Solar neutrino event spectra: Tuning SNO to equalize Super-Kamiokande

The Super-Kamiokande (SK) and the Sudbury Neutrino Observatory (SNO) experiments are monitoring the flux of B-8 solar neutrinos through the elect;on energy spectrum from the reactions nu (e,mu,tau)+ e(-)--> nu (e,mu,tau)+e(-) and nu (e)+d-->p+p+e(-), respectively. We show that the SK detector response to B-8 neutrinos in each bin of the electron energy spectrum (above 8 MeV) can be approximated, with good accuracy, by the SNO detector response in an appropriate electron energy range (above 5.1 MeV). For instance, the SK response in the bin [10, 10.5] MeV is reproduced ("equalized") within similar to2% by the SNO response in the range [7.1, 11.75] MeV. As a consequence, in the presence of active neutrino oscillations, the SK and SNO event rates in the corresponding energy ranges turn out to be linearly related, for any functional form of the oscillation probability. Such equalization is not spoiled by the possible contribution of hep neutrinos (within current phenomenological limits). In perspective, when the SK and the SNO spectra will both be measured with high accuracy, the SK-SNO equalization can be used to determine the absolute B-8 neutrino flux, and to cross-check the (non)observation of spectral deviations in SK and SNO. At present, as an exercise, we use the equalization to "predict" the SNO energy spectrum, on the basis of the current SK data. Finally, we briefly discuss some modifications or limitations of our results in the case of sterile nu oscillations and of relatively large Earth matter effects.