198 Dark matter and particle physics
wherenνis the number density of the relic neutrinos,M is the solar mass.
Therefore the first structures to form are superclusters and smaller structures
such as galaxies arise from fragmentation in a typical top-down scenario.
Unfortunately in these schemes one obtains too many structures at superlarge
scales. The possibility of improving the situation by adding the seeds for
small-scale structure formation using topological defects (cosmic strings) are
essentially ruled out at present [21,22]. Hence schemes of pure HDM are strongly
disfavoured by the demand of a viable mechanism for large-structure formation.
5.5 Low-energy SUSY and DM
Another kind of DM, widely studied, called cold DM (CDM) is made of particles
which were non-relativistic at their decoupling. Natural candidates for such
DM are Weakly Interacting Massive Particles (WIMPs), which are very heavy
if compared to neutrinos. The SM does not have non-baryonic neutral particles
which can account for CDM and therefore we need to consider extensions of the
SM as supersymmetric SM in which there are heavy neutral particles remnants of
annichilations such as neutralinos (for a review see [36]).
5.5.1 Neutralinos as the LSP in SUSY models
One of the major shortcomings of the SM concerns the protection of the scalar
masses once the SM is embedded into some underlying theory (and at least at
the Planck scale such new physics should set in to incorporate gravity into the
game). Since there is no typical symmetry protecting the scalar masses (while
for fermions there is the chiral symmetry and for gauge bosons there are gauge
symmetries), the clever idea which was introduced in the early 1980s to prevent
scalar masses from becoming too large was to have a supersymmetry (SUSY)
unbroken down to the weak scale. Since fermion masses are chirally protected
and as long as SUSY is unbroken there must be a degeneracy between the fermion
and scalar components of a SUSY multiplet; then, having a low-energy SUSY,
it is possible to have an ‘induced protection’ on scalar masses (for a review
see [34, 35]).
However, the mere supersymmetrization of the SM faces an immediate
problem. The most general Lagrangian contains terms which violate baryon and
lepton numbers producing a proton decay which is too rapid. To prevent this
catastrophic result we have to add some symmetry which forbids all or part of
these dangerous terms withLorBviolations. The most familiar solution is the
imposition of a discrete symmetry, calledRmatter parity, which forbids all these
dangerous terms. It reads over the fields contained in the theory:
R=(− 1 )^3 (B−L)+^2 s. (5.36)Ris a multiplicative quantum number reading−1 over the SUSY particles and
+1 over the ordinary particles. Clearly in models withRparity the lightest