A New Type of Resonant Neutrino Conversions Induced by Magnetic Fields

We consider resonant neutrino conversions in magnetised matter, such as a degenerate electron gas. We show how magnetisation effects caused by axial vector interactions of neutrinos with the charged leptons in the medium can induce a new type of resonant neutrino conversion which may occur even in situations where the MSW effect does not occur, such as the case of degenerate or inverted neutrino mass spectra. Our new resonance may simultaneously affect anti-neutrino $\bar{\nu_a} \leftrightarrow \bar{\nu}_b$ as well as neutrino $\nu_{a} \leftrightarrow \nu_b$ flavour conversions, and therefore it may substantially affect supernova neutrino energy spectra. Using SN1987A data we conclude that only laboratory experiments with long baseline such as ICARUS or MINOS are likely to find neutrino oscillations due to their sensitivity to small $\Delta m^2$. We also comment on the possibility of resonant conversions induced by Majorana neutrino transition moments and mention the case of sterile neutrinos $\nu_s$.