No abstract

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 .

New data are reported from the operation of a 2 liter C3F8 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 C3F8 provides excellent electron-recoil and alpha rejection capabilities at very low thresholds. The chamber exhibits an electron-recoil sensitivity of <3.5×10(-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.

We have directly measured the energy threshold and efficiency for bubble nucleation from iodine recoils in a CF3I bubble chamber in the energy range of interest for a dark matter search. These interactions cannot be probed by standard neutron calibration methods, so we develop a new technique by observing the elastic scattering of 12 GeV/c negative pions. The pions are tracked with a silicon pixel telescope and the reconstructed scattering angle provides a measure of the nuclear recoil kinetic energy. The bubble chamber was operated with a nominal threshold of (13.6 ± 0.6) keV. Interpretation of the results depends on the response to fluorine and carbon recoils, but in general we find agreement with the predictions of the classical bubble nucleation theory. This measurement confirms the applicability of CF3I as a target for spin-independent dark matter interactions and represents a novel technique for calibration of superheated fluid detectors. PACS numbers: 29.40.-n, 95.35.+d, 95.30.Cq, FERMILAB-PUB-10-318-A-CD-ERecent years have seen a resurgence in the use of superheated liquids and bubble chambers as continuously sensitive nuclear recoil detectors searching for dark matter in the form of Weakly Interacting Massive Particles (WIMPs) [1][2][3]. At a low degree of superheat, bubble chambers are insensitive to minimum ionizing backgrounds that normally plague WIMP searches but retain sensitivity to the nuclear recoils that would be characteristic of WIMP scattering. In a superheated liquid the process of radiation-induced bubble nucleation is described by the classical "hot spike" model [4]. For the phase transition to occur, the energy deposited by the particle must create a critically sized bubble, requiring a minimum energy deposition in a volume smaller than the critical bubble. Under mildly superheated conditions, the latter requirement renders the bubble chamber insensitive to minimum ionizing particles.The radius of the critical bubble is given by the condition that the bubble be in (unstable) equilibrium with the surrounding superheated fluid [5]. This demands the pressure balancewhere P b is the pressure inside the bubble, P l is the pressure in the liquid, σ is the bubble surface tension, and r c is the critical bubble radius. The pressure P b is fixed by the condition that the chemical potential inside and outside the bubble be equal, givingwhere P sat is the pressure in a saturated system at the given temperature, and ρ l and ρ v are the liquid and vapor densities in the saturated system [6].In Seitz's "hot spike" model for bubble nucleation, the entire energy necessary to create the critical bubble must come from the particle interaction that nucleates the bubble. This is in contrast to earlier models that required only the work (free energy) to come from the particle interaction, with the remaining bubble-formation energy supplied by heat flowing in from the surrounding superheated fluid [7]. As the name "hot spike" implies, the nucleation site in Seitz's model begins as a hightemperature seed, so i...