Stars as Laboratories for Fundamental Physics - MPP Theory Group

276 Chapter 7
The best limits on µ νe are based on the use of reactor neutrinos
as a source. The measurement of the ν e -e scattering cross section by
Reines, Gurr, and Sobel (1976) was interpreted by Kyuldjiev (1984)
to yield a bound of µ νe < 1.5×10 −10 µ B . Since then, the reactor ν e
spectrum has been much better understood. Vogel and Engel (1989)
stressed that a literal interpretation of the old results by Reines, Gurr,
and Sobel (1976) would actually yield evidence for a dipole moment of
about 2−4×10 −10 µ B . There is, however, a more recent limit of
µ νe < 2.4×10 −10 µ B (7.28)
from the Kurchatov Institute (Vidyakin et al. 1992).
When interpreting this bound as a limit on the ν e -ν τ transition moment,
recall that for an experimentally allowed ν τ mass in the 10 MeV
regime this limit would not apply as reactor neutrinos have relatively
low energies (below about 10 MeV). Therefore, it remains useful to consider
the bounds from the neutrino beam at LAMPF with a ν e endpoint
energy of 52.8 MeV
µ νe < 10.8×10 −10 µ B ,
µ νµ < 7.4×10 −10 µ B (7.29)
(Krakauer et al. 1990). A similar limit of µ νµ < 8.5×10 −10 µ B was
obtained by Ahrens et al. (1990).
An improvement of the bound on µ νe by an order of magnitude
or more can be expected from a new reactor experiment currently in
preparation (“MUNU experiment,” Broggini et al. 1990). However,
results will not become available before a few years from now.
The transition moments between ν τ and other sequential neutrinos
are bounded by the above experiments involving initial-state ν e ’s or
ν µ ’s or their antiparticles. The diagonal ν τ magnetic dipole moment,
however, is much less constrained because no strong ν τ sources are available
in the laboratory; the ν τ -e cross section has never been measured.
However, the calculated ν τ flux produced in a proton beam dump from
the decay of D s mesons can be used to derive an upper limit on the
ν τ -e cross section which can be translated into a bound
µ ντ < 5.4×10 −7 µ B , (7.30)
(Cooper-Sarkar et al. 1992). Of course, in this range the ν τ electromagnetic
cross section would far exceed a typical weak interaction one.