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

tel-00724955, version 1 - 23 Aug 2012
1.8 Dark Matter particle candidates 23
Figure 1.15: The bullet cluster (right) passing through the cluster on the left. The
hot gas that is stripped off the colliding clusters is colored in red-yellow. The white
bar indicates 200 kpc at the distance of the cluster. Shown in green contours are
the levels surfaces of gravitational lensing convergence. The two peaks of this do not
coincide with those of the gas X-ray image which is mostly all the known mass, but
point in the direction of the galaxy concentrations. Figure from [48].
1.8.1 Axions
The axion is a dark matter particle candidate that arises as a consequence of a
theory by Peccei and Quinn [52] that proposes a dynamical mechanism to conserve
strong CP symmetry. This symmetry, according to the Standard Model of particle
physics, should be violated and therefore produce a neutron electric dipole moment
10 orders of magnitude larger than the current experimental lower limit of 10−26 ecm
[53]. Therefore, this upper limit requires a mechanism to preserve CP symmetry.
The axion is described by the spontaneous symmetry breaking scale of the Peccei-
Quinn symmetry, fa, and is related to the mass, ma, as
12 10 GeV
ma 6µeV
(1.26)
and the axion-photon coupling gaγγ is defined as
fa
gaγγ ≡ αgγ
πfa
(1.27)
where α is the fine structure constant and gγ is a dimensionless model dependent
coupling parameter.
Axions may be created in the early universe as a non-relativistic condensate
through the “misalignment” mechanism [54]. The average axion relic density is
Ωah 2 ∼
fa
1012GeV 7/6 (1.28)
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