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

tel-00724955, version 1 - 23 Aug 2012
2
32 Detecting the WIMP
Figure 2.1: “First light” image of the gamma ray sky from the Fermi Gamma-ray
Space Telescope. Image from [85].
resolution, and an improved anti-coincidence shield to limit the self-vetoing which
plagued EGRET. Fermi has already released its first image of the gamma ray sky,
see Figure 2.1.
2.2.2 Neutrino detectors
Neutrinos are another leading tracer for WIMP annihilation. Like gamma rays,
neutrinos retain directional and spectral information over cosmological distances.
Their extremely low scattering cross section renders them vastly more difficult to
detect, but also allows them to penetrate dense matter. This makes neutrinos ideal
probes of WIMP annihilation in the cores of massive objects such as the Sun and
the Earth [69, 70]. Neutrinos are also a possible signature of WIMP annihilation
in the galactic center, but the observable signal is greatly suppressed by the small
solid angle of the Earth as seen from the galactic center.
The SuperKamiokande neutrino detector [86] or the IceCube experiment [87]
can set limits on the rate of WIMP annihilations in the Sun. As particles of dark
matter travel through the solar system, some will elastically scatter inside the Sun.
If they lose enough energy to become gravitationally bound, they will eventually
settle into the center of the Sun, where the WIMP density can grow high enough for
annihilations to occur. Among other products, the annihilation can yield neutrinos.
If a high-energy (∼ 1 GeV) muon neutrino reaches the Earth, it may scatter in material
near the surface to produce an energetic muon, that will propagate all the way
through the SuperK chamber, for example. This daughter muon will be emitted in
a direction well-aligned with that of the incident neutrino, thus retaining directional
information about the annihilation source. Upward-going muons of this sort are easily
distinguished from solar neutrinos by their far greater energies (solar neutrinos
have energies characteristic of nuclear processes: keV-MeV) and from atmospheric
neutrinos by their directional correlation with the Sun’s position in the sky.
When completed, the IceCube experiment will possess a full square kilometer of
effective area and kilometer depth, and will be sensitive to muons above approximately
50 GeV [88]. The Deep Core extension of IceCube will be sensitive down
to 10 GeV. The Super-Kamiokande detector, in contrast, has 10 −3 times the effective
area of IceCube and a depth of only 36.2 meters [89]. For low mass WIMPs,