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

G Angloher,S Wawoczny,F Reindl,M Willers,M Bauer,F Pröbst,F Petricca,J Jochum,C Bucci,G Deuter,W Potzel,I Usherov,K Rottler,C Strandhagen,M Kiefer,L Stodolsky,I Bavykina,C Isaila,A Tanzke,M V Sivers,J Schmaler,C Sailer,M Kimmerle,W Seidel,S Pfister,A Zöller,F Von Feilitzsch,A Bento,C Ciemniak,R Strauß,P Huff,D Hauff,K Schäffner,J.‐C Lanfranchi ,Stephan Scholl ,Stephan V Roth

doi:10.1140/epjc/s10052-012-1971-8

Abstract

The CRESST-II cryogenic Dark Matter search, aiming at detection of WIMPs via elastic scattering off nuclei in CaWO4 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/γ-band (2) “leakage” from the α-particle band (3) neutrons and (4) 206Pb recoils from 210Po decay. Using a maximum likelihood analysis, we find, at a statistical significance of more than 4σ, 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.

Highlights

Several experiments currently aim for direct detection of Dark Matter, mostly focusing on a particular class of particles, the so-called WIMPs (Weakly Interacting Massive Particles)
Monte Carlo simulations suggest that neutrons from s.f. and (α, n) reactions in the rock outside the experiment only constitute a negligible background at the level of 10−5 events per kg d [4]
In analogy to the α-background, we study the lead recoil background in a region where the Pb-band is free of other sources of events. This indicates that the energy spectrum of this background has a decreasing tail towards lower energies which we model by an exponential starting at 90 keV, on top of a constant contribution: dNPb dE

Summary

Introduction

Several experiments currently aim for direct detection of Dark Matter, mostly focusing on a particular class of particles, the so-called WIMPs (Weakly Interacting Massive Particles). Rare interactions with ordinary matter would possibly be detectable in earthbound experiments One such project is CRESST-II (Cryogenic Rare Event Search with Superconducting Thermometers), located at the Laboratori Nazionali del Gran Sasso in Italy. Monte Carlo simulations suggest that neutrons from s.f. and (α, n) reactions in the rock outside the experiment only constitute a negligible background at the level of 10−5 events per kg d [4] Such neutrons are a possibly relevant background only if they are emitted inside the neutron shielding, e.g. by s.f. of 238U in the lead shielding, or by (α, n) reactions or s.f. in the copper shielding. This leads to events with similar characteristics as when the shower is directly induced by a muon inside the PE shielding

Objectives

Methods

Conclusion