Proc. Neutrino Astrophysics - MPP Theory Group

170
Neutrinos in Astrophysics
José W. F. Valle
Instituto de Física Corpuscular—C.S.I.C.
Departament de Física Teòrica, Universitat de València, 46100 Burjassot, València,
Spain (http://neutrinos.uv.es)
Theory of Neutrino Mass
Neutrinos are the only apparently massless electrically neutral fermions in the Standard Model
(SM). Nobody knows why they are so special when compared with the other fermions. The
masslessness of neutrinos is imposed in an ad hoc fashion, not dictated by an underlying
principle such as gauge invariance. Moreover, massless neutrinos may be in conflict with
present data on solar and atmospheric neutrino observations, as well as cosmological data on
the amplitude of primordial density fluctuations [1]. The latter suggest the need for hot dark
matter in the Universe. On the other hand, if massive, neutrinos would pose another puzzle,
namely, why are their masses so much smaller than those of the charged fermions? The key
to the answer may lie in the fact that neutrinos could be Majorana fermions. Such fermions
are the most fundamental ones. If neutrinos are majorana particles the suppression of their
mass could be related to the feebleness of lepton number violation. This, in turn, would point
towards physics beyond the SM.
Lepton number, or B − L can be part of the gauge symmetry [2] or, alternatively, can be
a spontaneously broken global symmetry. In the latter case there is a physical pseudoscalar
Goldstone boson generically called majoron [3]. One can construct an enormous class of such
models [4] which may have important implications not only in astrophysics and cosmology
but also in particle physics [5].
Many mechanisms exist to explain the small masses of neutrinos as a result of the violation
of lepton number. The first is the seesaw mechanism, which is based on the existence of some
relatively large mass scale. However, neutrinos could acquire their mass radiatively. In this
case the smallness of the mass holds even if all the new particles required to generate the
neutrino masses are light and therefore accessible to present experiments [6]. The seesaw and
the radiative mechanisms of neutrino mass generation may be combined. Supersymmetry
with broken R-parity provides a very elegant mechanism for the origin of neutrino mass
[7] in which the tau neutrino ντ acquires a mass due to the mixing between neutrinos and
neutralinos. This happens in a way similar to the seesaw mechanism, in which the large mass
scale is now replaced by a supersymmetry breaking scale characterizing the neutralino sector.
In supergravity models with universal soft breaking scalar masses, this effective neutralinoneutrino
mixing is induced only radiatively [7]. As a result the Majorana ντ mass is naturally
suppressed, even though there is no large mass scale present. From this point of view, the
mechanism is a hybrid between the see-saw idea and the radiative mechanism. Despite the
small neutrino mass, many of the corresponding R-parity violating effects can be sizeable. An
obvious example is the fact that the lightest neutralino decay will typically decay inside the
detector, unlike the case of the minimal supersymmetric model.
Other than the seesaw scheme, none of the above models requires a large mass scale. In all
of them one can implement the spontaneous violation of the lepton number symmetry leading