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Dark-matter hunt, part III

Zeplin-III will join Zeplin-II, already operating in an English mine, in the hunt for dark matter.
Provided by the Royal Astronomical Society, United Kingdom
Abell 1060
Ninety percent of the material in a galaxy cluster, such as Abell 1060 in Hydra, seen here, consists of dark matter.
Anglo-Australian Observatory
April 30, 2007
The race for the first direct detection of dark matter will move into a new phase in the coming months as the ZEPLIN-II instrument is joined by ZEPLIN-III, the world's most sensitive dark matter detector.
Zeplin
This cut-away drawing shows the inner workings of Zeplin-II, which is searching for dark matter in England's Boulby mine.
Science and Technology Facilities Council
Dr Alexander Murphy, who presented the first results from the ZEPLIN-II detector at the RAS National Astronomy Meeting in Preston on 18th April said, "ZEPLIN-II is beginning its second search for dark matter particles, deep underground in a salt and potash mine in North Yorkshire, and we have been pouring through the first data looking for possible interactions with dark matter. Now, just last week, we've had the go-ahead to start operating our next generation detector, ZEPLIN-III. We will be tweaking both detectors to improve their sensitivity all the time and, over the next few months, we'll be able to see signals that are many times fainter. This will give us a fantastic chance of making the first direct detection of a dark matter particle."

The ZEPLIN-II instrument holds 31 kg of liquid xenon, cooled to a temperature of -110º Celsius. Theory suggests that, from time to time, a dark matter particle will scatter from the xenon leaving a very small signal behind. Extremely sensitive light detectors view the xenon looking for such a telltale sign. ZEPLIN-II, has proved the world's most sensitive detector of this type (noble liquid technology) and is surpassed only by the Cryogenic Dark Matter Search (CDMS), based in Minnesota, which uses a semiconductor technology. With a few tweaks, the team expects ZEPLIN-II to be able to match the sensitivity of CDMS within a few months.
Composition of the universe
This diagram shows the ingredients of the universe. Astronomers have recently realized that the main ingredient is "dark energy," a mysterious form of energy that exists between galaxies and forces the universe to expand at an ever-increasing rate. The next-largest constituent is "dark matter," which is an unknown form of matter. The rest of the universe consists of ordinary matter, most of which is locked up in stars and clouds of gas. A tiny fraction of this matter is composed of heavier elements, the stuff of which humans and planets are made.
Ann Field (STScI)
The upgraded ZEPLIN-III, although not significantly bigger than ZEPLIN-II, will be able to achieve a sensitivity that is a factor of 30 better than CDMS, although it should take about two years to reach this level of operation. This factor of 30 is especially important because the theoretical models predict that this is the level of sensitivity needed to have a realistic chance of seeing a signal.

The major benefit of noble liquid technology over semi-conductor technology is that it is more easily scalable, which means that it should allow for bigger detectors in the future. Features of ZEPLIN-III include a much better ability to reject background events, lower radioactivity of materials used in construction to minimise contamination and spurious signals, and the use of higher electric fields to improve discrimination against any remaining background.
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