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

Data from the operation of a bubble chamber filled with 3.5 kg of ${\mathrm{CF}}_3\mathrm{I}$ 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 $28.1\mathrm{kg}\mathrm{day}$, consistent with a neutron background. This observation provides strong direct detection constraints on weakly interacting massive particle (WIMP)-proton spin-dependent scattering for WIMP masses $>20\mathrm{GeV}/c^2$.

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 data sets (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 ${}^{222}\mathrm{Rn}$ early in the run, which reproduce the second peak in the untagged data set. 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 $291\mathrm{kg}\mathrm{day}$ 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 was to measure the $dP/dt$ associated with the 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 $28.1\mathrm{kg}\mathrm{day}$ WIMP search, plotted as in Fig. 2. 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 [5] is shown for comparison. Direct detection limits from the PICASSO experiment [17], cyan curve, and the KIMS experiment [18], green curve, are shown. Limits on neutralino annihilation in the Sun from the IceCube [19], magenta curve, and Super-Kamiokande [20], black curve, 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 [16]. The blue, dashed-dotted line is the expected cross section for “maverick” dark matter with $\Omega h^2=0.1$ [21].