In 1968, Bruno Pontecorvo showed that if neutrinos are not considered massless, then ν e (produced in the sun) can transform into some other neutrino species (ν μ or ν τ), to which Homestake detector was insensitive. This explained the deficit in the results of the Homestake experiment. The final confirmation of this solution to the solar neutrino problem was provided in April 2002 by the SNO (Sudbury Neutrino Observatory) collaboration, which measured both ν e flux and the total neutrino flux.