Results from 730 kg days of the CRESST-II Dark Matter Search

TL;DR

The CRESST-II experiment collected 730 kg days of data to search for WIMPs, finding events that could suggest light WIMP interactions beyond known backgrounds, using cryogenic detectors with scintillation discrimination.

Contribution

This study presents the first detailed analysis of CRESST-II data with a focus on light WIMPs, highlighting potential signals not explained by background alone.

Findings

67 events observed in the WIMP acceptance region

Background sources insufficient to explain the data statistically

Possible indication of light WIMP interactions

Abstract

The CRESST-II cryogenic Dark Matter search, aiming at detection of WIMPs via elastic scattering off nuclei in CaWO$_4$ crystals, completed 730 kg days of data taking in 2011. We present the data collected with eight detector modules, each with a two-channel readout; one for a phonon signal and the other for coincidently produced scintillation light. The former provides a precise measure of the energy deposited by an interaction, and the ratio of scintillation light to deposited energy can be used to discriminate different types of interacting particles and thus to distinguish possible signal events from the dominant backgrounds. Sixty-seven events are found in the acceptance region where a WIMP signal in the form of low energy nuclear recoils would be expected. We estimate background contributions to this observation from four sources: 1) "leakage" from the e/\gamma-band 2) "leakage" from the \alpha-particle band 3) neutrons and 4) Pb-206 recoils from Po-210 decay. Using a maximum likelihood analysis, we find, at a high statistical significance, that these sources alone are not sufficient to explain the data. The addition of a signal due to scattering of relatively light WIMPs could account for this discrepancy, and we determine the associated WIMP parameters.