First Dark Matter Constraints from a SuperCDMS Single-Charge Sensitive Detector

We present the first limits on inelastic electron-scattering dark matter and dark photon absorption using a prototype SuperCDMS detector having a charge resolution of 0.1 electron-hole pairs (CDMS HVeV, a 0.93 g CDMS high-voltage device). These electron-recoil limits significantly improve experimental constraints on dark matter particles with masses as low as $1\mathrm{MeV}/c^2$. We demonstrate a sensitivity to dark photons competitive with other leading approaches but using substantially less exposure (0.49 g d). These results demonstrate the scientific potential of phonon-mediated semiconductor detectors that are sensitive to single electronic excitations.

Figure 1

Laser calibration data showing a resolution of $\sim 0.07$ $e^{-}{h}^{+}$ pairs for a short laser-triggered acquisition at 150 V. In the series shown in this figure, a lower DR temperature allowed for a 30% improvement in energy resolution as compared to the average value during the science exposure. Both the between-peak event rate and the energy resolution are significantly improved compared to the previous result in Ref. [14]. For this calibration series, the mean photon number ($\lambda$) was 1.0 to increase statistics near zero in the short acquisition, while the science exposure used $\lambda \approx 2$ to cover the full energy range of interest. The model curve is a maximum likelihood fit of photon distribution and charge transport parameters, with results described in the text and Ref. [14].

Figure 2

Top: Event rate as a function of bias before and after prebias. Bottom: Event rate during the science exposure as a function of the time. Neutralization [14] was performed at hour 70 (solid line), and the polarity was reversed at hour 90 (dashed line). Data points represent blocks with a fixed number of events to ensure uniform vertical error bars, with large horizontal error bars corresponding to runs separated by gaps in data taking.

Figure 3

Top: Event rate for calibration (black) and science exposure (magenta) with live time and quality cuts applied. Also shown are an impact ionization background Monte Carlo model (orange) and the signal distribution for an excluded dark photon model (dotted line) assuming $m_V=9.4\mathrm{eV}$ and ${ϵ}_{\mathrm{eff}}=5\times {10}^{-13}$ ($ϵ\approx 2{ϵ}_{\mathrm{eff}}$ at 9.4 eV); the ERDM signals excluded have a similar form. Bottom: Measured cut efficiency as a function of the number of $e^{-}{h}^{+}$ pairs along with the efficiency model used in sensitivity estimates. The dashed line in both plots shows the 50% analysis efficiency at 0.7 $e^{-}{h}^{+}$ pairs.

Figure 4

Top: Limits on dark photon kinetic mixing compared to the results from DAMIC, XENON10, and XENON100 (see [16] and references therein). Middle (bottom): Limit on DM interacting with electrons via a heavy dark photon ($F_{\mathrm{DM}}=1$) [ultralight dark photon ($F_{\mathrm{DM}}\propto 1/q^2$)] compared to the XENON10 results [17]. The red line is the limit curve with a Fano factor of 0.155. The salmon-colored region indicates the systematic uncertainties due to varying the Fano factor in the ionization model between the lowest mathematically possible value and 1, as well as from uncertainties in the photoelectric cross section for dark photon absorption. For signal models as well as additional astrophysical constraints, see Ref. [1].