This is a list of hypothetical particles.
Hypothetical particles are proposed subatomic or composite entities arising in theoretical particle physics and cosmology that have not been experimentally confirmed. They are typically introduced to address limitations of the Standard Model, unify fundamental interactions, or explain unresolved observations such as dark matter, neutrino masses, baryon asymmetry, or cosmic inflation. [1] [2] Many are mathematically well defined within quantum field theory or its extensions and serve as mediators or constituents in speculative but testable frameworks beyond the Standard Model. [3]
Prominent classes include gauge or symmetry-related particles such as the graviton or graviphoton in quantum gravity and extra-dimensional theories, and supersymmetric partners (e.g., neutralinos or charginos) predicted by supersymmetry. [4] [5] Others address specific phenomena, including hidden-sector bosons such as dark photons or light force carriers proposed to explain nuclear anomalies. [6]
Hypothetical particles also encompass predicted bound states (e.g., glueballs), topological objects such as magnetic monopoles, and unconventional statistical entities such as anyons or tachyons. [7] [8] Collectively, they form a central component of theoretical model building, guiding experimental searches for physics beyond currently known particles.
Some theories predict the existence of additional elementary bosons and fermions that are not found in the Standard Model.
| Name | Spin | Notes |
|---|---|---|
| axion | 0 | A pseudoscalar particle introduced in Peccei–Quinn theory to solve the strong-CP problem. [9] |
| dilaton | 0 | Predicted in some string theories. [10] |
| graviphoton | 1 | Also known as "gravivector". [11] It appears in Kaluza–Klein theory. [12] |
| graviton | 2 | Massless boson associated to gravitation. Included in many beyond the Standard Model theories. [13] |
| dual graviton | 2 | Has been hypothesized as dual of graviton under electric–magnetic duality in supergravity. [14] |
| graviscalar | 0 | Also known as "radion". It appears in Kaluza–Klein theory. [15] |
| hyperphoton | 0 | Hypothetical photon-like particle related to CP violations in kaon decay. [16] [17] [18] |
| inflaton | 0 | Unidentified scalar force-carrier that is presumed to have physically caused cosmic inflation. [19] [20] [21] |
| majoron | 0 | Predicted to understand neutrino masses by the seesaw mechanism. [22] |
| sterile neutrino | 1 /2 | Right-handed neutrinos are compatible with the Standard Model but have never been observed. [23] |
| dual photon | 1 | Dual of the photon under electric–magnetic duality [24] [25] [26] [27] [28] |
| magnetic photon | 1 | Hypothetical particle similar to the photon in the presence of magnetic monopoles. [29] |
| pressuron | 0 | hypothetical scalar particle which couples to both gravity and matter theorized in 2013. [30] |
| symmetron | 0 | Mediates the fifth force of the hypothetical symmetron field. [31] |
| X and Y bosons | 1 | These leptoquarks are predicted by Grand Unified Theories to be heavier equivalents of the W and Z. [32] |
| W′ and Z′ bosons | 1 | Predicted by several extension of the electroweak interaction. [33] |
Supersymmetry predicts the existence of superpartners to particles in the Standard Model, none of which have been confirmed experimentally. [34] The sfermions (spin is 0) include:
| Name | Symbol | Superpartner of | Symbol |
|---|---|---|---|
| sup squark | up quark | ||
| sdown squark | down quark | ||
| scharm squark | charm quark | ||
| sstrange squark | strange quark | ||
| stop squark [35] | top quark | ||
| sbottom squark | bottom quark |
| Name | Symbol | Superpartner of | Symbol |
|---|---|---|---|
| selectron | electron | ||
| selectron sneutrino | electron neutrino | ||
| smuon | muon | ||
| smuon sneutrino | muon neutrino | ||
| stau | tau | ||
| stau sneutrino | tau neutrino |
Another hypothetical sfermion is the saxion, superpartner of the axion. It forms a supermultiplet, together with the axino and the axion, in supersymmetric extensions of Peccei–Quinn theory. [36]
The predicted bosinos (spin 1⁄2) are
| Name | superpartner of: | Notes |
|---|---|---|
| axino | axion | Forms a supermultiplet, together with the saxion and axion, in supersymmetric extensions of Peccei–Quinn theory. [37] |
| dilatino | dilaton | Combines with the axion to form a complex scalar field [38] |
| gluino | gluon | Majorana fermions that interact via the strong force as a color octet. |
| gravitino | graviton | Predicted by supergravity (SUGRA). |
| higgsino | Higgs boson | For supersymmetry there is a need for several Higgs bosons, neutral and charged, according with their superpartners. |
| photino | photon | Mixing with zino and neutral Higgsinos for neutralinos. |
| gaugino (wino, zino) | W and Z bosons | The charged wino mixing with the charged Higgsino for charginos, for the zino see line above. |
Just as the photon, Z and W± bosons are superpositions of the B0, W0, W1, and W2 fields, the photino, zino, and wino± are superpositions of the bino0, wino0, wino1, and wino2. No matter if one uses the original gauginos or this superpositions as a basis, the only predicted physical particles are neutralinos and charginos as a superposition of them together with the Higgsinos.
