200 Dark matter and particle physics
If|μ|>M 1 ,M 2 thenχ ̃ 10 is mainly a gaugino and, in particular, a bino if
M 1 >mZ,ifM 1 ,M 2 >|μ|thenχ ̃ 10 is mainly a higgsino. The corresponding
phenomenology is drastically different leading to different predictions for CDM.
Forfixedvaluesoftanβone can study the neutralino spectrum in the (μ,M 2 )
plane. The major experimental inputs to exclude regions in this plane are the
request that the lightest chargino be heavier thanmZ/2 and the limits on the
invisible width of the Z hence limiting the possible decays Z→ ̃χ 10 χ ̃ 10 ,χ ̃ 10 χ ̃ 20.
Moreover, if the GUT assumption is made, then the relation (5.38) betweenM 2
andm ̃gimplies a severe bound onM 2 from the experimental lower bound onmg ̃
of CDF (roughlymg ̃>220 GeV, hence implyingM 2 >70 GeV); the theoretical
demand that the electroweak symmetry be broken radiatively, i.e. due to the
renormalization effects on the Higgs masses when going from the superlarge scale
of supergravity breaking down toMW, further constrains the available (μ,M 2 )
region. The first important outcome of this analysis is that the lightest neutralino
mass exhibits a lower bound of roughly 30 GeV. The actual bound on the mass of
the lightest neutralinoχ ̃ 10 from LEP2 is:
mχ ̃ 0
1
≥40 GeV (5.40)foranyvalueoftanβ. This bound becomes stronger if we put further constraints
on the MSSM. The strongest limit is obtained in the so-called Constrained MSSM
(CMSSM) where we have only four independent SUSY parameters plus the sign
of theμparameter (see equation (5.37)):mχ ̃ 0
1
≥95 GeV [29].
There are many experiments already running or approved to detect WIMPS;
however, they rely on different techniques:
(i) DAMA and CDMS use the scattering of WIMPS on nuclei measuring the
recoil energy; in particular DAMA [31] exploits an annual modulation of the
signal which could be explained in terms of WIMPS;
(ii) ν-telescopes (AMANDA) are held to detectνfluxes coming from the
annihilation of WIMPS which accumulate in celestial bodies such as the
Earth or the Sun;
(iii) experiments (AMS, PAMELA) which detect low-energy antiprotons andγ-
rays fromχ ̃ 10 annihilation in the galactic halo.5.5.3 Thermal history of neutralinos andCDM
Let us focus now on the role played byχ ̃ 10 as a source of CDM. The lightest
neutralinoχ ̃ 10 is kept in thermal equilibrium through its electroweak interactions
not only forT>mχ ̃ 0
1
, but even whenTis belowmχ ̃ 0
1
.HoweverforT<mχ ̃ 0
1
the number ofχ ̃ 10 s rapidly decreases because of the appearance of the typical
Boltzmann suppression factor exp(−mχ ̃ 0
1
/T).WhenTis roughlymχ ̃ 0
1
/20 the
number ofχ ̃ 10 diminishes so much so that they no longer interact, i.e. they
decouple. Hence their contribution toCDM ofχ ̃ 10 is determined by two