New results are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 52 kg of ${\mathrm{C}}_3{\mathrm{F}}_8$ located in the SNOLAB underground laboratory. As in previous PICO bubble chambers, $\mathrm{PICO}\text{−}60{\mathrm{C}}_3{\mathrm{F}}_8$ 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 $3.4\times {10}^{-41}{\mathrm{cm}}^2$ for a $30\text{−}\mathrm{GeV}{\mathrm{c}}^{-2}$ WIMP, more than 1 order of magnitude improvement from previous PICO results.

New data are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 36.8 kg of ${\mathrm{CF}}_3\mathrm{I}$ 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 livedays, 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 ${\mathrm{C}}_3{\mathrm{F}}_8$ 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 weakly interacting massive particles 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 the operation of a 2 liter ${\mathrm{C}}_3{\mathrm{F}}_8$ bubble chamber in the SNOLAB underground laboratory, with a total exposure of 211.5 kg days at four different energy thresholds below 10 keV. These data show that ${\mathrm{C}}_3{\mathrm{F}}_8$ provides excellent electron-recoil and alpha rejection capabilities at very low thresholds. The chamber exhibits an electron-recoil sensitivity of $<3.5\times 10^{-10}$ and an alpha rejection factor of $>98.2\%$. These data also include the first observation of a dependence of acoustic signal on alpha energy. Twelve single nuclear recoil event candidates were observed during the run. The candidate events exhibit timing characteristics that are not consistent with the hypothesis of a uniform time distribution, and no evidence for a dark matter signal is claimed. These data provide the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date, with significant sensitivity at low WIMP masses for spin-independent WIMP-nucleon scattering.

New data are reported from a second run of the 2-liter PICO-2L ${\mathrm{C}}_3{\mathrm{F}}_8$ 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 spin-dependent scattering to date for WIMP masses $<50\mathrm{GeV}/{\mathrm{c}}^2$.

There are currently several existing and proposed experiments designed to probe sub-GeV dark matter (DM) using electron ionization in various materials. The projected signal rates for these experiments assume that this ionization yield arises only from DM scattering directly off electron targets, ignoring secondary ionization contributions from DM scattering off nuclear targets. We investigate the validity of this assumption and show that if sub-GeV DM couples with comparable strength to both protons and electrons, as would be the case for a dark photon mediator, the ionization signal from atomic scattering via the Migdal effect scales with the atomic number $Z$ and 3-momentum transfer $\mathbf{q}$ as $Z^2{\mathbf{q}}^2$. The result is that the Migdal effect is always subdominant to electron scattering when the mediator is light, but that Migdal-induced ionization can dominate over electron scattering for heavy mediators and DM masses in the hundreds of MeV range. We put these two ionization processes on identical theoretical footing, address some theoretical uncertainties in the choice of atomic wave functions used to compute rates, and discuss the implications for DM scenarios where the Migdal process dominates, including for XENON10, XENON100, and the recent XENON1T results on light DM scattering.

We present a novel unifying interpretation of excess event rates observed in several dark matter direct-detection experiments that utilize single-electron threshold semiconductor detectors. Despite their different locations, exposures, readout techniques, detector composition, and operating depths, these experiments all observe statistically significant excess event rates of $\sim 10\mathrm{Hz}/\mathrm{kg}$. However, none of these persistent excesses has yet been reported as a dark matter signal because individually, each can be attributed to different well-motivated but unmodeled backgrounds, and taken together, they cannot be explained by dark matter particles scattering elastically off detector nuclei or electrons. We show that these results can be reconciled if the semiconductor detectors are seeing a collective inelastic process, consistent with exciting a plasmon. We further show that plasmon excitation could arise in two compelling dark matter scenarios, both of which can explain rates of existing signal excesses in germanium and, at least at the order of magnitude level, across several single-electron threshold detectors. At least one of these scenarios also yields the correct relic density from thermal freeze-out. Both dark matter scenarios motivate a radical rethinking of the standard interpretations of dark matter-electron scattering from recent experiments.