First Dark Matter Limits from a Large-Mass, Low-Background, Superheated Droplet Detector

Abstract

We report on the fabrication aspects and calibration of the first large active mass (ϳ15 g) modules of SIMPLE, a search for particle dark matter using superheated droplet detectors (SDDs). While still limited by the statistical uncertainty of the small data sample on hand, the first weeks of operation in the new underground laboratory of Rustrel -Pays d'Apt already provide a sensitivity to axially coupled weakly interacting massive particles (WIMPs) competitive with leading experiments, confirming SDDs as a convenient, low-cost alternative for WIMP detection. PACS numbers: 95.35.+d, 05.70.Fh, The rupture of metastability by radiation has been historically exploited as a method for particle detection. Perhaps its most successful application is the bubble chamber, where ionizing particles deposit enough local energy in a superheated liquid to produce vaporization along their wake. Apfel extended this concept in the form of superheated droplet detectors [1] (SDDs, aka bubble detectors), in which small drops (radius ϳ10 mm) of the liquid are uniformly dispersed in a gel or viscoelastic medium. In a SDD the gel matrix isolates the fragile metastable system from vibrations and convection currents, while the smooth liquid-liquid interfaces impede the continuous triggering on surface impurities that occurs in bubble chambers. The lifetime of the superheated state is extended, allowing for new applications: SDDs are increasingly popular as neutron dosimeters, where the nucleated visible bubbles provide a reading of the radiation exposure. SIMPLE (superheated instrument for massive particle searches) aims to detect particle dark matter using SDDs. We report here on the sensitivity attained at the early prototype stage, already comparable to the best achieved with competing technologies.In the moderately superheated industrial refrigerants used in SDDs, bubbles are produced only by particles having elevated stopping powers (dE͞dx * 200 keV͞mm) such as nuclear recoils. This is understood in the framework of the "thermal spike" model [2], common to bubble chambers: for the transition to occur, a vapor nucleus of radius .r c must be created, while only the energy deposited along a distance comparable to this critical radius r c is available for its formation. Hence, a double threshold is imposed: the deposited energy E must be larger than the work of formation of the critical nucleus, E c , and this energy must be lost over a distance O͑r c ͒, i.e., a minimum dE͞dx is required. More formally [3,4] E . E c 4pr 2 c g͞3e ,where r c 2g͞DP, g͑T ͒ is the surface tension, DP P V 2 P, P V ͑T ͒ is the vapor pressure, P and T are, respectively, the operating pressure and temperature, e varies in the range ͓0.02, 0.06͔ for different liquids [4,5], and a͑T ͒ ϳ O͑1͒ [6]. Both thresholds can be tuned by changing the operating conditions: keV nuclear recoils like those expected from scattering of weakly interacting massive particles (WIMPs) (currently the favored galactic dark matter candidates [7]) are detectable at room T and atmospheric ...

First results from dark matter search experiment with LiF bolometer at Kamioka underground laboratory

“…3 is close to UKDMC's limits, we should also be able to compare our limits in the a p − a n plane with DAMA's spin-dependent annual modulation allowed region when DAMA publishes relevant data to calculate the allowed region in the a p − a n plane. [13,14,15,16] are shown in a thin solid and dotted lines, respectively. DAMA's allowed region investigated in a mixed coupling framework is shown as "case c" in Ref.…”

Astrophysics and Cosmology

“…Very recently there has been the interesting proposal of a completely new method based on a Superheated Droplet Detector (SDD), which claims to have already a similar sensitivity as the more standard methods described above, see Ref. [61].…”

“…Very recently there has been the interesting proposal of a completely new method based on a Superheated Droplet Detector (SDD), which claims to have already a similar sensitivity as the more standard methods described above, see Ref. [61].There exist other indirect methods to search for galactic WIMPs [62]. Such particles could selfannihilate at a certain rate in the galactic halo, producing a potentially detectable background of high energy photons or antiprotons.…”

Astrophysics in 2001

“…• WIMPs with masses of and cross sections of 2 1-10 GeV/c a few picobarns (Collar et al 2000). A barn, recall, was the unit of cross section in nuclear physics when not being able to hit the broad side of one was an insult.…”