New results are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 52 kg of C_{3}F_{8} located in the SNOLAB underground laboratory. As in previous PICO bubble chambers, PICO-60 C_{3}F_{8} exhibits excellent electron recoil and alpha decay rejection, and the observed multiple-scattering neutron rate indicates a single-scatter neutron background of less than one event per month. A blind analysis of an efficiency-corrected 1167-kg day exposure at a 3.3-keV thermodynamic threshold reveals no single-scattering nuclear recoil candidates, consistent with the predicted background. These results set the most stringent direct-detection constraint to date on the weakly interacting massive particle (WIMP)-proton spin-dependent cross section at 3.4×10^{-41}  cm^{2} for a 30-GeV c^{-2} WIMP, more than 1 order of magnitude improvement from previous PICO results.

Final results are reported from operation of the PICO-60 C3F8 dark matter detector, a bubble chamber filled with 52 kg of C3F8 located in the SNOLAB underground laboratory. The chamber was operated at thermodynamic thresholds as low as 1.2 keV without loss of stability. A new blind 1404-kg-day exposure at 2.45 keV threshold was acquired with approximately the same expected total background rate as the previous 1167-kg-day exposure at 3.3 keV. This increased exposure is enabled in part by a new optical tracking analysis to better identify events near detector walls, permitting a larger fiducial volume. These results set the most stringent direct-detection constraint to date on the WIMP-proton spin-dependent cross section at 2.5 × 10 −41 cm 2 for a 25 GeV WIMP, and improve on previous PICO results for 3-5 GeV WIMPs by an order of magnitude.

We report direct-detection constraints on light dark matter particles interacting with electrons. The results are based on a method that exploits the extremely low levels of leakage current of the DAMIC detector at SNOLAB of 2-6×10 −22 A cm −2 . We evaluate the charge distribution of pixels that collect < 10 e − for contributions beyond the leakage current that may be attributed to dark matter interactions. Constraints are placed on so-far unexplored parameter space for dark matter masses between 0.6 and 100 MeV c −2 . We also present new constraints on hidden-photon dark matter with masses in the range 1.2-30 eV c −2 .There is overwhelming astrophysical and cosmological evidence for Dark Matter (DM) as a major constituent of the universe. Still, its nature remains elusive. The compelling Weakly Interacting Massive Particle (WIMP) dark matter paradigm [1] -implying DM is made of hitherto unknown particles with mass in the GeV-TeV scale -has been intensely scrutinized during the last two decades by detectors up to the tonne-scale looking for nuclear recoils induced by coherent scattering of WIMPs. Despite the impressive improvements in sensitivity, notably by noble liquid experiments [2], WIMPs have so far escaped detection. Other viable candidates include DM particles from a hidden-sector [3], which couple weakly with ordinary matter through, for example, mixing of a hidden-photon with an ordinary photon [4]. A phenomenological consequence is that hiddensector DM particles also interact with electrons, with sufficiently large energy transfers to be detectable down to DM masses of ≈ MeV [5]. Also, eV-mass hidden-photon DM particles can be probed through absorption by electrons in detection targets [The DAMIC (Dark Matter in CCDs) experiment [7] is well-suited for a sensitive search of this class of DM candidates. DAMIC detects ionization events induced in the bulk silicon of thick, fully depleted Charge Coupled Devices (CCDs). By exploiting the charge resolution of the CCDs (≈ 2 e − ) and their extremely low leakage current (≈ 4 e − mm −2 d −1 ), DAMIC has already placed constraints on hidden-photon DM with masses in the range 1.2-30 eV c −2 [8] with data collected during the experiment's commissioning phase. In this Letter we apply a similar approach to explore DM-e − interactions with high-quality data from the DAMIC science run at the SNOLAB underground laboratory. We also present improved limits on hidden-photon DM particles.To model DM-e − interactions we follow Ref.[9] where the bound nature of the electrons and crystalline band structure of the target are properly taken into account. The differential event rate in the detector for a DM mass m χ , with transferred energy E e , and momentum q is parametrized as dR dE e ∝σ e dq q 2 η(m χ , q, E e )|F DM (q)| 2 |f c (q, E e )| 2 , (1) whereσ e is a reference cross section for free electron arXiv:1907.12628v1 [astro-ph.CO]

New data are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 36.8 kg of CF3I and located in the SNOLAB underground laboratory. PICO-60 is the largest bubble chamber to search for dark matter to date. With an analyzed exposure of 92.8 live-days, PICO-60 exhibits the same excellent background rejection observed in smaller bubble chambers. Alpha decays in PICO-60 exhibit frequency-dependent acoustic calorimetry, similar but not identical to that reported recently in a C3F8 bubble chamber. PICO-60 also observes a large population of unknown background events, exhibiting acoustic, spatial, and timing behaviors inconsistent with those expected from a dark matter signal. These behaviors allow for analysis cuts to remove all background events while retaining 48.2% of the exposure. Stringent limits on WIMPs interacting via spin-dependent proton and spin-independent processes are set, and most interpretations of the DAMA/LIBRA modulation signal as dark matter interacting with iodine nuclei are ruled out.

New data are reported from a second run of the 2-liter PICO-2L C 3 F 8 bubble chamber with a total exposure of 129 kg-days at a thermodynamic threshold energy of 3.3 keV. These data show that measures taken to control particulate contamination in the superheated fluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear-recoil event was observed in the data, consistent both with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron-recoil and alpha decay rejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle (WIMP)-proton spindependent scattering to date for WIMP masses < 50 GeV=c 2 .