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 because they are
confined within
hadrons, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment.
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, 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 lattices 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). 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. ==See also==