Dark Matter Spin-Dependent Limits for WIMP Interactions on 19-F by PICASSO

TL;DR

PICASSO reports a spin-dependent WIMP search on $^{19}$F using a new generation of superheated-droplet detectors at SNOLAB, achieving strong background discrimination and reduced alpha contamination. By employing acoustic variables $p_{var}$ and $f_{var}$ and temperature-dependent cuts, they extract SD limits with two detectors, finding no signal and setting $\sigma_F = 13.9$ pb (90% C.L.) at $M_W=24$ GeV/$c^2$, which translates to $\sigma_p = 0.16$ pb and $\sigma_n = 2.60$ pb. The results constrain the DAMA/LIBRA SD interpretation in the proton sector and demonstrate the method’s potential for substantial sensitivity gains as the full detector set is analyzed. These findings highlight the efficacy of bubble-detector architectures in probing SD WIMP interactions with low recoil thresholds.

Abstract

The PICASSO experiment at SNOLAB reports new results for spin-dependent WIMP interactions on $^{19}$F using the superheated droplet technique. A new generation of detectors and new features which enable background discrimination via the rejection of non-particle induced events are described. First results are presented for a subset of two detectors with target masses of $^{19}$F of 65 g and 69 g respectively and a total exposure of 13.75 $\pm$ 0.48 kgd. No dark matter signal was found and for WIMP masses around 24 GeV/c$^2$ new limits have been obtained on the spin-dependent cross section on $^{19}$F of $σ_F$ = 13.9 pb (90% C.L.) which can be converted into cross section limits on protons and neutrons of $σ_p$ = 0.16 pb and $σ_n$ = 2.60 pb respectively (90% C.L). The obtained limits on protons restrict recent interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent interactions.

Figures (7)

• Figure 1: Relation between neutron energy and threshold temperature for superheated C$_4$F$_{10}$ (1 bar). Mono-energetic neutrons were produced at the Montreal Tandem accelerator using the nuclear reactions $^{7}$Li(p,n)$^{7}$Be (black triangles) and $^{51}$V(p,n)$^{51}$Cr (red circles). In the case of $^{51}$V five resonances were selected yielding mono-energetic neutrons with sub-keV widths at E$_{n}$ = 4.8, 40, 50, 61 and 97 keV, respectively. Recoil energies of $^{19}$F at threshold are related to the neutron threshold energies by E$_{th}^F$(T) = 0.19E$_{th}^n$. Solid curve: prediction of the thermodynamical model.
• Figure 2: Detector response to different types of particles as a function of temperature for detectors loaded with C$_4$F$_{10}$ droplets of $\sim$200$\mu$m in diameter. From left to right: alpha particles of 5.6 MeV in a detector spiked with $^{226}$Ra (fit to data points represented by continuous line); nuclear recoils from fast neutrons of an AmBe source compared to simulations (dotted line); expected response for nuclear recoils following scattering of a 50 GeVc$^{-2}$ WIMP (continuous line); response to 1.275 MeV gamma rays of a $^{22}$Na source (dashed line). All responses are normalized to one at full detection efficiency. Temperatures are converted into $^{19}$F threshold energies (upper x-axis) by using relation (\ref{['eq:2']}) in the text. If not visible, experimental errors are smaller than the symbols.
• Figure 3: Distribution of the integrated signal power pvar recorded in calibrations with poly-energetic neutrons from an AmBe source. For a given event pvar is constructed by averaging over the waveforms of at least six transducers per detector. Neutron induced recoils accumulate in a peak (the same peak where WIMP induced recoils are expected); this peak is well separated from acoustic and electronic noise (left). The measured temperature dependence of the recoil peaks (right) serves to define a temperature dependent cut (95% acceptance) to the left of the centroid in order to reject non-particle induced events.
• Figure 4: The signal energy and frequency related variables pvar and fvar allow the classification of events into distinct categories. Shown are data from n- calibration (black crosses) and WIMP runs (red circles) for det. 71 used in this analysis. Neutron induced nuclear recoils from AmBe calibrations are located in region A; alpha particle events from WIMP runs appear displaced to the right in region A; electronic and acoustic noise, such as mine blasts, populate the lower region B; fracture events produced by primary events in the polymer would be located in region C, but do not occur in this detector. Data were recorded at 35$^{\circ}$C.
• Figure 5: Count rates as a function of temperature. The rates are normalized by the active mass of $^{19}$F and indicate the different levels of $\alpha$-background. Top: detector 93 (dots) with its higher $\alpha$-background serves as a reference to define the $\alpha$-threshold; detectors 71 (squares) and 72 (triangles) are used in this analysis. Bottom: zoom of the rates of det. 71 and 72. Also shown are the expected response curves for WIMP induced nuclear recoils for M$_W$ = 10 GeV c$^{-2}$ (broken), 30 GeV c$^{-2}$ (dashed) and 100 GeV c$^{-2}$ (dotted); a cross section of $\sigma_p$ = 1 pb was assumed for clarity.