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Inparticle physics, amassless particle is anelementary particle whoseinvariant mass is zero. At present the only confirmed massless particle is thephoton.
| Name | Symbol | Antiparticle | Charge (e) | Spin | Interaction mediated | Experimentally confirmed |
|---|---|---|---|---|---|---|
| Photon | γ | Self | 0 | 1 | Electromagnetism | Confirmed to exist. Confirmed massless. |
| Gluon | g | Self | 0 | 1 | Strong interaction | Indirectly confirmed to exist. |
| Graviton | G | Self | 0 | 2 | Gravitation | Never observed / entirely hypothetical |
Thephoton (carrier ofelectromagnetism) is one of two knowngauge bosons thought to be massless. The photon is well-known from direct observation to exist and be massless.
The other massless gauge boson is thegluon (carrier of thestrong force) whose existence has been inferred fromparticle collision decay products; it is expected to be massless, but a zero mass has not been confirmed by experiment.Although there are compelling theoretical reasons to believe thatgluons are massless, they can never be observed as free particles due tobeing confined withinhadrons, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment.[1][2]
The only other observed gauge bosons are theW and Z bosons, which are known from experiment to be extremely massive.
Thegraviton is a hypotheticaltensor boson proposed to be the carrier ofgravitational force in somequantum theories of gravity, but no such theory has been successfully incorporated into theStandard Model, so the Standard Model neither predicts any such particle nor requires it, and no gravitational quantum particle has been indicated by experiment. Whether a graviton would be massless if it existed is likewise an open question.
TheWeyl fermion discovered in 2015 is also expected to be massless,[3][4] but these are not actual particles. At one time neutrinos were thought to perhaps be Weyl fermions, but when they were discovered to have mass, that left no fundamental particles of the Weyl type.
The Weyl fermions discovered in 2015 are merelyquasiparticles – composite motions found in the structure of molecular latices that have particle-like behavior, but are not themselves real particles. Weyl fermions in matter are likephonons, which are also quasiparticles. No real particle that is a Weyl fermion has been found to exist, and there is no compelling theoretical reason that requires them to exist.
Neutrinos were originally thought to be massless. However, because neutrinos changeflavour as they travel, at least two of the types of neutrinos must have mass (and cannot be Weyl fermions).[5] The discovery of this phenomenon, known asneutrino oscillation, led to Canadian scientistArthur B. McDonald and Japanese scientistTakaaki Kajita sharing the 2015Nobel Prize in Physics.[6]
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