimal Supersymmetric Standard Model The Minimal Supersymmetric Standard Model (MSSM) is an extension to the Standard Model that realizes supersymmetry. MSSM is the minimal supersymmetrical model as it considers only "the inimumnumber of new particle states and new interactions ...
(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 In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
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 In particle physics, the hypercharge (a portmanteau of hyperonic and charge (physics), charge) ''Y'' of a subatomic particle, particle is a quantum number conserved under the strong interaction. The concept of hypercharge provides a single charg ...
gauge boson In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles whose interactions are described by a gauge theory interact with each other by the exchange of gauge ...
field B), with some admixture of neutral wino (superpartner of the
weak isospin In particle physics, weak isospin is a quantum number relating to the electrically charged part of the weak interaction: Particles with half-integer weak isospin can interact with the bosons; particles with zero weak isospin do not. Weak isospin ...
gauge boson field W0) and/or neutral Higgsino. *''Gravitino dark matter'' is a possibility in supersymmetric models in which the scale of supersymmetry breaking is low, around 100 TeV. In such models the gravitino is very light, of order an eV. 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

*
Dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
* Darkon (unparticle) *
List of hypothetical particles This is a list of hypothetical subatomic particles in physics. Elementary particles Some theories predict the existence of additional elementary bosons and fermions that are not found in the Standard Model. Particles predicted by supersy ...
*
Supersymmetry Supersymmetry is a Theory, theoretical framework in physics that suggests the existence of a symmetry between Particle physics, particles with integer Spin (physics), spin (''bosons'') and particles with half-integer spin (''fermions''). It propo ...
* Weakly interacting slender particle


References

{{Dark matter Dark matter Supersymmetric quantum field theory Hypothetical particles Weight