SuperCDMS Background Models for Low-Mass Dark Matter Searches

Abstract

An abundance of astrophysical and cosmological evidence indicates the existence of a non-luminous, non-baryonic form of matter, called dark matter, that is approximately a quarter of all energy in the universe. One promising candidate for dark matter is the Weakly Interacting Massive Particle (WIMP) which interacts with baryonic matter at most on the scale of the weak force. The Cryogenic Dark Matter Search (CDMS) experiment aims to detect the nuclear recoils induced by the elastic scattering of WIMPs off of germanium nuclei. This is a rare signal and difficult to detect, especially the low-energy recoils that are produced by low-mass dark matter. The CDMS project operated at the Soudan Underground Laboratory from 2003--2015, with an upgrade to the SuperCDMS experiment in 2012. The germanium detectors were operated at 50~mK and able to measure both the ionization and athermal phonons produced in a particle interaction. Measuring two signals enables discrimination between electron recoil and nuclear recoil events. An alternative operating mode for the detectors is called the CDMS low ionization threshold experiment (CDMSlite), where a higher bias was applied to the detectors and only the phonon signal analyzed. This method increased sensitivity to low-mass dark matter interactions, but sacrificed discrimination capability. The CDMSlite spectrum had a large contribution from electron recoil background events. From the information gained during the first two CDMSlite Runs, a background model was developed for the third and final CDMSlite Run. Analytical descriptions were identified for those backgrounds that were theoretically known, e.g. tritium $\beta$-spectrum, and Geant simulations were used to understand and predict the low-energy spectra from other sources, e.g. Compton scattering. Multiple new models were developed for detectors operated in CDMSlite at Soudan. These include the analytical formula for Compton scattering, and empirical models for surface backgrounds from $^{210}$Pb contamination of the germanium crystals and detector housing. In order to accurately describe the surface events, a new detector response function was developed that included information about the electric field and energy resolution of the detector. These models were essential to the implementation of a profile likelihood analysis of the CDMSlite Run 3 data, which improved on the sensitivity to dark matter over the Run 2 optimum interval analysis for WIMP masses above 2.5~GeV/$c^2$. This demonstrated a successful application of a likelihood analysis to the high-voltage operating mode, and the potential for these analyses in the future SuperCDMS SNOLAB experiment. For the SuperCDMS SNOLAB experiment, the change in background rates from radiogenic neutrons was considered as additional towers of detectors were added, and the feasibility of an active neutron veto as a potential upgrade for large payloads was studied. This veto could be constructed of plastic scintillator with layers of gadolinium resin, and would aid in reducing the nuclear recoil single scatter background that is indistinguishable from the WIMP signal.