Dark Matter Search Results from the PICO-60 C$_3$F$_8$ Bubble Chamber

TL;DR

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.

Abstract

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 1 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 WIMP-proton spin-dependent cross section at 3.4 $\times$ 10$^{-41}$ cm$^2$ for a 30-GeV$\thinspace$c$^{-2}$ WIMP, more than one order of magnitude improvement from previous PICO results.

Figures (6)

• Figure 1: Spatial distribution of single-bubble events in the WIMP search data. Z is the reconstructed vertical position of the bubble, and R$^{2}$/R$_{\mathrm{jar}}$ is the distance from the center axis squared, normalized by the nominal jar radius (145 mm). The fiducial cut is represented by the dashed line. Red squares are the 106 single bulk bubbles passing all cuts prior to acoustic unblinding and grey dots are all rejected single-bubble events.
• Figure 2: Top: AP distributions for AmBe and $^{252}$Cf neutron calibration data (black) and WIMP search data (red) at 3.3 keV threshold. Bottom: AP and NN score for the same dataset. The acceptance region for nuclear recoil candidates, defined before WIMP search acoustic data unmasking using neutron calibration data, are displayed with dashed lines and reveal no candidate events in the WIMP search data. Alphas from the $^{222}$Rn decay chain can be identified by their time signature and populate the two peaks in the WIMP search data at high AP. Higher energy alphas from $^{214}$Po are producing larger acoustic signals.
• Figure 3: The 90% C.L. limit on the SD WIMP-proton cross section from PICO-60 C$_3$F$_8$ plotted in thick blue, along with limits from PICO-60 CF$_3$I (thick red) 30l_13, PICO-2L (thick purple) 2l_15, PICASSO (green band) PICASSOFinallimit, SIMPLE (orange) SIMPLE, PandaX-II (cyan) PANDAX-II, IceCube (dashed and dotted pink) ICECUBElimit, and SuperK (dashed and dotted black) SKlimitSKlimit2. The indirect limits from IceCube and SuperK assume annihilation to $\tau$ leptons (dashed) and b quarks (dotted). The purple region represents parameter space of the constrained minimal supersymmetric model of SDblob. Additional limits, not shown for clarity, are set by LUX LUX_SD and XENON100 XENON100 (comparable to PandaX-II) and by ANTARES Ant1Ant2 (comparable to IceCube).
• Figure 4: The 90% C.L. limit on the SI WIMP-nucleon cross-section from PICO-60 C$_3$F$_8$ plotted in thick blue, along with limits from PICO-60 CF$_3$I (thick red) 30l_13, PICO-2L (thick purple) 2l_15, LUX (yellow) LUX2017, PandaX-II (cyan) PANDAX-II_SI, CRESST-II (magenta) CRESST, and CDMS-lite (black) CDMSlite. While we choose to highlight this result, LUX sets the strongest limits on WIMP masses greater than 6 GeV/c$^2$. Additional limits, not shown for clarity, are set by PICASSO PICASSOFinallimit, XENON100 XENON100, DarkSide-50 DarkSide50, SuperCDMS SuperCDMS, CDMS-II CDMSII, and Edelweiss-III Edelweiss.
• Figure 5: PICO-60 constraints (blue) on the effective spin-dependent WIMP-proton and WIMP-neutron couplings, $a_p$ and $a_n$, for a 50 GeV/c$^2$ WIMP mass. Parameter space outside of the band is excluded. Also shown are results from PANDAX-II (cyan) PANDAX-II, LUX (yellow) LUX_SD, PICO-2L (purple) 2l_15, and PICO-60 C$_3$FI (red) 30l_13.