Proc. Neutrino Astrophysics - MPP Theory Group

calibration is estimated to be ±2.4% based on the detailed analysis of cosmic-ray muons,
muon-decay electrons and electrons from a linac. The estimated momentum resolutions for
electrons and muons are 2.5%/ � E(GeV) + 0.5% and +3%, respectively.
Results
The data reduction, single-ring selection and lepton identification have been done by 2 groups
independently and the results were found to agree very well. Three simulated samples have
been compared with the data. The simulations used 2 different flux calculations, 3 different
neutrino interaction models and 2 codes for particle and light propagation in the detector.
All the simulated samples were processed by the same procedures as the data. The differences
were used in estimates of systematic errors. The results discussed below come from one data
analysis and Monte Carlo simulations based on flux calculations of Ref. [1].
Out of about 300 million triggers recorded during 320 days of detector operation between
May 1996 and June 1997, about 5000 events were classified as fully-contained events, i.e.
events with no significant energy deposit outside of the inner detector. Using the photon
time-arrival information recorded by PMTs, 2708 events with energy Evis > 30 MeV had
vertices reconstructed in the fiducial volume. The visible energy Evis is defined as the energy
of an electron which would produce the same number of Cherenkov photons.
From Monte Carlo simulation it is estimated that 83.2% of all charged-current neutrino
interactions in the fiducial volume are retained in the current sample. The remaining
fraction of 16.8% consists mostly of events with exiting particles (9.3%) or energy lower
than 30 MeV (5.8%).
To study the neutrino flavor composition we select a sample of single-ring events. Quasielastic
interactions, for which the information about neutrino flavor is provided by the charged
lepton identification, consist 75 ± 3% of the sample. Particle identification is based on the
pattern of Čerenkov rings. Diffuse, showering patterns associated with electromagnetic cas-
cades (e-like events), are separated from sharper, non-showering rings caused by stopping
muons (µ-like events). The misidentification probability for single-ring events is estimated
to be 0.8 ± 0.1%.
The results for events with Evis < 1.33 GeV (Sub-GeV sample) are presented in Table 1.
For single-track events the electron and muon momenta are greater than 100 and 200 MeV/c,
respectively. From these data one obtains: R = 0.63 ± 0.03(stat) ± 0.05(sys). The systematic
error of 8% comes mostly form uncertainty in the calculation of the νµ/νe ratio (5%), uncertainties
in interaction cross sections and nuclear effects in 16 O (4.1%), single-ring selection
(4%) and particle identification (2%).
An independent signature of muon neutrino interactions is provided by electrons from
muon decays. The detection efficiency for muon decay is estimated to be 0.80 for µ + and
0.63 for µ + . The fraction of µ-like events with a decay electron is 67.5 ± 1.7% for the data
and 68.2 ± 1.0% for MC. For e-like events the corresponding fractions are 9.3 ± 1.1% and
8.0 ± 0.3%.
Angular distributions of the reconstructed track directions for single-ring events have also
been studied. Figure 1 displays the distributions of zenith angles for µ-like and e-like events.
The double ratio R dependence on the zenith angle is shown in Figure 2.
Preliminary results of the analysis of the Multi-GeV sample of events with Evis > 1.33
GeV is shown in Table 2. The resulting double ratio is R = 0.60 ± 0.06(stat) ± 0.07(sys).
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