Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 389
10.8 Neutrino Decay
A deficit of solar neutrinos measured at Earth can be related to neutrino
decay. However, the in-flight decay of neutrinos does not provide the
required deformation of the spectrum because the decay rate in the
laboratory system involves a Lorentz factor m ν /E ν so that low-energy
neutrinos decay faster. If ν e is a mixture of mass eigenstates only the
heavier one decays. It is possible that the heavy admixture decays fast
so that the spectrum is reduced by a constant factor. Even this extreme
case does not provide a good fit to the data. A detailed analysis of
different cases was performed by Acker and Pakvasa (1994) who found
that the in-flight decay solution was ruled out at the 98% CL, even
when allowing for the solar model uncertainties. 59
Neutrino decays can yield a solar ν e flux which is, in principle,
measurable. Such a flux can be produced if neutrinos are Majorana
particles, and if they couple to majorons χ (Sect. 15.7). Some fraction
of ν → ν ′ + χ decays flip the helicity of the neutrino so that the ν ′ is
effectively a ν ′ . Thus after an MSW conversion ν e → ν µ,τ one could
have decays ν µ,τ → ν e + χ (Raghavan, He, and Pakvasa 1988). Even
without oscillations one can have matter-induced decays of the form
ν e → ν e + χ as discussed in Sect. 6.8. Detailed predictions for the
ν e flux for this type of scenario were worked out by Berezhiani et al.
(1992) and by Berezhiani, Moretti, and Rossi (1993). The Kamiokande
detector has already produced limits on solar ν e ’s (Fig. 10.14), with
much better limits to be expected from Superkamiokande. However,
in view of other limits on the neutrino-majoron coupling Berezhiani,
Moretti, and Rossi (1993) found that it seemed unrealistic to hope for
a detectable solar ν e signal.
Malaney, Starkman, and Butler (1994) showed that in decays of
the form ν → ν ′ + boson, final-state stimulation effects (“neutrino
lasing”) could enhance the decay rate. However, the best-motivated
case is that of majoron decays which involve a γ 5 coupling. The shape
of the resulting majoron spectrum is such that the crucial emission of
low-momentum bosons and thus the lasing effect is suppressed whence
Acker and Pakvasa’s conclusions remain valid. For models involving
scalar or vector bosons a detailed new analysis is required.
At the present time it looks rather unconvincing that the solar neutrino
problem is related to some form of neutrino decays.
59 See Acker and Pakvasa (1994) for references to earlier discussions of neutrino
decay as a potential solution to the solar neutrino problem.