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 ν_μ→ν_τ oscillations (OSC's). Our statistical analysis shows that a pure NSI mechanism is now ruled out at 99%, while the standard ν_μ→ν_τ 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 ε and the diagonal nonuniversality neutrino parameter ε^' are confined to -0.05<ε<0.04 and |ε^'|<0.17 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 Δm²=2.4×10^-3 eV², sin²(2θ)=0.99, ε=-9.1×10^-3, and ε^'=-1.9×10^-3, although the χ² function is quite flat in the ε and ε^' directions for ε,ε^'→0.