Stars as Laboratories for Fundamental Physics - MPP Theory Group

290 Chapter 8
the analysis was always based on the assumption that two-flavor oscillations
dominate. The disappearance experiments, of course, also
constrain oscillations into hypothetical sterile neutrinos.
Even though the experimental results look very impressive, a glance
on the CKM matrix Eq. (7.6) reveals that one could not yet have expected
to see oscillations in the ν e ↔ ν τ or ν µ ↔ ν τ channel if the
neutrino mixing angles are comparably small. It is very encouraging
that the NOMAD and CHORUS experiments which are currently taking
data at CERN (DiLella 1993; Winter 1995) anticipate a range of
sensitivity (curve g in Fig. 8.5) which is promising both in view of the
possible cosmological role of a m ν in the 10 eV range and the small
mixing angles probed. Other future but less advanced projects for terrestrial
oscillation searches were reviewed by Schneps (1993, 1995).
At the time of this writing the LSND Collaboration has reported a
signature that is consistent with the occurrence of ν µ → ν e oscillations
(Athanassopoulos et al. 1995). If this interpretation is correct, the
corresponding ∆m 2 would exceed about 1 eV 2 , while sin 2 2θ would be a
few 10 −3 . The status of this claim is controversial at the present time—
see, e.g. Hill (1995). No doubt more data need to be taken before one
can seriously begin to believe that neutrino oscillations have indeed
been observed.
8.2.5 Atmospheric Neutrinos
Besides reactors and accelerators, one may also use atmospheric neutrinos
as a source to search for oscillations. Primary cosmic ray protons
produce hadronic showers when interacting with atmospheric nuclei
(A). Neutrinos are subsequently produced according to the simple
scheme
p + A → n + π/K + . . .
π/K → µ + (µ − ) + ν µ (ν µ )
µ + (µ − ) → e + (e − ) + ν e (ν e ) + ν µ (ν µ ).
(8.25)
Therefore, one expects twice as many ν µ ’s as ν e ’s, and equally many
neutrinos as antineutrinos of both flavors. At a detector, the neutrino
flux is approximately isotropic except at energies below about
1 GeV where geomagnetic effects become important. Because the neutrinos
come from anywhere in the atmosphere, from directly overhead
or from as far as the antipodes, oscillation lengths between about 10