The dark side of the solar neutrino parameter space

Results of neutrino oscillation experiments have always been presented on the (sin(2)2 theta, Delta m(2)) parameter space for the case of two-flavor oscillations. We point out, however, that this parameterization misses the half of the parameter space pi/4 < theta less than or equal to pi/2 ('the dark side'), which is physically inequivalent to the region 0 less than or equal to theta less than or equal to pi/4 ('the light side') in the presence of matter effects. The MSW solutions to the solar neutrino problem can extend to the dark side, especially if we take the conservative attitude to allow higher confidence levels, ignore some of the experimental results in the fits, or relax theoretical predictions. Furthermore, even the so-called 'vacuum oscillation' solution distinguishes the dark and the light sides. We urge experimental collaborations to present their results on the entire parameter space. (C) 2000 Elsevier Science B.V. All rights reserved.