  Quasi Elastic Scattering (QEL) is the dominant mechanism for producing both
Fully Contained Events and Partially Contained Events in the SuperKamiokande
(SK) detector for atmospheric neutrinos in the range 0.1GeV to 10GeV. In the
analysis of SK events, it is assumed that the zenith angle of the incident
neutrino is the same as that of the detected charged lepton. In the present
paper, we derive the distribution function for the scattering angle of charged
leptons and derive the average scattering angles as well as their standard
deviation due to QEL. Thus, it is shown that the SK assumption for the
scattering angle of the charged lepton in QEL is not valid. Further, for a real
analysis of the experimental data, we should examine the correlations between
the zenith angle of the charged leptons and those of their incident neutrinos,
taking into account the effect of the azimuthal angle of the charged particle
over its zenith angle. We conclude that it is not possible to reliably assign
the zenith angle of the incident neutrino to that of the charged lepton, and so
the zenith angle distribution of charged leptons does not reflect that of the
incident neutrinos under the SK assumption. This result has clear implication
for attempts to detect neutrino oscillations from an analysis of Fully
Contained Events and Partially Contained Events in SuperKamiokande.
