Particle/Antiparticle Nature of Light Dark Matter in Direct Detection

Abstract

In the last century, astronomers have concluded that the luminous mass in the Universe impossibly can be responsible for the huge gravitational pull observed in stellar and galactic systems. The explanation for this discrepancy between luminous mass and gravitational e ects is believed to be dark matter { an unknown particle species that does not emit or absorb light at detectable wavelengths. Although extensive e orts have been made to detect this mysterious particle, it remains undiscovered, and its nature is one of the greatest unsolved questions in fundamental physics. This thesis investigates the prospects of discriminating between Dirac and Majorana dark matter, if dark matter is to be found in direct detection experiments. The Dirac or Majorana nature of a particle corresponds to the existence or absence of a distinct antiparticle, which for the invisible, and therefore probably neutral, dark matter particle is a property of major importance. The theoretical framework is fermionic dark matter at the sub-GeV mass scale, interacting with electrons in direct detection experiments with argon, xenon and germanium targets. The coupling constant parameter space of photon mediated interactions is explored, and the regions where statistical rejection of a Majorana hypothesis could be possible in the future are determined. It is found that the discrimination signi cance for rejecting a Majorana hypothesis, given simulated Dirac-like experimental signals, reaches values of >4 standard deviations for a substantial part of the coupling constant parameter space for germanium targets, whereas argon and xenon targets entail stronger restrictions on the discrimination parameter space. The discrimination signi cance is highly dependent on the available detection energy region, and the dark matter mass.