Light Dark Matter Searches and Modeling of Charge Propagation Effects in SuperCDMS Cryogenic Silicon R&D Detectors with eV-Scale Sensitivity

Many cosmological and astrophysical observations point to the existence of Dark Matter (DM) in our and other galaxies. However, despite decades-long history of direct searches for DM particles, there is no confirmed detection to this day. In view of very stringent experimental constraints on DM candidates with masses above 1 GeV/$c^2$, lighter DM candidates have been attracting more attention in recent years. Searches for such DM candidates require detectors with either a low energy threshold or a mechanism to amplify the interaction energy. SuperCDMS cryogenic silicon R&D detectors called HVeV detectors possess both of these properties.

HVeV detectors, consisting of silicon crystals instrumented with phonon sensors, have been operated in two modes, differing by the voltage applied across the crystal: the High-Voltage (HV) mode and the 0V mode. Particles interacting with the detector material generate both phonon and ionization signals. In the HV mode, the ionization signal is amplified by the electric field and converted into an additional comparatively large phonon signal, which is then measured by the phonon sensors. In the 0V mode, however, the ionization signal is not amplified, and the sensors only measure the initial interaction energy.
... mehr

In the work described in this dissertation, we start with developing an improved model of the amount of energy amplification generated by charges moving in the electric field when detectors are operated in the HV mode. We note that the expected detector response is different depending on whether particle interactions occur in the bulk of the detector or on the detector's surface. This property can be used in future DM searches for statistical discrimination between DM and background events.

After that, we take advantage of the versatility and eV-scale sensitivity of HVeV detectors conducting two DM search analyses. In the first analysis we derive the first HVeV exclusion limit on the strength of Light DM interactions with nuclei using the data acquired in the 0V mode in an unshielded facility. We observe a spectrum of background events, steeply rising towards the energy threshold which we set below 10 eV. This unknown background prevents us from reaching unexplored DM parameter space.

In the second analysis, we study the HV data from the first underground HVeV run with multiple detectors operated in the same housing. We apply an anti-coincidence event selection to suppress external backgrounds and derive exclusion limits on the strength of various DM-electron interactions, namely the DM-electron recoils via heavy and light mediator, Dark Photon absorption and Axion-Like Particle (ALP) absorption. We improve the limits obtained in previous HVeV analyses by up to a factor of 20 and reach ALP absorption parameter space, previously unexplored by direct searches.