60 articles
Long-lived particles
Long-lived particles (LLPs) are particles that are stable or live long enough for their decays to be experimentally distinguishable in time or position from where they were produced.
Magnetic monopole
A magnetic monopole, or Dirac monopole, is a hypothetical particle carrying an isolated magnetic charge, proposed by Paul Dirac in 1931 and required by grand unified theories, though no observation has confirmed it.
Majorana fermion
A Majorana fermion, or Majorana particle, is a hypothetical spin-1/2 particle identical to its own antiparticle, proposed by Italian physicist Ettore Majorana in 1937; no elementary one has been confirmed.
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model, or MSSM, is the simplest supersymmetric extension of the Standard Model: every particle gets a superpartner, and two Higgs doublets are required.
Mirror matter
Mirror matter is a hypothetical form of matter whose particles mirror the known Standard Model particles but interact with ordinary matter almost exclusively through gravity.
Nambu–Jona-Lasinio model
The Nambu–Jona-Lasinio model (NJL model) is a field theory of fermions interacting through a contact four-fermion term, proposed in 1961 to explain how particle masses arise dynamically.
Naturalness
Naturalness is a criterion in particle physics that a theory's parameters should not require extreme fine-tuning, underlying the Higgs hierarchy problem but called into question after the LHC found no expected new physics.
Neutralino
A neutralino is a neutral Majorana fermion predicted by supersymmetry, formed by mixing the bino, neutral wino, and higgsinos, and its lightest state is a leading WIMP dark matter candidate.
Next-to-Minimal Supersymmetric Standard Model
The Next-to-Minimal Supersymmetric Standard Model, or NMSSM, is a supersymmetric extension of the Standard Model that adds a gauge-singlet superfield, dynamically generating the Higgs mass term.
Peccei–Quinn symmetry
Peccei–Quinn symmetry is a hypothetical chiral symmetry proposed by Roberto Peccei and Helen Quinn in 1977 to solve the strong CP problem, implying a light particle called the axion.
Preon
A preon, also called a prequark or subquark, is a hypothetical subatomic particle proposed as a building block of quarks and leptons; no experiment has found quark substructure.
Proton decay
Proton decay is a hypothetical process, forbidden by the Standard Model, in which a proton decays into lighter particles such as a positron and a neutral pion.
Pure 4D N = 1 supergravity
Pure 4D N = 1 supergravity is the locally supersymmetric extension of general relativity in four dimensions, containing a graviton and one spin-3/2 gravitino and no matter multiplets.
R-hadron
An R-hadron is a hypothetical composite particle in which a long-lived gluino or squark binds with ordinary quarks or gluons; none has been observed, though LHC searches set multi-TeV mass limits.
Rishon (particle)
A rishon is a hypothetical spin-1/2 subparticle proposed in 1979 by Haim Harari and Michael Shupe, which builds quarks and leptons from triplets of two constituents, T and V; none has been observed.
Sfermion
A sfermion is the hypothetical scalar superpartner of a Standard Model fermion in supersymmetry, named with a prefixed s (selectron, stop, stau); no sfermion has been observed, with squarks excluded below roughly 2 TeV.
Slepton
A slepton is the hypothetical spin-0 superpartner of a Standard Model lepton in supersymmetric extensions of particle physics, including the selectron, smuon, stau, and sneutrinos; none has been observed.
Sneutrino
The sneutrino is the hypothetical scalar (spin-0) superpartner of the neutrino in supersymmetric extensions of the Standard Model, predicted for each neutrino flavor; no sneutrino has been observed, but collider searches constrain its mass.
Squark
A squark is the hypothetical spin-0 superpartner of a quark predicted by supersymmetry; LHC searches have found no squarks, excluding masses up to about 1.85 TeV.
Stop squark
The stop squark, or top squark (symbol t̃), is the hypothetical spin-zero superpartner of the top quark predicted by supersymmetry; its loops would cancel the top quark's contribution to the Higgs mass.
Strong CP problem
The strong CP problem is the unexplained smallness of CP violation in quantum chromodynamics, where experiments constrain the parameter to at least ten orders of magnitude below its natural size.
Tachyon condensation
Tachyon condensation is the string theory process in which a tachyonic field, signaling instability, rolls from a potential maximum to a minimum and decays unstable D-branes.
Technicolor (physics)
Technicolor is a class of theories beyond the Standard Model in which a new strong interaction breaks electroweak symmetry, proposed by Leonard Susskind in 1979.
Two-Higgs-doublet model
The two-Higgs-doublet model (2HDM) is one of the simplest extensions of the Standard Model, adding a second Higgs doublet to yield five physical Higgs bosons instead of one.
Vector-like lepton
A vector-like lepton (VLL) is a hypothetical heavy, non-chiral lepton in beyond-Standard-Model theories, predicted to help explain Higgs flavor hierarchies; ATLAS excludes doublet VLLs below about 1220 GeV.
Vector-like quark
A vector-like quark is a hypothetical heavy quark whose left- and right-handed chiralities transform identically, giving it a mass that does not require the Higgs field.
W′ and Z′ bosons
W′ and Z′ bosons are hypothetical heavy partners of the Standard Model W and Z bosons, predicted in many extensions and searched for at the LHC.
Weakly interacting slim particles (WISPs)
Weakly interacting slim particles (WISPs), also called weakly interacting sub-eV particles, are hypothetical particles with sub-eV masses and extremely feeble couplings to matter, including the QCD axion and axion-like particles.
Wino (particle)
In particle physics, the wino is the hypothetical supersymmetric partner of the weak-force W bosons, whose charged and neutral states form a benchmark WIMP dark matter candidate.
X17 boson
The X17 boson is a hypothetical neutral particle of about 17 MeV, proposed to explain anomalous electron-positron pairs in beryllium-8 decays reported by the ATOMKI institute in Hungary in 2015.