Complex sentence: 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".
Split into simpler sentences: With the discovery of neutrino oscillations, which imply that neutrinos have mass, the only observed massless particle is the photon. Hence, these are the only two particles now known with "absolute chirality".