202 Dark matter and particle physics
radiative breaking further reduces this number to four. Finally, in simple
supergravity realizations the soft parametersAandBare related. Hence we end
up with only three new, independent parameters. One can use the constraint that
the relicχ ̃ 10 abundance provides a correctCDMto restrict the allowed area in
this three-dimensional space. Or, at least, one can eliminate points of this space
whichwouldleadtoχ ̃ 0
1
1, hence overclosing the universe. Forχ ̃ 10 masses
up to 150 GeV it is possible to find sizable regions in the SUSY parameter space
whereχ ̃ 0
1
acquires interesting values for the DM problem. The interested reader
can find a thorough analysis in the review [36] and the original papers therein
quoted.
Finally a comment on models withoutRparity. From the point of view
of DM, the major implication is that in this context the LSP is no longer a viable
CDM candidate since it decays. There are very special circumstances under which
this decay may be so slow that the LSP can still constitute a CDM candidate. The
very slow decay ofχ ̃ 10 may have testable consequences. For instance in some
schemes the LSP could decay emitting a neutrino and a photon. The negative
result of the search for such neutrino line at Kamiokande resulted in an improved
lower bound on theχ ̃ 10 lifetime.
5.5.4 CDM models and structure formation
In the pure CDM model, almost all of the energy density needed to reach the
critical one (the remaining few percent being given by the baryons) was provided
by CDM alone. However, some observational facts (in particular the results of
COBE) put this model into trouble, showing that it cannot correctly reproduce the
power spectrum of density perturbations at all scales. At the same time it became
clear that some CDM was needed anyway in order to obtain a successful scheme
for large-scale structure formation.
A popular option is that of a flat universe realized with the total energy
density mostly provided by two different matter components, CDM and HDM in
a convenient fraction. These models, which have been called mixed DM (MDM)
[33], succeeded in fitting the entire power spectrum quite well. A little amount
of HDM has a dramatic effect on CDM models because the free-streaming of
relativistic neutrinos washes out any inhomogeneities in their spatial distribution
which will become galaxies. Therefore their presence slows the growth rates of
the density inhomogeneities which will lead to galaxies.
Another interesting possibility for improving CDM models consists in the
introduction of some late-time decaying particle [50]. The injection of non-
thermal radiation due to such decays and the consequent increase in the horizon
length at the equivalence time could lead to a convenient suppression of the
excessive power at small scales (hence curing the major disease of the pure
CDM SM). As appealing as this proposal may be from the cosmological point of
view, its concrete realization in particle physics models meets several difficulties.
Indeed, after considering cosmological and astrophysical bounds on such late