New observations with atmospheric neutrinos from the underground experiments
SuperKamiokande, Soudan 2, and MACRO, together with earlier results from
Kamiokande and IMB, are reviewed. The most recent observations reconfirm
aspects of atmospheric flavor content and of zenith angle distributions which
appear anomalous in the context of null oscillations. The anomalous trends,
exhibited with high statistics in both sub-GeV and multi-GeV data of the
SuperKamiokande water Cherenkov experiment, occur also in event samples
recorded by the tracking calorimeters. The data are well-described by
disappearence of nu_mu flavor neutrinos arising in oscillations with dominant
two-state mixing, for which there exists a parameter region allowed by all
experiments. In a new analysis by SuperKamiokande, nu_mu -> nu_tau is favored
over nu_mu -> nu_s as the dominant oscillation based upon absence of
oscillation suppression from matter effects at high energies. The possibility
for sub-dominant nu_mu -> nu_e oscillations in atmospheric neutrinos which
arises with three-flavor mixing, is reviewed, and intriguing possibilities for
amplification of this oscillation by terrestrial matter-induced resonances are
discussed. Developments and future measurements which will enhance our
knowledge of the atmospheric neutrino fluxes are briefly noted.Comment: Plenary talk at the XIX Int. Symposium on Lepton and Photon
Interactions at High Energies, Stanford, Aug. 1999, 28 pages, 16 figures;
added a reference for section 1