3-FLAVOR ATMOSPHERIC NEUTRINO ANOMALY

We investigate the indications of flavor oscillations that come from t he anomalous flavor composition of the atmospheric neutrino flux obser ved in some underground experiments. We study the information coming f rom the neutrino-induced mu-like and e-like events both in the sub-GeV energy range (Kamiokande, IMB, Frejus, and NUSEX experiments) and in the multi-GeV energy range (Kamiokande experiment). First we analyze a ll the data in the limits of pure nu(<mu)><->nu(tau) and nu(mu)<->nu(e ) oscillations. We obtain that nu mu<->v nu(e) oscillations provide a better fit, in particular, to the multi-GeV ata. Then we perform a thr ee-flavor analysis in the hypothesis of dominance of one neutrino squa re mass difference m(2) implying that the neutrino mixing is parametri zed by two angles (psi,phi)is an element of[0,pi/2]. We explore the sp ace (m(2),psi,phi) exhaustively, and find the regions favored by the o scillation hypothesis. The results are displayed in a form suited to t he comparison with other flavor oscillation searches at accelerator, r eactor, and solar v experiments. In the analysis, we pay particular at tention to the Earth matter effects, to the correlation of the uncerta inties, and to the symmetry properties of the oscillation probability.