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Short Sharp Science: A New Scientist blog

Particle quest: Hunting for Italian WIMPs underground

Roger Highfield, editor, New Scientist magazine


(Image: Enrico Sacchetti)

At the forefront of the hunt for elusive particles is the Gran Sasso National Laboratory, the largest underground facility of its kind.

Located under a mountain between L'Aquila and Teramo in Italy, the facility is hidden under 1400 metres of dolomite that not only contains relatively little radioactive uranium and thorium but also shields it from cosmic rays so that ultrasensitive detectors can "see" the signals produced by rare and exotic particles. Here, some 750 scientists from 22 countries work on 15 experiments.

Of these experiments, CRESST is hunting for dark matter, the mysterious form thought to explain why visible matter alone is not enough to account for the gravitational effects seen in the cosmos. The right-hand photo shows the electronics that process signals from the experiment's detectors, which are cooled to 0.01 kelvin.

The detectors contain nine calcium tungstate crystals. If struck by a dark matter candidate - hypothetical particles called Weakly Interacting Massive Particles, or WIMPs - the crystals should warm by about 0.00001 kelvin and scintillate. Researchers would know WIMPs were behind the change as they are calculated to convert around 0.1 per cent of the energy into light, one tenth of the conversion rate seen with radioactivity, says CRESST spokesman Wolfgang Seidel. So far, no WIMPs have been detected.

Another experiment is ICARUS, which is hunting for ghostly particles called neutrinos, whether made by the particle accelerator in CERN, the sun, or from the far reaches of the cosmos.

ICARUS's detector, shown in the left-hand photo, consists of two tanks containing 600 tonnes of liquid argon. As a neutrino decays in the argon, it generates charged particles. Along the path of each particle, electrons and positive ions are created. A strong electric field causes these to drift toward wire chambers along the side of the tanks, leaving a signal. Since the facility opened in May 2010, 144 neutrinos have been recorded.

The detection method, devised by the Nobel laureate Carlo Rubbia in 1977, is the modern-day equivalent of the bubble chambers that were once used to track particles.

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