The Super - Kamiokande Day - Night Effect Data and the MSW Solutions of the Solar Neutrino Problem

Abstract

The current Super-Kamiokande data on the D-N asymmetry between the the day event rate and the {\it Night} and {\it Core} event rates, produced by solar neutrinos which respectively cross the Earth along any trajectory and cross the Earth core before reaching the detector, imply rather stringent constraints on the MSW small mixing angle (SMA) $ν_e \to ν_{μ(τ)}$ solution of the solar neutrino problem. A simplified analysis shows, in particular, that a substantial subregion of the SMA solution region is disfavored by these data. The {\it Core} D-N asymmetry data alone allow to rule out at 99.7% C.L. a part of this subregion. The constraints on the MSW large mixing angle and LOW $ν_e \to ν_{μ(τ)}$ solutions as well as on the MSW $ν_e \to ν_{s}$ solution, following from the data on the {\it Night} and {\it Core} D-N asymmetries are also discussed.

8 pages latex text + 6 ps-files; includes 6 figures; new results based on the Super-Kamiokande data on the {\it Mantle} D-N asymmetry included; the text is changed accordingly and the 6 figures are substituted with new ones including the constraints from the {\it Mantle} asymmetry data