Spin Dependent Discrimination between Majorana and Dirac Dark Matter

Examensarbete för masterexamen
Master Thesis
Physics and astronomy (MPPAS), MSc
Johansson, Erik
A large number of observations reveal deviations from expected gravitational behaviour in astrophysical systems. This suggests that something is missing in our understanding of the Universe. A well established candidate for explaining these deviations is dark matter, a kind of matter that is not subject to photon interactions and thus can not be detected by absorbed or emitted light - it is dark. Even though we can not observe it at any detectable wavelength, dark matter is postulated to interact via gravitation. These interactions would then account for the shortcoming of gravitational pull from visible mass, explaining the observed irregular gravitational behaviour. One leading hypothesis is that the proposed dark matter component of the Universe is actually made out of massive, weakly interacting particles. As of today, there has not yet been any detection of such a particle, leaving the fundamental properties of dark matter unknown. There are currently several ongoing experimental projects searching for dark matter particles, and more are being planned. Furthermore, their accuracy is higher than ever, and if the dark matter particle hypothesis is correct, it is reasonable to expect a detection signal in the foreseeable future. The different searches take complementary approaches and exploit either annihilation, production or direct detection experiments. The latter, which is the focus of this thesis, aims to detect dark matter by measuring the recoil of a target nucleus in a detector when a dark matter particle scatters off of it. In case of positive detection at direct detection experiments, a model describing the interaction between dark matter particles and baryonic matter is needed in order to be able to draw conclusions about the dark matter particle properties. Under the assumption that dark matter is a spin 1/2 particle that only interacts with baryonic matter via spin dependent interactions, this thesis studies dark matter-nucleus scattering in order to see if it is feasible to discriminate between Majorana and Dirac dark matter. I find that, if dark matter particles are detected at three different experiments, the Majorana dark matter hypothesis can be rejected in favour of an alternative hypothesis in which dark matter is a Dirac particle. Restrictions on target elements that are of interest for this test are presented, and the test procedure is studied for setups containing some of them, namely: 131Xe, 127I, 73Ge, 23Na and 19F.
Fysik , Physical Sciences
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