Atmospheric neutrino anomaly without maximal mixing? art. no. 033001

We consider a pattern of neutrino masses in which there is an approximate m ass degeneracy between the two mass eigenstates most coupled to the v(mu) a nd v(tau) flavor eigenstates. Earth-matter effects can lift this degeneracy and induce an effectively maximal mixing between these two generations. Th is occurs if v(e)'s contain comparable admixtures of the degenerate eigenst ates, even if they are not large. This provides an explanation of the atmos pheric neutrino anomaly in which the ab initio introduction of a large mixi ng angle is not required. To test this possibility we perform a detailed an alysis of the 52 kiloton-year SuperKamiokande data, and we find that in a l arge region of parameter space the corresponding confidence levels are exce llent. The most recent results from the CHOOZ reactor experiment, however, severely curtail this region, so that the conventional scenario with nearly maximal mixing angles, which we also analyze in detail, is supported by th e data.