Constraints on neutrino oscillation parameters from the measurement of   day-night solar neutrino fluxes at Super-Kamiokande
  A search for day-night variations in the solar neutrino flux resulting from neutrino oscillations has been carried out using the 504 day sample of solar neutrino data obtained at Super-Kamiokande. The absence of a significant day-night variation has set an absolute flux independent exclusion region in the two neutrino oscillation parameter space. 
The Super-Kamiokande Collaboration: Y.Fukuda, T.Hayakawa, E.Ichihara,   K.Inoue