Stars as Laboratories for Fundamental Physics - MPP Theory Group

292 Chapter 8
ing the detector. (Electrons from ν e interactions range out much faster
in the rock and so one expects mostly muons from ν µ ’s.) This method is
sensitive to the high-energy spectral regime of the atmospheric ν µ flux.
Moreover, one may select upward going muons which are produced from
ν µ ’s which traversed the entire Earth and thus have a large oscillation
length available. The IMB detector excludes range c by this method
(Becker-Szendy et al. 1992). Also, one may determine the fraction of
muons stopped within the detector to those which exit, allowing one to
constrain a spectral deformation caused by the energy dependence of
the oscillation length. Range d is excluded by this method according
to the IMB detector (Becker-Szendy et al. 1992).
However, several detectors see a substantial deficit of atmospheric
ν µ ’s relative to ν e ’s, a finding usually expressed in terms of a “ratio of
ratios,” i.e. the measured over the expected ratio of e-like over µ-like
events (Fig. 8.7). While this procedure is justified because it is largely
free of the uncertain absolute flux normalization, one must be careful
at interpreting the significance of the flux deficit. The error of a measured
ratio does not follow a Gaussian distribution; a representation
like Fig. 8.7 tends to overemphasize the significance of the discrepancy
(Fogli and Lisi 1995).
Fig. 8.7. Measured ratio of the atmospheric ν µ /ν e fluxes relative to the
expected value (“ratio of ratios”) in five detectors. Where two results are
shown they refer to different signatures or data samples. (See Goodman
1995 for references.)