Editing Massless particle (section)

===Standard Model gauge bosons===
The [[photon]] (carrier of [[electromagnetism]]) is one of two known [[gauge boson]]s 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 collider|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 [[colour confinement|being confined]] within [[hadron]]s, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment.<ref>
{{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
}}
</ref><ref>
{{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
}}
</ref>

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.