Discovery of neutrino oscillations

Neutrinos are very special particles. Studies of these particles have played a crucial role in the understanding of the laws of elementary particles and their interactions. Until recently, there was no evidence that neutrinos have masses, and therefore the standard model in elementary particle physics assumed that neutrinos are massless. Small but non-zero masses of neutrinos have been discovered by studying neutrinos produced by cosmic ray interactions in the atmosphere. The small neutrino masses have profound implications for our understanding of particle physics and the universe. This paper discusses the discovery of neutrino masses.