Proc. Neutrino Astrophysics - MPP Theory Group

to neutrino masses that vanish as the lepton-number scale goes to zero, in contrast to the seesaw
model. Such low-scale models are very attractive and lead to a richer phenomenology,
as the extra particles required have masses at scales that could be accessible to present
experiments. One remarkable example is the possibility of invisibly decaying Higgs bosons [5].
The large diversity of neutrino mass schemes and the lack of a theory for the Yukawa couplings
imply that present theory is not capable of predicting the scale of neutrino masses any
better than it can fix the masses of the other fermions, such as the muon: one should turn to
observations as means for constraining their properties. Direct laboratory observations leave
a lot of room for massive neutrinos, except beta decay experiments which are more restrictive.
A complementary approach to the problem of neutrino mass tries to get information from astrophysics
and cosmology. It constitutes a promising interdisciplinary field, now often-called
astroparticle physics. In this talk I will briefly mention recent work devoted to the constraints
that one can place on neutrino properties from cosmological and astrophysical observations.
Limits from Cosmology
The first cosmological bound on neutrino masses follows from avoiding the overabundance of
relic neutrinos [8] � mνi < ∼ 92 Ωνh 2 eV , (1)
where Ωνh 2 ≤ 1 and the sum runs over all stable species of isodoublet neutrinos with mass
less than O(1 MeV). Here Ων = ρν/ρc, where ρν is the neutrino contribution to the total
density and ρc is the critical density. The factor h 2 measures the uncertainty in the present
value of the Hubble parameter, 0.4 ≤ h ≤ 1, and Ωνh 2 is smaller than 1. For the νµ and ντ
this bound is much more stringent than the laboratory limits.
Apart from the experimental interest, a heavy tau neutrino (say in the MeV range) could
also be interesting from the point of view of structure formation [9]. According to eq. (1)
such neutrino can not exist if it has only the interactions prescribed by the SM. However
a heavy tau neutrino is theoretically viable if in models with spontaneous violation of total
lepton number [3] since these contain new interactions of neutrinos majorons which may cause
neutrinos to decay into a lighter neutrino plus a majoron, for example [4],
or have sizeable annihilations to these majorons,
171
ντ → νµ + J . (2)
ντ + ντ → J + J . (3)
The possible existence of fast decay and/or annihilation channels could eliminate relic neutrinos
and therefore allow them to have higher masses, as long as the lifetime is short enough to
allow for an adequate red-shift of the heavy neutrino decay products. These 2-body decays
can be much faster than the visible modes, such as radiative decays of the type ν ′ → ν + γ.
Moreover, the majoron decays are almost unconstrained by astrophysics and cosmology (for
a detailed discussion see ref. [8]).
A general method to determine the majoron emission decay rates of neutrinos was first
given in ref. [10]. The resulting decay rates are rather model-dependent and will not be
discussed here. Explicit neutrino decay lifetime estimates are given, e.g. in ref. [4, 11]. The
conclusion is that there are many ways to make neutrinos sufficiently short-lived and that all
mass values consistent with laboratory experiments are cosmologically acceptable.