Improved Spin-Dependent WIMP Limits from a Bubble Chamber

TL;DR

An ultraclean, room-temperature bubble chamber containing 1.5 kilograms of superheated CF3I, a target maximally sensitive to spin-dependent and -independent weakly interacting massive particle (WIMP) couplings, showed extreme intrinsic insensitivity to the backgrounds that commonly limit direct searches for dark matter.

Abstract

Bubble Chambers provided the dominant particle detection technology in accelerator experiments for several decades, eventually falling into disuse with the advent of other techniques. We report here on the first period of operation of an ultra-clean, room-temperature bubble chamber containing 1.5 kg of superheated CF$_{3}$I, a target maximally sensitive to spin-dependent and -independent Weakly Interacting Massive Particle (WIMP) couplings. An exposure in excess of 250 kg-days is obtained, with a live-time fraction reaching 80%. This illustrates the ability to employ bubble chambers in a new realm, the search for dark matter particles. Improved limits on the spin-dependent WIMP-proton scattering cross section are extracted from this first period. An extreme intrinsic insensitivity to the backgrounds commonly limiting these experiments (a rejection factor for photon-induced electrons of $\sim10^{-10}$) has been measured in operating conditions leading to the detection of low-energy nuclear recoils such as those expected from WIMPs.

Figures (6)

• Figure 1: Instantaneous stopping power vs. energy for different particles in CF$_{3}$I, including its three recoiling species. Plotted as horizontal and vertical lines are the calculated dE/dx and energy thresholds for bubble nucleation at T=40 C and two different operating pressures. According to the "Hot Spike" nucleation model seitz only radiations in the top right (colored) quadrants can lead to bubbles. Notice the absence of this possibility for electrons even toward the end of their range, in conditions that nonetheless lead to a sensitivity to recoils of just a few keV, such as those expected from WIMP interactions. Alpha particles and their recoiling daughters can induce bubble nucleations and the presence of their emitters must therefore be avoided.
• Figure 2: Families of events in a 1.5 kg CF$_{3}$I bubble chamber. At high degrees of superheat ($\sim$60 C and atmospheric pressure for this compound), minimum ionizing cosmic ray events reminiscent of those observed in early bubble chambers glaser are visible (left). At moderate superheats ($\sim$30 C, 1 atm) the chamber is sensitive strictly to high $dE/dx$ radiation such as nuclear recoils. Whereas neutrons can give rise to simultaneous separate bubbles each corresponding to a scatter (center), WIMPs are expected to produce single bubbles only (right), due to their extremely small probability of interaction. The mean free path between scatters is of just a few cm for neutrons, leading to an excellent ability to reject them in large chambers.
• Figure 3: Response of the chamber to an intense $^{137}$Cs gamma source. The expected gamma interaction rate with the superheated liquid is 3.9$\times10^{6}$ per second, with gamma-induced electron energies reaching up to 662 keV. Inset: Intrinsic gamma rejection factor (fraction of interacting gammas inducing bubbles) obtained from the exposure to the source (see text).
• Figure 4: Blind absolute comparison between expected bubble nucleation rate (lines) and observations (points) in the active presence of the switchable Am/Be neutron source ( top inset). The two lines correspond to largely different values of a nucleation parameter "a" jap2, the single free factor in the classical theory used to predict bubble nucleation thresholds seitz. A fit to the data provides an excellent agreement with theoretical expectations (a = 6) bell. Including the uncertainty in the predictions (see text) the same fit simultaneously yields an efficiency in the response to this source of $81\pm^{63}_{33}$% ($51\pm^{40}_{18}$%) at 40 C (30 C) (errors are 90% confidence levels). Calculated thresholds for recoil-induced nucleation are expressed in keV along the top axis. Bottom inset: Spectrum of nuclear recoil energies produced by the source (MCNP-PoliMi simulation polimi), similar to typical expected WIMP recoil spectra.
• Figure 5: Top: Distribution of times between consecutive bulk events in the chamber. A model (solid line) including the effect of dead time and fiducial volume cuts, based on the triple alpha emission from $^{222}$Rn and progeny, successfully reproduces this distribution (see text). Bottom: Distribution of single bulk bubble nucleation rate vs. operating pressure, for two different running temperatures. Rates display a flat behavior up to a pressure endpoint. This is characteristic of the response to monochromatic alphas and $\sim$100 keV alpha recoils (here from $^{222}$Rn emanations, see text). Colored arrows along the bottom axes indicate the predicted onset of sensitivity to these particles, in good agreement with observations. As a reference, solid lines correspond to the expected signal rate from WIMPs with a mass of 10 and 50 GeV/$c^{2}$ and a 3 pb cross section for their spin-dependent coupling to protons. Also shown is the response function to $^{222}$Rn and progeny (dashed lines). Calculated energy thresholds (in keV) for bubble nucleation by fluorine recoils are shown along the top axes.