Other superpartner categories include:
The following hypothetical particles have been proposed to explain dark energy:
| Name | Description |
|---|---|
| Chameleon | Couples to matter more weakly than gravity, with non-linear variable effective mass. [39] postulated as a dark energy candidate. [40] |
| Acceleron | Particle that relates neutrino masses to dark energy. [41] [42] [43] |
| Quintessence particle | Ultralight scalar boson whose vacuum expectation value evolves cosmologically; its coherent zero-momentum state produces negative pressure and late-time acceleration consistent with slowly rolling dark-energy models. [44] [45] |
| Phantom particle | Scalar boson with wrong-sign kinetic term yielding equation-of-state below −1; quantum excitations represent phantom dark energy causing super-accelerated expansion and possible future Big-Rip cosmology. [46] [47] |
| K-essence particle | Quantum of a scalar with noncanonical kinetic structure; attractor dynamics drive cosmic acceleration without fine-tuned potential, behaving as effective vacuum energy at late times. [48] [49] |
| Quintaxion | Axion-like pseudoscalar with extremely shallow periodic potential; misalignment energy evolves slowly and mimics quintessence, linking dark energy to Peccei–Quinn-type symmetries or string compactifications. [50] [51] |
| Ultralight axion | Axion-like particle with mass near Hubble scale (~10⁻³³ eV); field remains frozen cosmologically so particle condensate behaves as nearly constant vacuum energy today. [52] [53] |
| String axion | One of many ultralight pseudoscalars predicted by string compactifications; lightest modes can remain overdamped until recent epochs, providing axion-like particle interpretation of dark-energy density. [54] [55] |
| Proca dark-energy boson | Massive vector boson with self-interaction potential; homogeneous temporal component acquires vacuum energy density, and particle excitations correspond to vector-dark-energy quanta driving acceleration. [56] [57] |
| Dark-energy vector boson | General cosmic vector particle whose coherent background breaks Lorentz symmetry cosmologically; effective negative pressure arises from potential energy dominating kinetic contributions at late times. [58] [59] |
| Graviton condensate excitation | Collective bosonic mode of a macroscopic graviton condensate; emergent quasiparticles encode vacuum energy of spacetime and can phenomenologically mimic cosmological-constant–like dark-energy behavior. [60] [61] |
The following categories are not unique or distinct: For example, either a WIMP or a WISP is also a FIP.
| Meaning | Abbreviation | Explanation | Candidates |
|---|---|---|---|
| Feebly interacting particle | FIP | Particles that interacts very weakly with conventional matter | Massive gravitons |
| Gravitationally interacting massive particle | GIMP | Massive particles that only interact with matter gravitationally | |
| Lightest supersymmetric particle | LSP | Predictions by supersymmetry | Sneutrino, gravitino, neutralino |
| Strongly interacting massive particle | SIMP | Particle that interact strongly between themselves and weakly with ordinary matter | |
| Stable massive particles | SMP | Long-lived particle with appreciable mass | |
| Weakly interacting massive particle | WIMP | Heavy particles that only interact with matter weakly | neutralino, sterile neutrino |
| Weakly interacting slender particle | WISP | Light particles that only interact with matter weakly | axion |
Hidden sector theories have also proposed forces that only interact with dark matter, like dark photons.
These hypothetical particles were claimed to be found or hypothesized to explain unusual experimental results. They relate to experimental anomalies but have not been reproduced independently or might be due to experimental errors (in chronological order):
| Name | Date of anomaly | Originator of the anomaly | Details |
|---|---|---|---|
| N-ray | 1903 | Prosper-René Blondlot | An unknown form of radiation. |
| Oops-Leon | 1976 | Fermilab | Resonance at 6 GeV |
| Valentine's day monopole | 1982 | Blas Cabrera Navarro | Single magnetic monopole detected on February 14, 1982. [62] |
| Meshugatron | 1989 | Fleischmann–Pons experiment | Predicted by Edward Teller in 1989 in an attempt to understand cold fusion claims [63] |
| Oh-My-God particle | 1991 | High Resolution Fly's Eye Cosmic Ray Detector | 320 EeV cosmic ray, most energetic ultra-high-energy cosmic ray detected as of 2015 |
| Leptoquark (B-anomaly) | 2012 | Large Hadron Collider | Hypothetical boson coupling quarks to leptons proposed to explain persistent flavor-universality violations in B-meson decays, notably RK and RD ratios, indicating potential new semileptonic interactions beyond Standard Model predictions. [64] [65] |
| Z′(Lμ−Lτ) | 2001 | Brookhaven E821 | Neutral gauge boson coupling to muon–tau lepton number difference proposed to explain muon anomalous magnetic-moment discrepancy and flavor anomalies while avoiding strong electron and quark coupling constraints. [66] [67] |
| 750 GeV diphoton | 2015 | Large Hadron Collider | Resonance at 750 GeV signature of a bosonic particle |
| X17 particle | 2015 | ATOMKI | Hypothesized new vector boson to explain nuclear experiments with beryllium. |
| Amaterasu particle | 2021 | Telescope Array Project | 240 EeV cosmic ray |