Can a mass inversion save solar neutrino oscillations from the Los Alamos neutrino?

Abstract

In the light of the $\nu_\mu\to\nu_e$ neutrino oscillations which may have been observed at the LSND experiment we explore the consequences of two inverted mass schemes where solar neutrino oscillations occur between $\nu_e$ and $\nu_\tau$. The favored LSND value $\Delta m^2=6\,\eV^2$ leads to $m_{\nu_e}\approx m_{\nu_\tau}\approx 2.5\,\eV$ and $m_{\nu_\mu}\approx0$ so that cosmology can benefit from a recently proposed ``cold plus hot dark matter'' structure formation scenario with two equal mass light neutrinos (C$\nu^2$DM). Solar neutrino oscillations ($\nu_e\to\nu_\tau$) can occur with one of the large mixing angle solutions so that a serious conflict with $\beta\beta$ decay Majorana mass limits is avoided without invoking Dirac masses. However, there is a problem with the SN~1987A signal because of resonant $\anti\nu_e\leftrightarrow\anti\nu_\mu$ oscillations which are expected to cause far higher $\anti\nu_e$ energies at the IMB and Kamiokande~II detectors than have been observed. A small value $\Delta m^2=0.5\,\eV^2$ at LSND, which allows for a relatively large $\nu_e$-$\nu_\mu$ mixing angle without conflicting with the KARMEN and BNL-E776 experiments, would indicate $m_{\nu_e}\approx m_{\nu_\tau}\approx 1.62\,\eV$ and $m_{\nu_\mu}\approx1.77\,\eV$. This scheme of C$\nu^3$DM maintains, and even may improve, the essential cosmological model implications for large-scale structure, leaving no conflict with SN r-process nucleosynthesis. It may improve the discordance between the SN~1987A neutrino spectra inferred from Kamiokande~II and IMB.