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.
Dr Alexander Murphy, who is presenting 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.
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.
What Is Dark Matter?
Since the 1930’s it has been apparent that the Universe is made up of more than just the things we can see. It is now widely accepted that a large fraction of the Universe consists of ‘dark matter’ in the form of a new type of fundamental particle. These dark matter particles constitute about 90% of the mass of our galaxy and are constantly passing through the Earth itself. Evidence for this comes from a diverse, yet consistent, array of astronomical observations and is supported by advanced theories of particle physics that seek a deeper symmetry to explain the forces of nature. Crucially however, no direct observation of these dark matter particles has yet been made.
Who Is Involved With The ZEPLIN Project?
The ZEPLIN Project team is composed of scientists from the Universities of Edinburgh, Oxford and Sheffield, Imperial College London and the Rutherford Appleton Laboratory. International collaborators include scientists from the University of Coimbra, Portugal, the University of California, Los Angeles, USA, Texas A&M University, USA, and the University of Rochester, USA. UK funding is provided by the Science and Technologies Facilities Council.
The project’s first detector, ZEPLIN-I, ran between 2001 and 2004. It was the first generation of this style of detector and had a target of 3.2 kg of liquid xenon.
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