We measured the nuclear--recoil ionization yield in silicon with a cryogenic phonon-sensitive gram-scale detector. Neutrons from a mono-energetic beam scatter off of the silicon nuclei at angles ...corresponding to energy depositions from 4\,keV down to 100\,eV, the lowest energy probed so far. The results show no sign of an ionization production threshold above 100\,eV. These results call for further investigation of the ionization yield theory and a comprehensive determination of the detector response function at energies below the keV scale.
Recent experiments searching for sub-GeV/\(c^2\) dark matter have observed event excesses close to their respective energy thresholds. Although specific to the individual technologies, the measured ...excess event rates have been consistently reported at or below event energies of a few-hundred eV, or with charges of a few electron-hole pairs. In the present work, we operated a 1-gram silicon SuperCDMS-HVeV detector at three voltages across the crystal (0 V, 60 V and 100 V). The 0 V data show an excess of events in the tens of eV region. Despite this event excess, we demonstrate the ability to set a competitive exclusion limit on the spin-independent dark matter--nucleon elastic scattering cross section for dark matter masses of \(\mathcal{O}(100)\) MeV/\(c^2\), enabled by operation of the detector at 0 V potential and achievement of a very low \(\mathcal{O}(10)\) eV threshold for nuclear recoils. Comparing the data acquired at 0 V, 60 V and 100 V potentials across the crystal, we investigated possible sources of the unexpected events observed at low energy. The data indicate that the dominant contribution to the excess is consistent with a hypothesized luminescence from the printed circuit boards used in the detector holder.
Two photo-neutron sources, \(^{88}\)Y\(^{9}\)Be and \(^{124}\)Sb\(^{9}\)Be, have been used to investigate the ionization yield of nuclear recoils in the CDMSlite germanium detectors by the SuperCDMS ...collaboration. This work evaluates the yield for nuclear recoil energies between 1 keV and 7 keV at a temperature of \(\sim\) 50 mK. We use a Geant4 simulation to model the neutron spectrum assuming a charge yield model that is a generalization of the standard Lindhard model and consists of two energy dependent parameters. We perform a likelihood analysis using the simulated neutron spectrum, modeled background, and experimental data to obtain the best fit values of the yield model. The ionization yield between recoil energies of 1 keV and 7 keV is shown to be significantly lower than predicted by the standard Lindhard model for germanium. There is a general lack of agreement among different experiments using a variety of techniques studying the low-energy range of the nuclear recoil yield, which is most critical for interpretation of direct dark matter searches. This suggests complexity in the physical process that many direct detection experiments use to model their primary signal detection mechanism and highlights the need for further studies to clarify underlying systematic effects that have not been well understood up to this point.
CDMSlite Run 2 was a search for weakly interacting massive particles (WIMPs) with a cryogenic 600 g Ge detector operated in a high-voltage mode to optimize sensitivity to WIMPs of relatively low mass ...from 2 - 20 GeV/\(c^2\). In this article, we present an effective field theory (EFT) analysis of the CDMSlite Run 2 data using an extended energy range and a comprehensive treatment of the expected background. A binned likelihood Bayesian analysis was performed on the recoil energy data, taking into account the parameters of the EFT interactions and optimizing the data selection with respect to the dominant background components. Energy regions within 5\(\sigma\) of known activation peaks were removed from the analysis. The Bayesian evidences resulting from the different operator hypotheses show that the CDMSlite Run 2 data are consistent with the background-only models and do not allow for a signal interpretation assuming any additional EFT interaction. Consequently, upper limits on the WIMP mass and coupling-coefficient amplitudes and phases are presented for each EFT operator. These limits improve previous CDMSlite Run 2 bounds for WIMP masses above 5 GeV/\(c^2\).
The Cryogenic Dark Matter Search low ionization threshold experiment (CDMSlite) achieved efficient detection of very small recoil energies in its germanium target, resulting in sensitivity to Lightly ...Ionizing Particles (LIPs) in a previously unexplored region of charge, mass, and velocity parameter space. We report first direct-detection limits calculated using the optimum interval method on the vertical intensity of cosmogenically-produced LIPs with an electric charge smaller than \(e/(3\times10^5\)), as well as the strongest limits for charge \(\leq e/160\), with a minimum vertical intensity of \(1.36\times10^{-7}\)\,cm\(^{-2}\)s\(^{-1}\)sr\(^{-1}\) at charge \(e/160\). These results apply over a wide range of LIP masses (5\,MeV/\(c^2\) to 100\,TeV/\(c^2\)) and cover a wide range of \(\beta\gamma\) values (0.1 -- \(10^6\)), thus excluding non-relativistic LIPs with \(\beta\gamma\) as small as 0.1 for the first time.
