Etude de bruit de fond induit par les muons dans l'expérience ...

tel-00724955, version 1 - 23 Aug 2012
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22 The Dark Matter problem
halo fraction f < 0.2 (95% CL) for objects in the mass range between 2 · 10 −7 M⊙
and 1 M⊙, dismissing earlier claims of a significant halo fraction of compact objects
by the MACHO collaboration [45]. Compact objects within this mass range cannot
account for more than 25% of a standard halo.
Another example of candidates for Dark Matter are the light neutrinos. They
are hot dark matter (HDM), rather than cold, and then would have substantial effects
on the formation of large-scale structure. The investigation of the still open
role of neutrino HDM in the evolution of large scale structure is one of the main
motivations for the proposed next-generation tritium β decay experiment KATRIN
[46], which is designed to measure the absolute mass of the electron neutrino with
sub-eV sensitivity (a discovery potential down to about 0.35 eV/c 2 ). Correspondingly,
KATRIN would be sensitive to a neutrino HDM contribution down to a value
of Ων = 0.025, thus significantly constraining the role of neutrino HDM in structure
formation. With its sensitivity, KATRIN will either detect a neutrino mass
of cosmological relevance or exclude (in case of a negative result) any significant
contribution of neutrinos to the universe’s matter content and structures.
There is also the possibility that the dark matter problem could be explained
by non-Newtonian gravity models, in which the strength of the gravitational force
decreases less rapidly than r −2 at large distance. However, gravitational lensing
by the colliding galaxy clusters 1E0657-56 is a convincing dynamic system giving
theory-independent proof of dark matter dominance at large scales [47]. It is made
up of two subclusters in the process of merging, which appear to have passed directly
through one another, shown in Figure 1.15. The space between galaxies is
large enough so that the galaxies in each subcluster have passed through the other
subcluster without collisions. The intracluster medium (ICM), on the other hand,
has been shocked and heated by the interaction, and remains concentrated closer to
the collision point. This sets the stage for a very beautiful test of the dark matter
model: If the mass of the clusters is dominated by collisionless dark matter, then
weak lensing will show two centers near the concentrations of galaxies. If, on the
other hand, the mass is mainly in the ICM, with the high mass-to-light ratio of
galaxies and clusters explained by modified gravity, then weak lensing should show
one center, near the gas. When this system is studied and mapped with weak lensing,
it shows that the lensing mass is concentrated in the two regions containing
the galaxies, rather than in the two clouds of stripped gas which contain most of
the baryonic mass and significantly separated from the highest intensity of X-rays
[47, 48]. Weakly interacting dark matter would move together with the galaxies, and
therefore explains the observed system. The bullet cluster of 1E0657-56 is not a sole
example, more cases have been studied since, see [49, 50, 51]. And all agree to a nonbaryonic
cold dark matter. These observations present a difficulty for alternative
gravity theories.
Nonetheless, there is a strong consensus in the astrophysics community that
non-baryonic cold dark matter best explains the wealth of cosmological observations
available to us, and is a real component of the universe in which we live. The leading
nonbaryonic cold-dark-matter candidates are axions and weakly interacting massive
particles (WIMPs). The following sections describe the properties of these two main
particle candidates for cold dark matter.