Silicon Detector Dark Matter Results from the Final Exposure of CDMS II (1304.4279v3)

Abstract: We report results of a search for Weakly Interacting Massive Particles (WIMPS) with the silicon detectors of the CDMS II experiment. This blind analysis of 140.2 kg-days of data taken between July 2007 and September 2008 revealed three WIMP-candidate events with a surface-event background estimate of 0.41^{+0.20}{-0.08}(stat.)^{+0.28}{-0.24}(syst.). Other known backgrounds from neutrons and 206Pb are limited to < 0.13 and <0.08 events at the 90% confidence level, respectively. The exposure of this analysis is equivalent to 23.4 kg-days for a recoil energy range of 7-100 keV for a WIMP of mass 10 GeV/c2. The probability that the known backgrounds would produce three or more events in the signal region is 5.4%. A profile likelihood ratio test of the three events that includes the measured recoil energies gives a 0.19% probability for the known-background-only hypothesis when tested against the alternative WIMP+background hypothesis. The highest likelihood occurs for a WIMP mass of 8.6 GeV/c2 and WIMP-nucleon cross section of 1.9e-41 cm2.

• The paper presents a blind analysis of 140.2 kg-days of silicon detector data, revealing three candidate events that may indicate low-mass WIMP interactions.
• It employs an array of ZIP detectors cooled to 40 mK to distinguish nuclear recoils from electron backgrounds using simultaneous ionization and phonon measurements.
• Statistical analysis yields a p-value of 0.19% for the WIMP+background hypothesis and sets competitive 90% confidence limits on the WIMP-nucleon cross section.

Evaluation of Silicon Detector Dark Matter Results from CDMS II Final Exposure

The paper presents results from the investigation of Weakly Interacting Massive Particles (WIMPs) using silicon detectors from the last phase of the Cryogenic Dark Matter Search (CDMS) II experiment. A detailed blind analysis of 140.2 kg-days of data obtained between July 2007 and September 2008 was conducted. The analysis identified three candidate events indicative of potential WIMP interactions; however, they correspond to a background estimate due to surface events of $$0.41_{-0.08}^{+0.20} (stat.)_{-0.24}^{+0.28} (syst.)$$. The upper 90% confidence limits on other backgrounds, such as neutrons, were constrained to less than 0.13 events.

Experimental Setup and Methodology

The CDMS II experiment, located at the Soudan Underground Laboratory, employed an array of semiconductor detectors cooled to 40 mK. This array comprised 30 Z-sensitive ionization and phonon (ZIP) detectors, including 19 germanium and 11 silicon detectors. Events were distinguished by the simultaneous detection of ionization and non-equilibrium phonons, which differentiates nuclear recoils potentially caused by WIMPS from electron recoils representing the majority of background events.

Of the silicon detectors, eight were used in the final analysis after excluding others due to technical deficiencies. The nuclear recoil detection efficacy was rigorously determined using experimental data from well-characterized calibration sources. The efficiency of the silicon detectors in identifying nuclear recoils was delineated across varying energy spectrums, confirming the robustness of the established event-selection criteria.

The three identified candidate events were observed with recoil energies of 8.2 keV, 9.5 keV, and 12.3 keV, reflecting a scenario where WIMPs might be of approximately 10 GeV/$c^2$ mass. The candidate events had a measureable probability of occurrence under the null hypothesis involving only known backgrounds.

Statistical Analysis and Results

A profile likelihood ratio test was conducted to evaluate the hypotheses involving either known-background-only or WIMP+background models. The test results favored the WIMP+background hypothesis with a notable, yet not conclusive, probability (p-value 0.19%). The maximum likelihood estimations aligned with a WIMP mass of 8.6 GeV/c$^2$ and a nucleon cross section of 1.9 $\times 10^{-41}$~cm$^2$. Nevertheless, the analysis did not achieve the statistical significance necessary to claim a discovery.

The resulting upper limits on the spin-independent WIMP-nucleon cross section at the 90% confidence level were competitive yet aligned with results from contemporaneous experiments like CDMS II (Ge), EDELWEISS, and XENON. Notably, these results addressed potential WIMP models with low mass, generating significant interest in the context of results from the DAMA/LIBRA, CoGeNT, and CRESST experiments suggesting low-mass WIMP contributions.

Implications and Future Work

The findings underscore intriguing prospects for low-mass WIMP models, notwithstanding constraints imposed by competitive experiments under standard assumptions. The paper suggests potential refinements in analyzing the silicon data at reduced thresholds to enhance sensitivity to low-mass WIMPs, alongside discussions on future experimental endeavors involving silicon detectors, notably within the SuperCDMS framework.

The CDMS collaboration's explorations expand understanding in dark matter research, with sustainable methodological advancements and the proposal for future inquiries set to refine the elucidation of WIMP properties and interactions. The collaboration also contemplates advanced detector technologies to further extend sensitivity to low-mass dark matter candidates, thereby enhancing the capacity for definitive direct detection endeavors.