Impact of two mass scale oscillations on the analysis of atmospheric and reactor neutrino data

An analysis of atmospheric and reactor neutrino data is presented in terms of three--neutrino oscillations where the effect of both mass differences is explicitly considered. We study the allowed parameter space resulting from this analysis as a function of the {\it mass splitting hierarchy parameter} $\alpha=\Delta m^2/\Delta M^2$ which parametrizes the departure from the one--dominant mass scale approximation. We consider schemes with both direct and inverted mass ordering. Our results show that the analysis of the atmospheric neutrino data does not provide any significant restriction on $\alpha$ but even for large deviations of the one--dominant mass scale approximation the derived range of the largest mass splitting, $\Delta M^2$, is stable while the allowed ranges of mixing angles $\sin^2\theta_{23}$ and $\sin^2\theta_{13}$ are wider than those obtained in the one--dominant mass scale approximation. Inclusion of the CHOOZ reactor data in the analysis favours small values of $\alpha$. This results into the reduction of the parameter space in particular for the mixing angles. As a consequence the final allowed ranges of parameters from the combined analysis are only slightly broader than when obtained in the one--dominant mass scale approximation.