Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CF$_3$I Bubble Chamber

TL;DR

Data from the operation of a bubble chamber filled with 3.5 kg of CF3I in a shallow underground site confirm that alpha decays generate a significantly louder acoustic emission than single nuclear recoils, leading to an efficient background discrimination.

Abstract

Data from the operation of a bubble chamber filled with 3.5 kg of CF$_{3}$I in a shallow underground site are reported. An analysis of ultrasound signals accompanying bubble nucleations confirms that alpha decays generate a significantly louder acoustic emission than single nuclear recoils, leading to an efficient background discrimination. Three dark matter candidate events were observed during an effective exposure of 28.1 kg-day, consistent with a neutron background. This observation provides the strongest direct detection constraint to date on WIMP-proton spin-dependent scattering for WIMP masses $>20$ GeV/c$^{2}$.

Figures (4)

• Figure 1: The acoustic parameter, defined in the text, is a measure of the loudness of the acoustic "plink" resulting from bubble nucleations. The red, dashed histogram in the main panel is from the nuclear recoil calibration datasets (sAmBe, NuMI, cosmic). The blue, solid histogram in the main panel is from all the untagged data, including data taken before the muon veto was commissioned. The inset histogram contains events from $^{222}$Rn early in the run, which reproduce the second peak in the untagged dataset. The acceptance of the nuclear recoil cut, indicated with arrows, is $88\%$.
• Figure 2: The two methods of measuring the bubble multiplicity are plotted for the full data set of $291$ kg-day exposure. The first method, based on automated video reconstruction software, searches for clusters of pixels changing between frames and is represented by the various symbols labeled in the legend. The second method of counting bubbles was to measure the dP/dt associated with an event. The abscissa is the pressure rise, scaled to the number of bubbles. The ordinate is the acoustic parameter $AP$ (see text).
• Figure 3: Data from a $28.1$ kg-day WIMP search, plotted as in Fig. \ref{['bubble_counting']}. In the alpha region (large dashed box) $21$ events with no evidence for muon activity were observed, with 3 events having coincident muon activity (labeled vetoed). The smaller dashed box indicates the WIMP signal region, containing $3$ candidate events and $47$ events coincident with muons in the veto. The neutron double scatter region, indicated with a dashed circle, contains $8$ vetoed events and $1$ unvetoed event, the latter indicating an irreducible neutron background contamination (see text).
• Figure 4: Improved limits on spin-dependent WIMP-proton elastic scattering from the data presented in this letter. A previous COUPP result COUPPscience is shown for comparison. Direct detection limits from the PICASSO experiment PICASSOlimit, cyan, and the KIMS experiment KIMSlimit, green, are shown. Limits on neutralino annihilation in the Sun from the IceCube ICECUBElimit, magenta, and Super Kamiokande SKlimit, black, neutrino observatories are also plotted. The indirect detection limits from the neutrino observations have additional dependence on the branching fractions of the annihilation products. The gold region indicates favored regions in cMSSM cMSSMregion. The blue, dashed-dotted line is the expected cross-section for "maverick" dark matter with $\Omega h^2=0.1$maverickdm.