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)-->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)-->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 epsilon and the diagonal nonuniversality neutrino parameter epsilon' are confined to -0.05 < epsilon < 0.04 and \ epsilon' \ < 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 Delta m(2) =2.4 x 10(-3) eV(2), sin(2)(2 theta) = 0.99, epsilon = -9.1 x 10(-3), and epsilon' = -1.9 x 10(-3), although the chi(2) function is quite flat in the epsilon and epsilon' directions for epsilon,epsilon'-->0.