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ANN ARBOR— A team of University of Michigan physicists are part of the PandaX international collaboration based in China who recently reported the first physics results from their 120 kg dark matter detector.
The U-M team includes Professor Wolfgang Lorenzon, Professor Gregory Tarlé, Research Scientist Michael Schubnell, Postdoctoral Fellow Kirill Pushkin, and Graduate Student Scott Stephenson.
PandaX is an acronym for “Particle and Astrophysical Xenon” detector. The experiment uses ultra-pure liquefied xenon, a heavy noble gas extracted from the air, as the detection medium for dark matter. The experiment started a month-long dark matter search this May in the China JinPing underground Laboratory (CJPL). During the initial 17 day run, in the central 37-kg target region, 46 events were observed and all were consistent with signals from background radiation, not from dark matter. The PandaX-I result places strong constraints on all previously reported positive signals from other direct detection experiments, and casts doubt on the interpretation of these dark matter-like signals as dark matter.
The goal of the first stage of the PandaX experiment was to examine previously reported dark matter-like signals. The scale of the PandaX-I experiment is only second to LUX, the currently largest dark matter experiment, located in a South Dakota mine in the United States. To shield the experiment from cosmic ray background, PandaX is located in the deepest underground laboratory in the world.
The main challenge of this type of experiment is to reject events due to the ubiquitous radioactivity on Earth. Cosmic rays constantly showering the earth must be shielded by kilometers of rock above a deep underground laboratory. For PandaX, a 100-ton passive shield containing polyethylene, lead, and high purity copper surrounds the experiment to protect it from low levels of natural radioactivity in the surrounding rocks and materials used in the construction of the laboratory. In addition, krypton contamination in the liquid xenon was reduced to below one part in ten billion to avoid radioactive signal contamination from a radioactive isotope of krypton.
“PandaX has reached the frontier in low mass dark matter searches and is now pushing forward to uncharted territory,” said Professor Wolfgang Lorenzon.
While the first stage PandaX detector continues to search for low mass dark matter with even higher sensitivity, a second stage PandaX detector with a 500 kg liquid xenon target is under construction. It will be used to search for heavy dark matter favored by theories that extend the Standard Model of particle physics.
The results have been accepted by SCIENCE CHINA Physics, Mechanics & Astronomy, and are available at http://arxiv.org/pdf/1408.5114.pdf.