massless particle
{{Short description|Particle whose invariant mass is zero}}
{{More citations needed|date=April 2022}}
In particle physics, a massless particle is an elementary particle whose invariant mass is zero. At present the only confirmed massless particle is the photon.
Other particles and quasiparticles
| class="wikitable sortable" style="margin:1em auto; text-align: center;" | ||||||
| Name | Symbol | Antiparticle | Charge (e) | Spin | Interaction mediated | Experimentally confirmed |
|---|---|---|---|---|---|---|
| Photon | {{mvar|γ}} | Self | 0 | 1 | Electromagnetism | Confirmed to exist. Confirmed massless. |
| Gluon | {{math|{{Subatomic particle|Gluon}}}} | Self | 0 | 1 | Strong interaction | Indirectly confirmed to exist. |
| Graviton | {{math|G}} | Self | 0 | 2 | Gravitation | Never observed / entirely hypothetical |
=Standard Model gauge bosons=
The photon (carrier of electromagnetism) is one of two known gauge bosons thought to be massless. The photon is well-known from direct observation to exist and be massless.
The other massless gauge boson is the gluon (carrier of the strong force) whose existence has been inferred from particle 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 that gluons are massless, they can never be observed as free particles due to being confined within hadrons, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment.
{{cite journal
|last=Valencia |first=G.
|year=1992
|title=Anomalous gauge-boson couplings at hadron supercolliders
|journal=AIP Conference Proceedings
|volume=272 |issue=2 |pages=1572–1577
|arxiv=hep-ph/9209237 |bibcode=1992AIPC..272.1572V
|doi=10.1063/1.43410 |s2cid=18917295
}}
{{cite journal
|last=Debrescu |first=B.A.
|year=2005
|title=Massless gauge bosons other than the photon
|journal=Physical Review Letters
|volume=94 |issue=15 |page=151802
|arxiv=hep-ph/0411004 |bibcode=2005PhRvL..94o1802D
|doi=10.1103/PhysRevLett.94.151802 |pmid=15904133 |s2cid=7123874
}}
The only other observed gauge bosons are the W and Z bosons, which are known from experiments to be extremely massive, even heavier than iron nuclei.
= Hypothetical graviton =
The graviton is a hypothetical tensor boson proposed to be the carrier of gravitational force in some quantum theories of gravity, but no such theory has been successfully incorporated into the Standard 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.
= Quasiparticles =
The Weyl fermion discovered in 2015 is also expected to be massless,
{{cite news
|title=After 85 year search, massless particle with promise for next-generation electronics found
|date=16 July 2015
|publisher=Princeton University
|website=phys.org
|url=https://phys.org/news/2015-07-year-massless-particle-next-generation-electronics.html
}}
{{cite journal
|author=Su-Yang Xu |author2=Ilya Belopolski |author3=Nasser Alidoust |author4=Madhab Neupane
|display-authors=etal
|date=16 July 2015
|title=Discovery of a Weyl fermion semimetal and topological Fermi arcs
|journal=Science
|volume=349 |issue=6248 |pages=613–617
|publisher=AAAS
|doi=10.1126/science.aaa9297 |pmid=26184916 |arxiv=1502.03807 |bibcode=2015Sci...349..613X |s2cid=206636457 |url=https://www.science.org/doi/10.1126/science.aaa9297
|access-date=2023-11-14
}}
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 merely quasiparticles – 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 like phonons, 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 change flavour as they travel, at least two of the types of neutrinos must have mass (and cannot be Weyl fermions).
{{cite news
|first=Robert |last=Garisto
|date=1 September 1998
|title=Neutrinos have mass
|series=Physical Review Letters
|department=Focus
|website=aps.org
|publisher=American Physical Society
|url=https://physics.aps.org/story/v2/st10
|access-date=2023-11-14
}}
The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita sharing the 2015 Nobel Prize in Physics.
{{cite news
|first = Charles |last = Day
|date = 2015-10-07
|title = Takaaki Kajita and Arthur McDonald share 2015 Physics Nobel
|magazine = Physics Today
|issn = 0031-9228 |doi = 10.1063/PT.5.7208
}}