Day - Night Effect Predictions for the SNO Detector

Detailed predictions for the day-night (D-N) asymmetry in the energyintegrated one year signals in the SNO detector in the case of the MSW νe → νμ(τ) and/or νe → νs transition solutions of the solar neutrino problem are presented. The asymmetries in the charged current (CC) and ν − e− elastic scattering (ES) event rates are calculated for both MSW solutions; in the case of the νe → νs transition solution the D-N asymmetry in the neutral current (NC) event rate are derived as well. The asymmetries are calculated for three night samples of events which are produced by the solar neutrinos crossing i) the Earth mantle only (Mantle), ii) the Earth core (Core) and iii) the Earth core and/or the mantle (Night). The effects of the uncertainties i) in the values of the cross-sections of the CC and NC neutrino-induced reactions on deuterium, and ii) in the value of the bulk matter density and/or the chemical composition of the Earth core, on the corresponding D-N asymmetry predictions are analyzed. It is shown, in particular, that that due to the strong enhancement of the transitions of the solar neutrinos crossing the Earth core, at sin 2θV ≤ 0.01 the corresponding one year average D-N asymmetry in the Core sample of CC events in the case of the νe → νμ(τ) solution can be larger by a factor of up to ∼ 8 than the asymmetry in the Night sample. In certain subregions of the MSW solution regions at small sin 2θV , the predicted magnitude of the Core D-N asymmetry in the CC sample is very sensitive to the value of the electron fraction number in the Earth core. Iso (D-N) asymmetry contours in the ∆m − sin 2θV plane for the SNO detector are derived in the region sin 2θV > ∼10−4 for the Core and Night samples of the CC, ES and NC ∗Also at: Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, BG–1784