Solutions to the Atmospheric Neutrino Problem

Neutrinos produced as decay products in hadronic showers from cosmic ray collisions with nuclei in the upper atmosphere have been observed by several detectors [1–7]. Although the absolute fluxes of atmospheric neutrinos are largely uncertain, the expected ratio (μ/e) of the muon neutrino flux (νμ + ν̄μ) over the electron neutrino flux (νe + ν̄e) is robust, since it largely cancels out the uncertainties associated with the absolute flux. In fact, this ratio has been calculated [8] with an uncertainty of less than 5% over energies varying from 0.1 GeV to 100 GeV. On this resides our confidence in the long-standing atmospheric neutrino anomaly. Super-Kamiokande high statistics observations [1] indicate that the deficit in the total ratio R(μ/e) is due to the number of neutrinos arriving in the detector at large zenith angles. The e-like events do not present any compelling evidence of a zenith-angle dependent suppression while the μ-like event rates are substantially suppressed at large zenith angles. The νμ → ντ as well as the νμ → νs [9,10] oscillation hypothesis provides a very good explanation for this smaller-than-expected ratio, which is also simple and well-motivated theoretically. This led the Super-Kamiokande Collaboration to conclude