In particle physics, a massless particle is an elementary particle whose invariant mass is zero. There are two known gauge boson massless particles: 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] In addition the Weyl semimetal or Weyl fermion discovered in 2015 is also massless.^[3][4]

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.^[5] 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.^[6]

• Note that although literal Weyl fermions have never been experimentally confirmed to physically exist, certain systems can act collectively such that they seem to contain Weyl fermion quasiparticles.