The Dark Side of the Solar Neutrino Parameter Space

Results of neutrino oscillation experiments have always been presented on the(sin^2 2theta, Delta m^2) parameter space for the case of two-flavoroscillations. We point out, however, that this parameterization misses the halfof the parameter space pi/4 < theta <= pi/2 (``the dark side''), which isphysically inequivalent to the region 0 <= theta <= pi/4 (``the light side'')in the presence of matter effects. The MSW solutions to the solar neutrinoproblem can extend to the dark side, especially if we take the conservativeattitude to allow higher confidence levels, ignore some of the experimentalresults in the fits, or relax theoretical predictions. Furthermore even theso-called ``vacuum oscillation'' solution distinguishes the dark and the lightsides. We urge experimental collaborations to present their results on theentire parameter space.