Final Analysis and Results of the Phase II SIMPLE Dark Matter Search

We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, with the second stage including an improved neutron shielding. The analyses include a refined signal analysis, and revised nucleation efficiency based on a reanalysis of previously reported monochromatic neutron irradiations. The combined results yield a contour minimum of ${\sigma }_p=5.7\times {10}^{-3}\mathrm{pb}$ at $35\mathrm{GeV}/c^2$ in the spin-dependent sector of weakly interacting massive particle (WIMP) proton interactions, the most restrictive to date for $M_W\le 60\mathrm{GeV}/c^2$ from a direct search experiment and overlapping, for the first time, with results previously obtained only indirectly. In the spin-independent sector, a minimum of $4.7\times {10}^{-6}\mathrm{pb}$ at $35\mathrm{GeV}/c^2$ is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.

Figure 1

Scatter plot of the amplitudes and frequency of the primary harmonic of each true nucleation event observed over the Stage 2 exposure, with the boxed event for pressures $\le 2.2\mathrm{bar}$.

Figure 2

Variation of the recoil and $\alpha$ energy thresholds with temperature for the three ${\mathrm{C}}_2{\mathrm{ClF}}_5$ constituents at 2.00 (solid lines), 2.50 (dashed lines), and 3.00 (dash-dotted line) bar, with $\Lambda =1.40$. The freon constituents are identified for 2.5 bar. The vertical line indicates the $9°\mathrm{C}$ measurement temperature; the horizontal line, a threshold recoil energy of 8 keV.

Figure 3

Various spin-dependent WIMP-proton exclusion contours for Phase II, together with the leading direct [11, 12, 13] and indirect SuperK [14] and IceCube [15] search results; shown are the Stage 2 result, the reanalyzed Stage 1 result, and a merging of the two. The region outlined in grey is favored by cMSSM [34].

Figure 4

Various spin-independent contours for Phase II, together with those of the leading [12, 13, 20, 21, 22, 23, 24, 25, 26, 27] spin-independent search results; shown are the Stage 2 result, the reanalyzed Stage 1 result, and a merging of the two. The partial contours (a) and (b) are taken from [21, 24], respectively. The closed areas identified as either CRESST-II [28], DAMA/LIBRA [29], or CoGeNT [30] represent the regions in which possible light mass WIMPS have been, respectively, reported.