MODERN COSMOLOGY

194 Dark matter and particle physics

5.4 Lepton number violation and neutrinos as HDM candidates

Neutrinos are the first candidate for DM we review which can account for
HDM [30]: particles that were relativistic at their decoupling from the thermal
bath when their rate of interaction became smaller than the expansion rate and
they froze out (or, to be more precise, at the time galaxy formation starts at
T ∼300 eV). The SM has no candidate for HDM; however, it is now well
established from experimental data that neutrinos are massive and very light.
Therefore they can account for HDM. We briefly discuss their characteristics.

5.4.1 Experimental limits on neutrino masses

The recent atmospheric neutrino data from Super-Kamiokande provide strong
evidence of neutrino oscillations which can take place only if neutrinos are
massive. The parameters relevant inν-oscillations are the mixing angleθand the
mass-squared differences which can be measured in atmospheric neutrinos, solar
neutrinos, short-baseline and long-baseline experiments (for a review see [9, 10]):

(i) In atmospheric neutrino experiments, to account for the deficit of theνμflux

expected towards theνeone from cosmic rays and its zenith dependence, it

is necessary to call forνμ→ντoscillations with

sin^22 θatm≥ 0. 82 (5.22)

m^2 atm(1.5–8.0)× 10 −^3 eV^2 (5.23)

from Super-Kamiokande data at 99% C.L. [11].

(ii) The solarνanomaly arises from the fact that theνeflux coming from the Sun

is sensibly less than the one predicted by the solar SM: this problem can also

be explained in terms of neutrino oscillations. The recent Super-Kamiokande

data [12] favour the LMA (large mixing angle) solution with

tan^2 θ 0.15–4 (5.24)

m^2 (1.5–10)× 10 −^5 eV^2 (5.25)

at 99% C.L., even if the small mixing angle (tan^2 θ ∼ 10 −^4 )andtheLOW
(tan^2 θ ∼ 0 .4–4) solutions cannot be excluded and the oscillations into
sterile neutrinos are strongly disfavoured.
(iii) Reactor [13] and short- and long-baseline experiments constrain further the
parameters and, in particular, the mixing angles.
(iv) Finally the LSND experiment has evidence ofνμ →νeoscillations with
m^2 LSND 0 .1–2 eV^2 , the Karmen experiment has no positive results for
the same oscillation and then restricts the LSND allowed region [14].
In the near future several long-baseline experiments will be held to testν-
oscillations directly and measure the relevant parameters: K2K in Japan is already