Atmospheric neutrinos as probes of neutrino-matter interactions

Neutrino oscillation experiments provide a unique tool for probing neutrino-matter interactions, especially those involving the tau neutrino. We describe the sensitivity of the present atmospheric neutrino data to these interactions in the framework of a three-flavor analysis. Compared to the two-flavor ${\nu }_{\mu }-{\nu }_{\tau }$ analyses, we find qualitatively new features, in particular, that large nonstandard interactions, comparable in strength to those in the Standard Model, can be consistent with the data. The existence of such interactions could imply a smaller value of the neutrino mixing angle and larger value of the mass-squared splitting than in the case of standard interactions only. This and other effects of nonstandard interactions may be tested in the next several years by MINOS, KamLAND, and solar neutrino experiments.