Probing neutrino nonstandard interactions with atmospheric neutrino data

We reconsider the atmospheric neutrino anomaly in light of the latest data from Super-Kamiokande contained events and from Super-Kamiokande and MACRO upgoing muons. We reanalyze the proposed solution to the atmospheric neutrino anomaly in terms of nonstandard neutrino-matter interactions (NSI’s) as well as the standard ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations (OSC’s). Our statistical analysis shows that a pure NSI mechanism is now ruled out at 99%, while the standard ${\nu }_{\mu }\to {\nu }_{\tau }$ OSC mechanism provides a quite remarkably good description of the anomaly. We therefore study an extended mechanism of neutrino propagation which combines both oscillation and nonstandard neutrino-matter interactions, in order to derive limits on flavor-changing and nonuniversal neutrino interactions. We obtain that the off-diagonal flavor-changing neutrino parameter $\varepsilon$ and the diagonal nonuniversality neutrino parameter ${\varepsilon }^{\prime }$ are confined to $-0.05<\mathrm{}\varepsilon <0.04\mathrm{}$ and $|{\varepsilon }^{\prime }|<0.17\mathrm{}$ at 99% C.L. These limits are model independent and they are obtained from pure neutrino-physics processes. The stability of the neutrino oscillation solution to the atmospheric neutrino anomaly against the presence of nonstandard neutrino interactions establishes the robustness of the near-maximal atmospheric mixing and massive-neutrino hypothesis. The best agreement with the data is obtained for $\Delta m^2=2.4\mathrm{}\times {10}^{-3}\hspace{0.5em}{\mathrm{eV}}^2,$ ${\sin }^2\left(2\mathrm{}\theta \right)=0.99,$ $\varepsilon =-9.1\times {10}^{-3},$ and ${\varepsilon }^{\prime }=-1.9\times {10}^{-3},$ although the ${\chi }^2$ function is quite flat in the $\varepsilon$ and ${\varepsilon }^{\prime }$ directions for $\varepsilon ,{\varepsilon }^{\prime }\to 0.$