[Paper Review] Direct Dark Matter Search with the CRESST II Experiment

Schieck, J, G. Angloher+41 more|arXiv (Cornell University)|Nov 7, 2016

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper presents the latest results from the CRESST II experiment, which uses low-temperature CaWO₄ cryogenic detectors to search for low-mass dark matter particles via nuclear recoils. By achieving a 100 eV energy threshold and enhanced radio-purity, CRESST II sets the most stringent direct detection limits below 1.8 GeV/c², significantly improving sensitivity to sub-GeV dark matter candidates.

ABSTRACT

The quest for the particle nature of dark matter is one of the big open questions of modern physics. A well motivated candidate for dark matter is the so-called WIMP - a weakly interacting massive particle. Recently several theoretically well-motivated models with dark matter candidates in a mass region below the WIMP mass-scale gained also a lot of interest, theoretically and experimentally. The CRESST II experiment located at the Gran Sasso laboratory in Italy is optimised for the detection of the elastic scattering of these low-mass dark matter particles with ordinary matter. We show the results obtained with an improved detector setup with increased radio purity and enhanced background rejection and the results obtained with a dedicated low-threshold analysis of a single conventional detector module. The limit achieved is the most stringent limit achieved for direct dark matter experiments in the mass region below 1.8 GeV/$c^{2}$. We will discuss the expected performance for new small CRESST-type detectors to be used during the next data taking phase. We conclude with an outlook of the future potential for direct dark matter detection using further improved CRESST CaWO$_{4}$ cryogenic detectors.

Motivation & Objective

• To improve sensitivity to low-mass dark matter particles (below 1.8 GeV/c²) using direct detection via nuclear recoils in cryogenic CaWO₄ crystals.
• To reduce background events from intrinsic radioactivity and electron/gamma scattering by enhancing detector radio-purity and light yield discrimination.
• To set stringent exclusion limits on momentum-dependent dark matter-nucleon cross sections in the low-mass region.
• To search for rare neutrinoless double electron capture (0ν EC) and two-neutrino double K-capture (2ν 2K) processes in ⁴⁰Ca and ¹⁸⁰W isotopes.
• To demonstrate the feasibility of future CRESST upgrades with 100 eV energy thresholds and 100× lower intrinsic background.

Proposed method

• Detection of nuclear recoils via phonon signals in CaWO₄ crystals operated at ~15 mK using superconducting transition-edge sensors.
• Simultaneous measurement of scintillation light to distinguish electron-like backgrounds from nuclear recoils via light yield (L/E) discrimination.
• Application of a low-threshold analysis on a single detector module (Lise) to enhance sensitivity to low-energy nuclear recoils.
• Use of a momentum-dependent dark matter-nucleon cross-section model to reinterpret limits across different momentum transfer dependencies.
• Analysis of 2009–2011 data for rare double-capture processes, focusing on events with light yield ≈1 and total energy deposition in the detector.
• Implementation of improved detector design with smaller crystals to increase phonon density and reduce energy threshold to 100 eV.

Experimental results

Research questions

• RQ1 What are the most stringent limits on elastic scattering of low-mass dark matter particles (500 MeV/c² to 1.8 GeV/c²) in direct detection experiments?
• RQ2 How does the performance of CRESST II compare to previous experiments in the sub-GeV dark matter mass range?
• RQ3 To what extent can light yield and phonon energy discrimination reduce background from electron and gamma events?
• RQ4 Can the CRESST II data set improve existing limits on neutrinoless double electron capture (0ν EC) and two-neutrino double K-capture (2ν 2K) in ⁴⁰Ca and ¹⁸⁰W?
• RQ5 What is the expected sensitivity gain from future CRESST upgrades with 100 eV threshold and 100× lower intrinsic radioactivity?

Key findings

• CRESST II achieves the most stringent direct detection limit for dark matter in the mass range below 1.8 GeV/c², with a 100 eV energy threshold in the latest analysis.
• The collaboration sets a lower limit on the 0ν EC lifetime of ⁴⁰Ca at >1.40×10²² years and ¹⁸⁰W at >9.39×10¹⁸ years, improving previous limits by factors up to 30.
• The 2ν 2K decay of ¹⁸⁰W is constrained to a lower limit of >3.13×10¹⁹ years, representing a 30-fold improvement over prior measurements.
• The best-fit momentum-dependent dark matter model (q² dependence) from previous work is excluded by the new CRESST II data set.
• The new CRESST II detector module design achieves a 100 eV energy threshold and reduces intrinsic radioactivity by a factor of 100, enabling future sensitivity improvements by several orders of magnitude.
• The analysis reveals four events in the signal region with light yield below expected background leakage, indicating a potential new background source not originating from electron or gamma scatters.