# Physics:Massless particle

Short description: Particle whose invariant mass is zero

In particle physics, a massless particle is an elementary particle whose invariant mass is zero. The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons.[1][2] Neutrinos were originally thought to be massless. However, because neutrinos change flavor as they travel, at least two of the types of neutrinos must have mass. 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.[3]

Name Symbol Antiparticle Charge (e) Spin Interaction mediated Existence
Photon γ Self 0 1 Electromagnetism Confirmed
Gluon g Self 0 1 Strong interaction Confirmed
Graviton G Self 0 2 Gravitation Unconfirmed

## References

1. Valencia, G. (1992). "Anomalous Gauge-Boson Couplings At Hadron Supercolliders". AIP Conference Proceedings 272 (2): 1572–1577. doi:10.1063/1.43410. Bibcode1992AIPC..272.1572V.
2. Debrescu, B. A. (2005). "Massless Gauge Bosons Other Than The Photon". Physical Review Letters 94 (15): 151802. doi:10.1103/PhysRevLett.94.151802. PMID 15904133. Bibcode2005PhRvL..94o1802D.
3. Day, Charles (2015-10-07). "Takaaki Kajita and Arthur McDonald share 2015 Physics Nobel". Physics Today. doi:10.1063/PT.5.7208. ISSN 0031-9228.