With the discovery of neutrino oscillations, which imply that neutrinos have mass, the only known "observed" massless particles are the photon and the gluon: the only two particles now known that have "absolute chirality".
The only known "observed" massless particles are the photon: the only two particles now known that have "absolute chirality". This was with the discovery of neutrino oscillations. The only known "observed" massless particles are the gluon: the only two particles now known that have "absolute chirality". This was with the discovery of neutrino oscillations. Neutrinos have mass. Neutrino oscillations imply this.