Results from a Low-Energy Analysis of the CDMS II Germanium Data

TL;DR

This work reanalyzes CDMS II data from eight Ge detectors with a lowered recoil-energy threshold of 2 keV to enhance sensitivity to WIMPs with masses $m_\chi \lesssim 10~\mathrm{GeV}/c^2$. The analysis characterizes backgrounds (zero-charge and surface events) and reconstructs nuclear-recoil energies via phonons with Neganov-Luke corrections, using calibration lines and an ionization-yield model, and sets 90% C.L. upper limits on the spin-independent WIMP-nucleon cross section $\sigma_{SI}$ with the Yellin optimum-interval method. These limits strengthen CDMS II constraints below about $m_\chi \lesssim 9~\mathrm{GeV}/c^2$ and disfavor interpretations of the DAMA/LIBRA and CoGeNT low-mass signals under standard halo assumptions. The results show incompatibility with a low-mass WIMP explanation for the CoGeNT excess without invoking larger systematic uncertainties in the background, and emphasize the importance of background modeling at low energy.

Abstract

We report results from a reanalysis of data from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Data taken between October 2006 and September 2008 using eight germanium detectors are reanalyzed with a lowered, 2 keV recoil-energy threshold, to give increased sensitivity to interactions from Weakly Interacting Massive Particles (WIMPs) with masses below ~10 GeV/c^2. This analysis provides stronger constraints than previous CDMS II results for WIMP masses below 9 GeV/c^2 and excludes parameter space associated with possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments.

Figures (8)

• Figure 1: (color online). Comparison of the energy spectra for the candidate events and background estimates, co-added over the 8 detectors used in this analysis. The observed event rate (error bars) agrees well with the electron-recoil background estimate (solid), which is a sum of the contributions from zero-charge events (dashed), surface events (+), bulk events (dash-dotted), and the 1.3 keV line (dotted). The selection efficiencies have been applied to the background estimates for direct comparison with the observed rate, which does not include a correction for the nuclear-recoil acceptance. The inset shows the measured nuclear-recoil acceptance efficiency, averaged over all detectors.
• Figure 2: (color online). Events in the ionization-yield versus recoil-energy plane for T1Z5. Events within the $(+1.25,-0.5)$$\sigma$ nuclear-recoil band (solid) are WIMP candidates (large dots). Events outside these bands (small, dark dots) pass all selection criteria except the ionization-energy requirement. The widths of the band edges denote variations between data runs. Events from the $^{252}$Cf calibration data are also shown (small, light dots). The recoil-energy scale assumes the ionization signal is consistent with a nuclear recoil, causing electron recoils to be shifted to higher recoil energies and lower yields.
• Figure 3: (color online). Top: comparison of the spin-independent (SI) exclusion limits from these data (solid) to previous results in the same mass range (all at 90% C.L.). Limits from a low-threshold analysis of the CDMS shallow-site data Akerib:2010rr (dashed), CDMS II Ge results with a 10 keV threshold CDMSScience:2010 (dash-dotted), recalculated for lower WIMP masses, and XENON100 with constant (+) or decreasing ($\square$) scintillation-efficiency extrapolations at low energy Aprile:2010xx are also shown. The filled regions indicate possible signal regions from DAMA/LIBRA Bernabei:2008yiHooper:2010ly (dark), CoGeNT (light) Aalseth:2010vxHooper:2010ly, and a combined fit to the DAMA/LIBRA and CoGeNT data Hooper:2010ly (hatched). Bottom: comparison of the WIMP-neutron spin-dependent (SD) exclusion limits from these data (solid), CDMS II Ge results with a 10 keV threshold (dash-dotted), XENON10 Angle:2008uq ($\triangle$), and CRESST Angloher:2002*Savage:2004 ($\ocircle$). The filled region denotes the 99.7% C.L. DAMA/LIBRA allowed region for neutron-only scattering Savage:2008er. An escape velocity of 544 km/s was used for the CDMS and XENON100 exclusion limits, whereas the other results assume an escape velocity from 600–650 km/s. Using the same halo parameters as assumed for the allowed regions would lead to slightly stronger limits (dotted).
• Figure 4: Efficiency-corrected phonon recoil-energy spectrum for electron recoils in T1Z5. The solid lines show fits to the location of the activation lines, which give mean values of 1.333$\pm$0.025 keVee and 10.391$\pm$0.022 keVee. The resolution of the 1.3 keV line is $\sim$100 eVee, consistent with the expected resolution from noise. The 10.4 keV line is broadened by position dependence of the phonon signal for which no correction has been applied. Cosmogenic $^{65}$Zn has decayed away sufficiently that its contribution to the low energy tail of the activation peaks is negligible Ahmed:2009rh. The relative intensity of the lines measured from the fits is 0.145$\pm$0.030, consistent with expectations.
• Figure 5: Events in the ionization energy vs. recoil energy plane for T1Z5. Events from the $^{252}$Cf calibration data (small, gray dots) and WIMP search data (large, black dots) are shown. The recoil energy scale is given by the total phonon energy minus the Neganov-Luke phonon contribution corresponding to the mean ionization for nuclear recoils. This scale, in units of keVnr, gives the correct recoil energy only for nuclear recoils, while electron recoils appear at higher recoil energy due to the larger contribution of Neganov-Luke phonons. The solid lines show fits to the mean of the nuclear recoils (green) and electron recoils (blue). The red dotted curves indicate lines of constant recoil energy when the ionization signal is used to determine the Neganov-Luke phonon contribution.