Neutral-current atmospheric neutrino flux measurement using neutrino-proton elastic scattering in Super-Kamiokande

Recent results show that atmospheric νμ oscillate with δm2≅3x10-3 eV2 and sin22θatm≅1, and that conversion into νe is strongly disfavored. The Super-Kamiokande (SK) Collaboration, using a combination of three techniques, reports that their data favor νμ→ντ over νμ→νsterile. This distinction is extremely important for both four-neutrino models and cosmology. We propose that neutrino-proton elastic scattering (ν+p→ν+p) in water Cerenkov detectors can also distinguish between active and sterile oscillations. This was not previously recognized as a useful channel since only about 2% of struck protons are above the Cerenkov threshold. Nevertheless, in the present SK data there should be about 40 identifiable events. We show that these events have unique particle identification characteristics, point in the direction of the incoming neutrinos, and correspond to a narrow range of neutrino energies (1-3 GeV, oscillating near the horizon). This channel will be particularly important in Hyper-Kamiokande, with ∼40 times higher rate. Our results have other important applications. First, for a similarly small fraction of atmospheric neutrino quasielastic events, the proton is relativistic. This uniquely selects νμ (not ν(bar sign)μ) events, useful for understanding matter effects, and allows determination of the neutrino energy and direction, useful for the L/E dependence of oscillations. Second, using accelerator neutrinos, both elastic and quasielastic events with relativistic protons can be seen in the K2K 1-kton near detector and MiniBooNE