[Paper Review] Results from the CDMS 5-Tower Operation

Jonghee Yoo|arXiv (Cornell University)|Oct 20, 2008

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper presents results from the CDMS five-tower detector array operating from October 2006 to July 2007, achieving a net exposure of 121.3 kg-days. No WIMP signal was observed, leading to a 90% confidence level upper limit on the WIMP-nucleon cross section of $4.6 \times 10^{-44}$ cm$^2$ for a 60 GeV/$c^2$ WIMP mass, representing the most sensitive direct detection limit above 42 GeV at the time.

ABSTRACT

Astrophysical observations strongly suggest that non-luminous, nonbaryonic components, so called "Dark Matter", may constitute most of the matter in the Universe. The Cryogenic Dark Matter Search (CDMS) experiment is designed to detect Dark Matter interaction events through nuclear recoils from elastic scattering. The detector is capable of reading out both phonon and ionization energy of an interaction in Ge or Si crystals. We present results from the CDMS five-tower detector arrays. The data were collected in the period between October 2006 and July 2007 (with an effective exposure of 121.3 kg-days). No WIMP signal was observed. The results, when combined with previous CDMS Soudan data, set a 90% confidence level upper bound on the WIMP-nucleon cross section of $4.6 imes 10^{-44}$ cm$^2$ at 60 Gev/c$^2$ WIMP mass.

Motivation & Objective

• To search for Weakly Interacting Massive Particles (WIMPs) via nuclear recoils in high-purity germanium and silicon detectors.
• To reduce background from electron recoils and neutron interactions using ionization and phonon signal discrimination.
• To achieve the highest possible sensitivity in direct dark matter detection by minimizing false positives and optimizing detector performance.
• To establish a stringent upper bound on the WIMP-nucleon cross section using data from the CDMS-II Soudan experiment.

Proposed method

• Detectors used cryogenic, ultra-pure germanium and silicon crystals cooled to 50 mK to detect nuclear recoils from WIMP scattering.
• Ionization and phonon signals were measured simultaneously using ZIP detectors and transition-edge sensors (TESs) with SQUID readout.
• A timing parameter based on phonon rise time was used to distinguish surface electron recoils from nuclear recoils.
• Ionization yield was used to discriminate electron recoils (high yield) from nuclear recoils (low yield), with cuts applied to isolate the signal region.
• A blind analysis was performed, with signal region masking until final cuts were defined to prevent bias.
• Monte Carlo simulations and calibration sources (133Ba, 252Cf) were used to estimate backgrounds from muon-induced neutrons and cosmogenic sources.

Experimental results

Research questions

• RQ1 What is the sensitivity of the CDMS five-tower detector array to WIMP-nucleon scattering at masses above 42 GeV/c²?
• RQ2 How effectively can ionization yield and phonon timing distinguish nuclear recoils from electron recoils in high-purity Ge and Si crystals?
• RQ3 What is the expected background level in the WIMP signal region from surface electron interactions and neutron backgrounds?
• RQ4 What upper limit can be set on the WIMP-nucleon cross section after combining this data with previous CDMS Soudan results?
• RQ5 Can the CDMS experiment achieve a background-free signal region, demonstrating the highest discovery potential for dark matter?

Key findings

• No WIMP signal was observed in the five-tower detector array during the 121.3 kg-days of effective exposure.
• The expected background in the signal region was estimated at $0.6 \pm 0.5$ events, with neutron backgrounds from cosmic rays and radioactivity contributing less than 0.1 event each.
• The 90% confidence level upper limit on the WIMP-nucleon cross section is $4.6 \times 10^{-44}$ cm$^2$ for a 60 GeV/$c^2$ WIMP mass.
• This result represents the most stringent limit on WIMP-nucleon cross section above 42 GeV at the time of publication.
• CDMS achieved a zero-background signal region, demonstrating the highest potential for dark matter discovery among direct detection experiments.
• The results are consistent with a null detection, supporting the conclusion that WIMPs, if they exist, must have a cross section below the current sensitivity threshold.