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Topic 3: "Matter and Radiation from the Universe"

Dark Matter Search

The new Standard Model of cosmology implies a dark Universe, where non-baryonic Dark Matter (DM) is the dominant form of gravitationally interacting matter. With a fraction of 26% of the total matter-energy density of the Universe (Planck result), DM is playing a key role in the formation and evolution of large scale structures. The non-baryonic matter of the Universe contains a smaller admixture of primordial neutrinos with sub-eV masses (Hot Dark Matter, HDM), but it is dominated by weakly interacting massive particles (WIMPs), which act as Cold Dark Matter (CDM) in structure evolution. The identification of CDM particles would thus be of major importance in our understanding of the Universe both at the largest and smallest scales.

 

Supersymmetry (SUSY) is a promising theoretical framework to explain the nature of CDM, where the lightest supersymmetric particle may be a neutral particle, the neutralino. Likely to be stable over cosmological time scales and only interacting weakly with ordinary matter, the neutralino is thus a natural particle candidate for CDM. Models of thermal neutralino production after the Big Bang give the correct relic density and a neutralino-matter interaction cross section below even the most stringent limits set by direct DM searches, for example by the EDELWEISS experiment. Other DM candidates entail additional free parameters or fine-tuning to be compatible with present data, but should nevertheless not be excluded a priori. Here, we focus on our experimental programme to detect WIMPs, either directly or indirectly via excess signatures in the flux of cosmic rays originating from either our galaxy or from extragalactic sources.

 

 

Challenges

The direct search for neutralino-like CDM is based on the elastic scattering off nuclei within a particle detector. The experimental challenges in the direct detection of WIMPs arise from very low event rates (less than 1 event per kg of target material per year) and from the small energy transfer to the recoiling nucleus (as little as a few keV). In addition, the unknown mass of a CDM particle, as well as different types of interactions, call for a variety of target materials and technologies to cover as much as possible of the unknown parameter space. These requirements translate to the need for dedicated underground facilities and a very high degree of background shielding against ambient gamma and neutron background. Additionally specific detectors with very low energy threshold (keV or even below), excellent energy resolution and active suppression of electron and gamma background of the order of 105 or better are required.

 

The indirect DM search is based on the measurement of secondary particles from the annihilation, decay or interaction of DM. Indirect searches are sensitive to all DM particles which produce a signal above the expected astrophysical background. This includes the WIMP, but also DM candidates such as axion-type particles. The secondary particles searched for are energetic photons, neutrinos and cosmic rays detected with gamma-ray, neutrino and charged particle telescopes. The annihilation or decay signal observable with gamma and neutrino observatories is expected to be strongest towards astrophysical regions of DM overdensity such as the Galactic Centre, halos of galaxies, dwarf spheroidal galaxies and massive objects like our Sun. Axion-like particles may be seen in the measurement of the absorption of gamma rays from very distant blazars and GRBs. The major experimental challenge for indirect searches is the detection of the very weak signal expected from DM interaction and the separation of background fluxes of astrophysical origin.