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Inparticle physics, the acronymWISP refers to a largely hypotheticalweakly interacting sub-eV particle, orweakly interactingslender particle, orweakly interactingslim particle – low-mass particles which rarely interact with conventional particles.

The term is used to generally categorize a type ofdark matter candidate, and is essentially synonymous withaxion-like particle (ALP).WISPs are generallyhypothetical particles.

WISPs are the low-mass counterpart ofweakly interacting massive particles (WIMPs).

Except for conventional, activeneutrinos, all WISPs are candidatedark matter constituents, and many proposed experiments to detect WISPs might possibly be able to detect several different kinds. "WISP" is most often used to refer to a low-masshypothetical particles which are viabledark matter candidates. Examples include:^[1]

• Axion – long-standing hypotheticalstrong force related light particle
• Sterile neutrino – never-observed particles explicitly excluded (if they exist) from theweak,strong andelectromagnetic interactions
• Supersymmetric particles, particularly thelightest supersymmetric particle which might be a
  • Neutralino –supersymmetricfermions that are electricallyneutral composites ofsuperpartners tobosons

Although ordinary "active"neutrinos (left-chiralneutrinos and right-chiral antineutrinos) are particles known to exist, and though active neutrinos do indeed technically satisfy the description of the term, they are often excluded from lists of "WISP" particles.

The reason that active neutrinos are oftennot included among WISPs is that they are no longer viable dark matter candidates: current estimated limits on their number density and mass indicate that their cumulative mass-density could not be high enough to account for the amount ofdark matter inferred from its observed effects, although they certainly do make somesmall contribution to dark matter density.^[1]

The various sources of WISPs could possibly include hotastrophysical plasma and energy transport in stars.^[1] Note however, that since they remain hypothetical (except foractive neutrinos), the means of creation of WISPs depends on the theoretical framework used to propose them.

• Axion
• Feebly interacting particle (FIP)
• Hot dark matter
• Light dark matter
• Lightest supersymmetric particle (LSP)
• Sterile neutrino
• Weakly interacting massive particle (WIMP)

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