Stars as Laboratories for Fundamental Physics - MPP Theory Group

454 Chapter 12
Fig. 12.2. Spectrum of reactor ν e ’s per fission of 239 Pu and 235 U (von Feilitzsch
et al. 1982; Schreckenbach et al. 1985). The width of the lines gives
the total error of the spectra.
able bound was inferred by Vogel (1984) from data taken at the Gösgen
reactor (Switzerland). The most recent analysis is, again, based on
data taken with a scintillation counter at Gösgen (Oberauer, von Feilitzsch,
and Mössbauer 1987). From a comparison of the “reactor on”
with the “reactor off” photon counts for several energy channels in the
MeV range and using the experimentally established neutrino spectrum
(Fig. 12.2), these authors found the 68% CL lower limits on the ν e radiative
decay times of τ γ /m νe > 22 s/eV for α = −1, 38 s/eV for α = 0,
and 59 s/eV for α = +1. It was assumed that ν ′ is massless so that
δ m = 1 in Eq. (12.5). With Eq. (7.12) the α = −1 constraint translates
into a bound on the effective electromagnetic transition moment of
µ eff < 0.092 µ B m −2
eV , (12.6)
not a very restrictive limit even if ν e saturates its upper mass bound of
about 5 eV.
Even this weak limit would cease to apply if ν e and ν ′ became
nearly degenerate. Therefore, Bouchez et al. (1988) performed an experiment
where they searched for optical decay photons at the Bugey
reactor (France). They excluded a certain region in the parameter
plane spanned by τ γ /m νe and δ m . Their greatest sensitivity was approximately
at δ m = 2×10 −5 where they found τ γ /m > νe ∼ 0.04 s/eV.
With Eq. (7.12) this is µ < eff ∼ 10 7 µ B /m 2 eV which, unfortunately, is irrelevant
as a constraint. Hence, for nearly degenerate neutrinos there