We present limits on spin-independent dark matter-nucleon interactions using a \(10.6\) \(\mathrm{g}\) Si athermal phonon detector with a baseline energy resolution of \(\sigma_E=3.86 \pm 0.04\) ...\((\mathrm{stat.})^{+0.19}_{-0.00}\) \((\mathrm{syst.})\) \(\mathrm{eV}\). This exclusion analysis sets the most stringent dark matter-nucleon scattering cross-section limits achieved by a cryogenic detector for dark matter particle masses from \(93\) to \(140\) \(\mathrm{MeV}/c^2\), with a raw exposure of \(9.9\) \(\mathrm{g}\cdot\mathrm{d}\) acquired at an above-ground facility. This work illustrates the scientific potential of detectors with athermal phonon sensors with eV-scale energy resolution for future dark matter searches.
This article presents an analysis and the resulting limits on light dark matter inelastically scattering off of electrons, and on dark photon and axion-like particle absorption, using a ...second-generation SuperCDMS high-voltage eV-resolution detector. The 0.93 gram Si detector achieved a 3 eV phonon energy resolution; for a detector bias of 100 V, this corresponds to a charge resolution of 3% of a single electron-hole pair. The energy spectrum is reported from a blind analysis with 1.2 gram-days of exposure acquired in an above-ground laboratory. With charge carrier trapping and impact ionization effects incorporated into the dark matter signal models, the dark matter-electron cross section \(\bar{\sigma}_{e}\) is constrained for dark matter masses from 0.5--\(10^{4} \)MeV\(/c^{2}\); in the mass range from 1.2--50 eV\(/c^{2}\) the dark photon kinetic mixing parameter \(\varepsilon\) and the axioelectric coupling constant \(g_{ae}\) are constrained. The minimum 90% confidence-level upper limits within the above mentioned mass ranges are \(\bar{\sigma}_{e}\,=\,8.7\times10^{-34}\) cm\(^{2}\), \(\varepsilon\,=\,3.3\times10^{-14}\), and \(g_{ae}\,=\,1.0\times10^{-9}\).
We present an analysis of electron recoils in cryogenic germanium detectors operated during the SuperCDMS Soudan experiment. The data are used to set new constraints on the axioelectric coupling of ...axion-like particles and the kinetic mixing parameter of dark photons, assuming the respective species constitutes all of the galactic dark matter. This study covers the mass range from 40 eV/\(c^2\) to 500 eV/\(c^2\) for both candidates, excluding previously untested parameter space for masses below ~1 keV/\(c^2\). For the kinetic mixing of dark photons, values below \(10^{-15}\) are reached for particle masses around 100 eV/\(c^2\); for the axioelectric coupling of axion-like particles, values below \(10^{-12}\) are reached for particles with masses in the range of a few-hundred eV/\(c^2\).
Direct dark matter detection experiments based on a liquid xenon target are leading the search for dark matter particles with masses above \(\sim\) 5 GeV/c\(^2\), but have limited sensitivity to ...lighter masses because of the small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreducible contribution from inelastic processes accompanying the elastic scattering, which leads to the excitation and ionization of the recoiling atom (the Migdal effect) or the emission of a Bremsstrahlung photon. In this letter, we report on a probe of low-mass dark matter with masses down to about 85 MeV/c\(^2\) by looking for electronic recoils induced by the Migdal effect and Bremsstrahlung, using data from the XENON1T experiment. Besides the approach of detecting both scintillation and ionization signals, we exploit an approach that uses ionization signals only, which allows for a lower detection threshold. This analysis significantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach.
We report constraints on light dark matter (DM) models using ionization signals in the XENON1T experiment. We mitigate backgrounds with strong event selections, rather than requiring a scintillation ...signal, leaving an effective exposure of \((22 \pm 3)\) tonne-days. Above \(\sim\!0.4\,\mathrm{keV}_\mathrm{ee}\), we observe \(<1 \, \text{event}/(\text{tonne} \times \text{day} \times \text{keV}_\text{ee})\), which is more than one thousand times lower than in similar searches with other detectors. Despite observing a higher rate at lower energies, no DM or CEvNS detection may be claimed because we cannot model all of our backgrounds. We thus exclude new regions in the parameter spaces for DM-nucleus scattering for DM masses \(m_\chi\) within \(3-6\,\mathrm{GeV}/\mathrm{c}^2\), DM-electron scattering for \(m_\chi > 30\,\mathrm{MeV}/\mathrm{c}^2\), and absorption of dark photons and axion-like particles for \(m_\chi\) within \(0.186 - 1 \, \mathrm{keV}/\mathrm{c}^2\).
