Oscillation Analysis from Spectrum and Zenith Angle Variation

Neutrino oscillations may cause three effects in Super-Kamiokande: a distortion of the energy spectrum, a zenith-angle dependend flux (day-night effect) and a seasonal dependence of the flux (seasonal variation). Super-Kamiokande has looked for distortions of the spectrum and zenith-angle variations of the flux.

The largest sensitivity to MSW oscillation effects is achieved by combining the zenith-angle flux variation with the spectral distortion. Super-Kamiokande realizes this combination by fitting oscillation predictions to seven spectra day and a night spectrum.

Definition of Zenith Angles:
The matter density inside the earth shows a sharp increase at about half the earth's radius (the core of the earth):

Unfortunately, Super-K's geographical location prevents collecting solar neutrinos that pass through the earth's inner core. We divide the data into 'Day' (neutrinos do not pass through the earth), 'Mantle' (neutrinos pass through the earth, but not through the core) and 'Core' (neutrinos pass through the core) according to the zenith angle of the sun. The Mantle data set is subdivided into 5 regions as shown above (Man 1-5). Between 5.5 and 16 MeV, the data in each energy bin can be subdivided in zenith angle bins:



Above 16 MeV and below 5.5 MeV, the data cannot be divided in zenith angle bins, since the statistical precision is too low. So these energy bins are integrated in zenith angle. The top panel of the left plot shows an 8 bin energy spectrum, the lower panels display the zenith angle variations of the six energy bins between 5.5 and 16 MeV. Superimposed in red is the +-1 sigma correlated error shape due to 8B spectrum uncertainty and energy scale and resolution systematic uncertainty. On the right hand side are the results of a chi^2 analysis of this `zenith angle spectrum'. The excluded red area results from the shape of the zenith angle spectrum alone, the allowed blue area arises from constraining the total rate to the standard solar mode. The green allowed area is a fit to only the rates of Gallex/GNO/SAGE, Homestake, Super-Kamiokande and SNO. The regions inside the dotted lines are allowed at 95% using the zenith angle spectrum and constraining the implied neutrino-deuteron charged-current interaction rate to the measurement by SNO. These dotted contours are INDEPENDENT of SSM neutrino flux predictions!

Global Analysis

Starting with the SK-SNO combined fit described above, we can add the rate information from the radio-chemical experiments. To minimize the dependence on SSM neutrino flux predictions, we do not constrain either 8B or Hep neutrino flux in this analysis. We show contours at the 1 sigma (68%), 2 sigma (95%) and 3 sigma (99.73%) confidence level. Solutions other than the LMA only appear at the three sigma level.

Published Results using 1258 Days of Data



Exclusion and allowed areas for two-flavour oscillation:


The red area is excluded at 95% C.L. where just the zenith angle spectrum is used. The blue area is allowed at 95% C.L. using all Super-Kamiokande information: Super-Kamiokande flux and zenith angle spectrum. The green areas are globally allowed at 95% C.L. using the Super-Kamiokande flux, the flux of the Gallium experiments and the flux of the Chlorine experiment.