Proc. Neutrino Astrophysics - MPP Theory Group

116
Atmospheric Neutrinos in Super-Kamiokande
Danuta Kie̷lczewska 1,2 (for the Super-Kamiokande Collaboration)
1 The University of California, Irvine, California 92717, USA
2 Warsaw University, Warsaw, Poland
Abstract
The measurements of atmospheric neutrino interactions during 326 days of the Super-Kamiokande
detector operation are reported. The measured ratio of muon and electron neutrinos
has been found smaller than expected from theoretical models. The angular distribution of
muon neutrinos is also inconsistent with expectations.
Introduction
Atmospheric neutrinos are produced in a layer of about 15 km as a result of hadronic cascades
originating from interactions of cosmic rays. A number of models have been developed to
calculate the fluxes of atmospheric neutrinos [1, 2] at energies below 5 GeV. The absolute
neutrino fluxes are predicted with uncertainties of 20%. However the ratio of νµ + ¯νµ to
νe + ¯νe fluxes is known to better than 5% and therefore experiments measure the double ratio
R ≡ (µ/e)DATA/(µ/e)MC, where µ/e denote the ratio of µ-like to e-like neutrino interactions
correspondingly in the data and simulated event samples.
The flavor composition of atmospheric neutrinos was studied in the earlier underground
experiments. The Kamiokande [3] and IMB [4] experiments, using water Cherenkov detectors,
and the Soudan [5] experiment, using iron calorimeters, have reported the double ratio to be
smaller than one. A dependence of R on the zenith angle, and hence on the neutrino path
length, was also observed in Kamiokande [6]. Those results have been often interpreted by
neutrino oscillations.
Here the preliminary results of measurements of atmospheric neutrino fluxes are presented
for 20.1 kton-year exposure of the Super-Kamiokande detector.
Super-Kamiokande Detector
Super-Kamiokande is a 50 kton water detector located in the mine near Kamioka town in
Japan. It is situated at a mean overburden of 1000 meters below the peak of Mt. Ikeno.
Cherenkov photons emitted by relativistic particles inside the inner cylindrical volume of
water, 16.9 m in radius and 36.2 m high, are recorded by 11146 photomultiplier tubes (PMTs)
of 50 cm diameter (40% photocathode coverage). Each inner PMT signal provides the first
photon arrival-time in 1.2 µs range at 0.3 ns resolution and the collected charge at 0.2 pC
resolution (equivalent to 0.1 p.e.). The outer layer of water, about 2.7 m thick, is instrumented
with 1885 outward facing PMTs of 20 cm diameter with wavelength shifting plates. The two
detector regions are optically separated. The outer detector tags events with incoming or
exiting particles.
Water transparency is measured using a dye laser and is about 100 m at wavelenth of
420 nm. The neutrino interactions were collected inside the fiducial volume 2 m from the
inner detector walls, comprising 22.5 ktons of water. The accuracy of the absolute energy