Primordial Black Holes and Cosmological Phase Transitions Report ...

PBHs and Cosmological Phase Transitions 46
like Ñ1 LSP; nearly degenerate wino–like Ñ2, ˜ C1; and higgsino–like Ñ3, Ñ4, ˜ C2.
The gluino is the heaviest superpartner. The squarks are all much heavier than
the sleptons, and the lightest sfermion is an stau (e.g. Martin, 2006). The mass
spectrum of supersymmetric particles and Higgs boson according to the SPS1a
scenario is represented in Figure 11 and in Table 11. Note that in this scenario
the masses of the second family coincide with the masses of the first family.
At the moment we only have lower limit masses for these particles (cf. Table
11) and a list of assumptions that we see as reasonable. For example, it is
perhaps not unlikely that (e.g. Martin, 2006):
• The LSP is the lightest neutralino Ñ1.
• The gluino will be much heavier than the lighter neutralinos and charginos.
• The squarks of the first and second families are nearly degenerate and
much heavier than the sleptons.
• The lighter stop ˜t1 and the lighter sbottom ˜ b1 are probably the lightest
squarks.
• The lightest charged slepton is probably a stau ˜τ.
• The left–handed charged sleptons are likely to be heavier than their righthanded
counterparts.
• The lightest neutral Higgs boson h 0 is lighter than about 150 GeV, and
may be much lighter than the other Higgs scalar mass eigenstates A 0 , H ± ,
H 0 .
Extensions of the MSSM can be introduced, where the Higgs sector is further
enlarged and the Higgs masses are less constrained. As an example we have the
so–called Next–to–Minimal Supersymmetric Standard Model (NMSSM), whose
Higgs sector includes not only two Higgs doublets, but also an additional singlet.
Such an extension may slightly decrease the level of fine–tuning required to
reconcile the present stringent lower bounds on supersymmetric particles and
Higgs boson masses with the measured value of the Fermi scale (e.g. Ellis et al.,
2007).
1.10 Degrees of freedom
In the early Universe collision and decay processes are continuously creating
and destroying particles. Considering thermal equilibrium (i.e., each process is
taking place at the same rate as its inverse) then the number of particles of a
given species i, per momentum state, is given by (e.g. Lyth, 1993)
f(p) =gi(T ) e E−µ −1 T ± 1
(95)
where gi(T ) counts the effective number of relativistic helicity degrees of freedom
of that particle species at a given photon temperature T ; p is the momentum, E