Bubble chambers were the dominant technology used for particle detection in accelerator experiments for several decades, eventually falling into disuse with the advent of other techniques. We report here on a new application for these devices. We operated 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. An extreme intrinsic insensitivity to the backgrounds that commonly limit direct searches for dark matter was measured in this device under operating conditions leading to the detection of low-energy nuclear recoils like those expected from WIMPs. Improved limits on the spin-dependent WIMP-proton scattering cross section were extracted during our experiments, excluding this type of coupling as a possible explanation for a recent claim of particle dark-matter detection.

The viability of using Bubble Chambers as dark matter particle detectors is considered. Techniques leading to the enhanced chamber stability needed for this new application are described in detail. Prototype trials show that sensitivity to the low-energy nuclear recoils induced by Weakly Interacting Massive Particles (WIMP) is possible in conditions of extreme insensitivity to minimum ionizing backgrounds. An understanding of detector response is demonstrated using existing theoretical models. We briefly comment on the prospects for detection of supersymmetric dark matter with large CF 3 I chambers. rThe positive identification of sporadic signals from among comparatively frequent backgrounds is common to any experiment at the forefront of particle physics. The challenge faced by direct searches for cold dark matter particles [1] is in this respect extraordinary: signal rates as small as one low-energy nuclear recoil (few keV) per ton of detector mass per year are predicted for the nuclear scattering of supersymmetric Weakly Interacting Massive Particle (WIMP) candidates, even if they comprise the bulk of dark matter halos able to explain galactic evolution and dynamics [2]. A number of detector techniques have been developed for this purpose over the last two decades [1]. Simplicity of design, optimal target materials, rapid scaling to the ton regime and an excellent background rejection are desirable qualities for the next-generation of detectors that should soon explore the vast range of WIMP masses and couplings still allowed.The use of moderately superheated liquids has been proposed as a possible fast route towards this goal [3,4]. A concentrated energy deposition from certain particles can lead in these to the rupture of metastability and the formation of visible bubbles. Two experiments, SIMPLE [5] and PICASSO [6] exploit this approach, benefiting from an intrinsic insensitivity to most backgrounds, discussed below. Both experiments implement the method using superheated droplet detectors [7] (SDDs, a.k.a. bubble detectors), where small drops ðr$10 mmÞ of the active liquid are dispersed in an insoluble gel or viscoelastic medium. In a SDD the gel provides a smooth liquid-liquid interface that impedes the continuous triggering (inhomogeneous nucleations) on surface defects, gaskets, motes, etc. that is traditionally observed even in the cleanest bubble chambers. As a result, the lifetime of the superheated state is considerably extended, to the point that a WIMP search can be performed. ARTICLE IN PRESSwww.elsevier.com/locate/nima 0168-9002/$ -see front matter r

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