Low-threshold analysis of CDMS shallow-site data

TL;DR

This work lowers the CDMS II shallow-site analysis threshold to probe light WIMPs by leveraging a dual- readout of phonons and ionization in ZIP detectors, calibrated thresholds, and detector-specific efficiencies. Using a serialized optimum-interval approach and detailed background characterization, the study places 90% CL upper limits on the spin-independent WIMP-nucleon cross section that challenge portions of the DAMA/LIBRA and CoGeNT favored regions for $m_{\chi}$ down to a few GeV, with new sensitivity around $3$–$4\ \mathrm{GeV}/c^2$. The analysis demonstrates robustness to systematic uncertainties in energy scale, trigger efficiency, and halo-velocity parameters, and confirms the value of low-threshold, background-limited searches in expanding the WIMP-mass reach. These results motivate continued low-threshold analyses with larger exposures and at deeper sites, including further CDMS data and next-generation low-threshold approaches such as cdmslite.

Abstract

Data taken during the final shallow-site run of the first tower of the Cryogenic Dark Matter Search (CDMS II) detectors have been reanalyzed with improved sensitivity to small energy depositions. Four ~224 g germanium and two ~105 g silicon detectors were operated at the Stanford Underground Facility (SUF) between December 2001 and June 2002, yielding 118 live days of raw exposure. Three of the germanium and both silicon detectors were analyzed with a new low-threshold technique, making it possible to lower the germanium and silicon analysis thresholds down to the actual trigger thresholds of ~1 keV and ~2 keV, respectively. Limits on the spin-independent cross section for weakly interacting massive particles (WIMPs) to elastically scatter from nuclei based on these data exclude interesting parameter space for WIMPs with masses below 9 GeV/c^2. Under standard halo assumptions, these data partially exclude parameter space favored by interpretations of the DAMA/LIBRA and CoGeNT experiments' data as WIMP signals, and exclude new parameter space for WIMP masses between 3 GeV/c^2 and 4 GeV/c^2.

Figures (12)

• Figure 1: (color online). Expected differential event rates for 5 GeV/$c^2$ (top panel) and 100 GeV/$c^2$ (bottom panel) WIMPs scattering from Ge (blue/solid) and Si (red/dashed) targets. All event rate calculations are based on the standard halo model described in halo1, for an arbitrarily chosen WIMP-nucleon cross section of 1x10$^{-41}$ cm$^{2}$. Each energy spectrum cuts off abruptly at a maximum recoil energy due to the assumed galactic escape velocity. We use the 544 km/s galactic escape velocity from rave1, while all other halo parameters are taken from halo2, and the local WIMP density is assumed to be 0.3 GeV/cm$^{3}$.
• Figure 2: (color online). Hardware trigger efficiency estimate (error bars) for a representative Ge detector (Z2 3V data) as a function of $Y_{\mathrm{NR}}$-corrected recoil energy. The efficiency is calculated in bins of 0.25 keV by dividing the distribution of energies correlated to the presence of logical pulses in the post-trigger history (green/light solid) by the distribution of all reconstructed energies (black/dark solid). The efficiency scale is given by the y-axis on the right (blue/dash-dotted grid lines), while the scale for the histograms is given by the y-axis on the left (black/dotted grid lines). A split-width error function (red/dashed) fits the efficiency estimate over the full range of energies, yielding a hardware threshold of 0.74 keV at 50% efficiency. For this detector the hardware trigger is 100% efficient for energies above $\sim$3 keV.
• Figure 3: (color online). Fits to binned estimates of the hardware (orange/light solid) and software (blue/dashed) phonon energy threshold efficiencies are multiplied, yielding the combined threshold efficiencies (black/dark solid) for typical Ge (top panel, Z5 6V data) and Si (bottom panel, Z4 3V data) detectors as a function of $Y_{\mathrm{NR}}$-corrected recoil energy.
• Figure 4: (color online). Ionization yield plotted as a function of recoil energy for representative Ge (Z3 6V data) $^{252}$Cf neutron calibration data, with the average ionization yield (blue/dashed), 2$\sigma$ nuclear-recoil band (blue/dark solid) and software phonon threshold (yellow/light solid) overlaid. Unvetoed single scatters passing the fiducial-volume and data-quality cuts are displayed for events consistent with the nuclear-recoil criterion (orange/light dots), and some that are not (black/dark dots). A substantial gamma-ray flux from the $^{252}$Cf source populates the "electron-recoil band" near an ionization yield of 1. Two lines at $\sim$10.4 keV and $\sim$66.7 keV resulting from decays of Ge isotopes can be distinguished among the electron-recoil events. As described in Section \ref{['sec:3c']}, the low number of events above the nuclear-recoil band between 2 keV and 6 keV recoil energy qualitatively demonstrates that channeling does not significantly diminish this experiment's efficiency for detecting low-mass WIMPs.
• Figure 5: (color online). The detection efficiencies associated with the hardware and software phonon thresholds (top panel, Si Z6 3V data) and the combined analysis cuts (bottom panel, Ge Z2 6V data) for representative detectors. The best-fit mean efficiency (orange/light solid) is compared in each case to its 90% statistical lower limit (blue/dark solid). The combined threshold efficiencies are plotted versus $Y_{\mathrm{NR}}$-corrected recoil energy, while the combined efficiency of the analysis cuts is plotted as a function of $Q$-corrected recoil energy. The step in the latter is due to the larger muon-veto window chosen for the low-energy events. Due to the uncertainty introduced by the efficiencies' statistical errors, we conservatively used the 90% lower limit (1.28$\sigma$ below the mean) when calculating upper limits on the WIMP-nucleon cross section.