Session M12: Dark Matter II

Recent Results from The Cryogenic Dark Matter Search II (CDMSII)

The Cryogenic Dark Matter Search II (CDMS II) operated a 4.75 kg array of Ge and Si ZIP detectors at the Soudan Underground Laboratory. Recent results using the Si detectors provide 3 events which are consistent with being from low-mass ($<$10~$GeV^2$) dark matter. To explore further this interesting region of parameter space we use Geant4 simulations, that demonstrate excellent agreement with recent SuperCDMS data, to simulate the CDMS II surface-event background. An accurate low-energy background model is constructed from the simulation in order to test a low-mass dark matter hypothesis in the lowest-energy Ge recoils using a maximum-likelihood technique. I will present results from this model that demonstrate a good understanding of our low-energy backgrounds. Additionally, I will discuss results obtained from extending the CDMS II background-free (higher-threshold) WIMP searches by lowering the energy thresholds for both the Ge and Si detectors (while maintaining near-zero background) to gain additional sensitivity to low-mass dark matter. The improvements in background modeling and analysis techniques presented here give important insights moving forward to the next generation of direct detection experiments.   

Energy Scale for Nuclear Recoils in CDMS II Silicon Detectors

Weakly Interacting Massive Particles (WIMPs) are a favored candidate for the dark matter in the Universe. The Cryogenic Dark Matter Search (CDMS) collaboration employs Z-sensitive Si and Ge crystals instrumented with Ionization and athermal Phonon sensors (ZIPs) attempting to directly detect WIMPs when they produce nuclear recoils. It is necessary to calibrate the detector response with low-energy nuclear recoils to translate results into an allowed WIMP region in mass and cross-section. We present measurements of the energy scale for nuclear recoils for the Si ZIP detectors, determined by comparing the observed energy spectra in Si detectors for 252-Cf neutron calibrations to those expected from Monte Carlo simulations. We quantify uncertainties in both the nuclear recoil spectra Monte Carlo input and results to estimate the nuclear recoil energy scale uncertainty. Implications on regions allowed by previous CDMS Si results are shown.   

Low energy threshold analysis of LUX data

LUX, a dual phase xenon time projection chamber with fiducial target mass greater than 100 kg, is currently the most sensitive direct dark matter search experiment. The initial null result limit on WIMP-nucleon cross section was released in late 2013. Signals from this type of detector include the primary scintillation light (S1) and a follow-up charge response (S2). In the initial analysis of the data, S1 pulses were required to have a signal in at least two PMTS and a total area larger than 2 photoelectrons. Additionally, the S2 size was required to have an area larger than 200 photoelectrons. If these thresholds are lowered, the sensitivity of the detector is expected to change. Here we present the investigation of lowering the thresholds.   

LUX HV Conditioning

The discrimination between nuclear and electron recoils in the LUX experiment's two-phase Xe dark matter search detector is influenced by the achievable voltages and electric fields on the grid wires. To improve the sensitivity of the detector, a series of high voltage conditioning tests have been performed to increase the grid voltages. Results from both the conditioning of the grids in liquid xenon prior to Run03 and the conditioning of the grids in gaseous xenon prior to Run04 will be presented in this talk.   

An analysis of LUX data in the S2-only mode

LUX is the world's largest two-phase Xe time-projection chamber, with an active fiducial target mass in excess of 100 kg. In 2013, we reported a null result from our search for WIMP dark matter and set the most stringent limits on WIMP-nucleon cross section for a wide range of WIMP masses. As the energy of the recoiling Xe nucleus decreases below a few keV, the primary scintillation signal is often too small to detect. An S2-only technique, which relies on detecting the liberated electrons that escape recombination and drift to the liquid surface, allows us to lower the energy threshold, albeit at the cost of rejection power. We will present techniques developed for this study, including novel pulse-finding algorithms and simulation methods. The prospects for LUX sensitivity in this search mode will be outlined.   

Two Years of SuperCDMS at Soudan

There is much cosmological evidence and theoretical motivation for particle dark matter. One such candidate is so-called Weakly Interacting Massive Particle (WIMP) dark matter. For two years of nearly continuous operation at the Soudan Underground Lab, the SuperCDMS experiment has been taking data with 15 state-of-the-art germanium interdigitated Z-sensitive Ionization and Phonon (iZIP) sensors in a direct-detection experiment. The iZIP has proven to be a versatile detector capable of discriminating against backgrounds over a wide energy range allowing searches for both low- and high-mass WIMPs. We will present a brief overview of the iZIP detector itself, illustrating its background rejection capabilities and the mass range it is capable of probing. Constraints on low-mass WIMPs from a high-voltage operation mode called CDMSlite will be presented, and the status and future plans of an on-going high-mass WIMP search will also be discussed.   

Backgrounds and Discrimination Algorithms for Low-energy SuperCDMS Soudan Data

The SuperCDMS experiment at Soudan uses an array of cryogenic germanium detectors called iZIPs to search for weakly interacting massive particles (WIMPs), a leading dark matter candidate. A key feature of the iZIP is its measurement of athermal phonons from WIMP interactions, which provides some position sensitivity and background rejection at energies near the $\sim$1.6 keVnr threshold of the experiment. This talk describes the detector and background models that allow us to simulate the expected WIMP signal and backgrounds for a dedicated low-mass (5-15 GeV/c$^2$) WIMP search. We then optimize background discrimination using rectangular cuts and boosted decisions tree classifiers. These algorithms are used to maximize the sensitivity of an analysis of low-energy SuperCDMS Soudan data.   

Dark matter search results from the first analysis of SuperCDMS Soudan low-threshold data

Recent dark matter search results from CDMS-II Si, CoGeNT, CRESST-II, and DAMA may be interpreted to favor the existence of weakly interacting massive particles (WIMPs) with masses in the 6-20 GeV range. We report results from the first search for low-mass WIMPs from the SuperCDMS Soudan iZIP detectors. This dataset employs 7 germanium iZIP detectors with an analysis threshold of $\sim$1.6 keVnr and a raw exposure of $\sim$800 kg-days. We present the results of a blinded WIMP-search analysis of this exposure and the resulting constraints on the WIMP-nucleon cross-section.