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In [[Physics:Particle physics|particle physics]], the '''lightest supersymmetric particle''' ('''LSP''') is the generic name given to the lightest of the additional hypothetical particles found in [[Physics:Minimal Supersymmetric Standard Model|supersymmetric models]]. In models with [[Physics:R-parity|R-parity]] conservation, the LSP is stable; in other words, the LSP cannot decay into any [[Physics:Standard Model|Standard Model]] particle, since all SM particles have the [[Additive inverse|opposite]] R-parity. There is extensive observational evidence for an additional component of the matter density in the Universe that goes under the name [[Astronomy:Dark matter|dark matter]]. The LSP of supersymmetric models is a dark matter candidate and is a [[Physics:Weakly interacting massive particle|weakly interacting massive particle]] (WIMP).<ref>{{cite journal |author1=Jungman, Gerard |author2=Kamionkowski, Marc |author3=Griest, Kim | title=Supersymmetric dark matter |volume=267 |issue=5–6 |pages=195–373 |journal=Phys. Rep. |doi=10.1016/0370-1573(95)00058-5|arxiv = hep-ph/9506380 |bibcode = 1996PhR...267..195J |year=1996 }}</ref>
==Constraints on LSP from cosmology==
The LSP is unlikely to be a charged [[Physics:Gaugino|wino]], charged [[Physics:Higgsino|higgsino]], slepton,
sneutrino, [[Physics:Gluino|gluino]], squark, or [[Physics:Gravitino|gravitino]] but is most likely a mixture of neutral higgsinos, the [[Physics:Gaugino|bino]] and the neutral [[Physics:Gaugino|winos]],<ref>{{cite journal |author1=Ellis, John R. | author2=Hagelin, J.S.| author3=Nanopoulos, Dimitri V. |author4=Olive, Keith A.| author5=Srednicki, M. |date=July 1983 | title=Supersymmetric Relics from the Big Bang|volume=B238 | issue=2|pages=453–476 |journal=Nucl. Phys. |doi=10.1016/0550-3213(84)90461-9 |bibcode = 1984NuPhB.238..453E }}</ref> i.e. a [[Physics:Neutralino|neutralino]]. 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.<ref>{{Cite journal|arxiv=hep-ph/0202252|last1=Byrne|first1=Mark|title=Bounds on Charged, Stable Superpartners from Cosmic Ray Production|journal=Physical Review D|volume=66|issue=7|pages=075007|last2=Kolda|first2=Christopher|last3=Regan|first3=Peter|date=2002|doi=10.1103/PhysRevD.66.075007|bibcode = 2002PhRvD..66g5007B |citeseerx=10.1.1.348.1389}}</ref> Searches for anomalous hydrogen in natural water<ref>{{cite journal
|author1=Smith, P.F. | author2=Bennett, J.R.J.| author3=Homer, G.J. |author4=Lewin, J.D. | author5=Walford, H.E.| author6=Smith, W.A. |date=November 1981 | title=A search for anomalous hydrogen in enriched D2O, using a time-of-flight spectrometer|volume=B206 | issue=3|pages=333–348 |journal=Nucl. Phys. |doi=10.1016/0550-3213(82)90271-1 |bibcode = 1982NuPhB.206..333S }}</ref> however have been without any evidence for such particles and thus put severe constraints on the existence of a charged LSP.

==Lightest Supersymmetric Particle 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-[[Physics:Color charge|colored]].<ref>{{cite journal |last=McGuire |first=Patrick C. |last2=Steinhardt |first2=Paul |date=May 2001 |title=Cracking open the window for strongly interacting massive particles as the halo dark matter |arxiv=astro-ph/0105567 |bibcode=2001ICRC....4.1566M |volume=4 |pages=1566 |journal=Proceedings of the 27th International Cosmic Ray Conference. 07-15 August}}</ref> 
With these constraints, the LSP could be the lightest [[Physics:Neutralino|neutralino]], the [[Physics:Gravitino|gravitino]], or the lightest sneutrino.

*''Sneutrino dark matter'' is ruled out in the [[Physics:Minimal Supersymmetric Standard Model|Minimal Supersymmetric Standard Model]] (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 [[Physics:Z boson|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.<ref>{{cite journal |last=Tucker-Smith |first= David. |author2=Weiner, Neal |date=February 2004 |title=The Status of inelastic dark matter |arxiv= hep-ph/0402065|bibcode = 2005PhRvD..72f3509T |doi = 10.1103/PhysRevD.72.063509 |journal=Physical Review D |volume=72 |issue=6 |pages= 063509 }}</ref>
*''Neutralino dark matter'' is the favored possibility. In most models the lightest neutralino is mostly [[Physics:Gaugino|bino]] (superpartner of the [[Physics:Weak hypercharge|hypercharge]] [[Physics:Gauge boson|gauge boson]] field B), with some admixture of neutral [[Physics:Gaugino|wino]] (superpartner of the [[Physics:Weak isospin|weak isospin]] gauge boson field W<sup>0</sup>) and/or neutral [[Physics:Higgsino|Higgsino]].
*''Gravitino dark matter'' is a possibility in supersymmetric models in which the scale of supersymmetry breaking is low, around 100 [[Physics:TeV|TeV]]. In such models the gravitino is very light, of order an [[Physics:Electronvolt|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 [[Astronomy:Extra dimensions|extra-dimensional theories]], there are analogous particles called ''LKP''s or ''Lightest Kaluza-Klein Particle''. These are the stable particles of extra-dimensional theories.<ref>{{cite journal |last=Servant |first= Geraldine. |author2=Tait, Tim M.P. |date=September 2003 |title=Is the Lightest Kaluza-Klein Particle a Viable Dark Matter Candidate? |arxiv= hep-ph/0206071|bibcode = 2003NuPhB.650..391S |doi = 10.1016/S0550-3213(02)01012-X |journal=Nuclear Physics B |volume=650 |issue= 1–2 |pages=391–419 }}</ref>

==See also==
*[[Physics:Supersymmetry|Supersymmetry]]
*[[Astronomy:Dark matter|Dark matter]]
*[[Physics:Neutralino|Neutralino]]
*[[Physics:Gravitino|Gravitino]]
*Sneutrino

==References==
<references/>

{{Dark matter}}

[[Category:Dark matter]]
[[Category:Supersymmetry]]
[[Category:Hypothetical particles]]



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