massless particle

In particle physics, a massless particle is a particle whose invariant mass is theoretically zero. the two known massless particles were 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. Neutrinos were, until recently, thought to be either massless or have a small mass. However, because neutrinos change flavour as they travel, at least two of the types of neutrinos must have mass. This was proven by Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita, which resulted in their sharing of the Nobel prize in Physics 2015.