Dark Matter Search Results from the PICO-2L ${\mathrm{C}}_3{\mathrm{F}}_8$ Bubble Chamber

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.

Figure 1

A schematic of the PICO-2L bubble chamber.

Figure 2

$AP$ distributions for neutron calibration data (black) and WIMP search data (red) at 4.4-keV threshold. Note that the $x$ axis shows $\ln (AP)$. As discussed in the text, alphas from the ${}^{222}\mathrm{Rn}$ decay chain can be identified by their time signature and populate the two peaks in the WIMP search data at high $AP$, with higher energy alphas from ${}^{214}\mathrm{Po}$ producing larger acoustic signals.

Figure 3

The green points show the observed rates of single and multiple bubbles for the calibration sources at a thermodynamic threshold of 3.2 keV. Green error bars indicate statistical uncertainties, and the black error bars at the bottom show the systematic uncertainty on the neutron flux (a flat percent uncertainty that is common to all multiplicities in a given data set at the 10%, 12%, and 30% levels for 61-keV, 97-keV and AmBe data, respectively). The black histograms show the predicted rates from the simulation given the best-fit efficiency model derived from all calibration data.

Figure 4

The best-fit fluorine (black) and carbon (red) efficiency curves for 3.2-keV data are shown by the solid lines. The dashed lines show the curves used to determine sensitivity for a 5 GeV SI WIMP. The light blue band shows the calculated Seitz threshold with the experimental and theoretical uncertainties from Table 1 added in quadrature.

Figure 5

The CDF of the TPNT for events with random timing (simulated WIMP-like events) and the 3.2-keV candidate events. The two distributions are not consistent with each other.

Figure 6

The 90% C.L. limit on the SD WIMP-proton cross section from PICO-2L is plotted in red, along with limits from COUPP (light blue region), PICASSO (dark blue), SIMPLE (green), XENON100 (orange), IceCube (dashed and solid pink), SuperK (dashed and solid black), CMS (dashed orange), and ATLAS (dashed purple) [7, 9, 10, 26, 27, 28, 29, 30]. For the IceCube and SuperK results, the dashed lines assume annihilation to $W$ pairs while the solid lines assume annihilation to $b$ quarks. Comparable limits assuming these and other annihilation channels are set by the ANTARES, Baikal, and Baksan neutrino telescopes [31, 32, 33]. The CMS and ATLAS limits assume an effective field theory, valid for a heavy mediator. The purple region represents parameter space of the constrained minimal supersymmetric model of [25].

Figure 7

The 90% C.L. limit on the SI WIMP-nucleon cross section from PICO-2L is plotted in red, along with limits from PICASSO (blue), LUX (black), CDMS-lite and SuperCDMS (dashed purple) [9, 34, 35, 36]. Similar limits that are not shown for clarity are set by XENON10, XENON100, and CRESST-II [37, 38, 39]. Allowed regions from DAMA (hashed brown), CoGeNT (solid green), and CDMS-Si (hashed pink) are also shown [40, 41, 42].