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Dark matter signal sparks interest, but falls short of discovery — Updated

detect_zoom3.pngAn underground experiment in France has announced the possible detection of dark matter, the mysterious substance that is hypothesized to make up four-fifths of the matter in the universe and that has yet to be definitively detected. But the strength of the signal seen falls short of the statistical significance needed to claim a discovery, according to the spokesman of a competing experiment.

EDELWEISS-II reported March 21 on the physics preprint archive that it had observed five candidate Weakly Interacting and Massive Particles (or WIMPS), over a data-collection period from April 2009 to May 2010. WIMPS are the leading candidate for dark matter, and EDELWEISS-II looks for their signature by collecting electric charge generated when candidate WIMPS collide with nuclei in germanium crystals in the underground Laboratoire Souterrain de Modane, which is located on the border between France and Italy at a depth of 1700 metres. The expected number of signals due to background (due to the decay of radioactive isotopes in the germanium, neutrons from the fission of uranium in the surrounding rock, and the decay of decay products of naturally-occurring radon gas) in the absence of WIMPS was three events, which means there is an excess of two – which is prompting considerable interest in the dark matter community. However the excess is far from statistically significant, says Bernard Sadoulet of the University of California, Berkeley, who is spokesman for the Cryogenic Dark Matter Search based in Soudan Mine in Minnesota. “It’s not a discovery. It’s important for the field and their data is sound, but it’s not a discovery,” he says.

The paper itself stops short of claiming to have discovered dark matter. Sadoulet sounds a note of caution based on his own experience. He points out that, in 2010, CDMS also reported two candidate WIMPS, but concluded they were indistinguishable from background. Although competitors, CDMS and EDELWEISS-II are now working on a joint paper to combine their datasets to see if a promising statistical significant signal emerges. However, because both groups have already looked at their data, the combined study inevitably won’t follow the standard practice in the field of dark matter detection of developing understanding of background level by looking at only a subset of data, while blinding oneself to the main dataset that might contain signals, preparing an analysis, and then unblinding the crucial data and feeding it into the analysis. Sadoulet adds that while the WIMP candidates seen by CDMS had energies of 12 and 15 keV, EDELWEISS-II candidates had energies of more than 20, so the signals are not evidence for the same kind of WIMPS. “I’d like to understand their background better,” he adds.


The spokesman for EDELWEISS-II, Gilles Gerbier of the Commissariat a l’Énergie Atomique’s Institut de Recherche sur les Lois Fondamentales de I’Univers in Saclay, France, confirms that the collaboration is not claiming the discovery of dark matter. But nor is their result negative. “We cannot say for sure that there is no signal. We are in the uncomfortable situation, same as CDMS. We may have a signal but we cannot make any claim now.” He says the next step will be to refine the experiment to further reduce the levels of background noise and its ratio to any genuine signal, by adding more shields to cut out incoming neutrons and by increasing the size of the germanium detectors from 400 grams to 800 grams.

Read this week’s overview of dark matter on Nature News.

Updated: This blog was updated on March 24 to reflect the comments of Gilles Gerbier, and to add additional information on the experiment’s sources of background and underground depth.

Image: EDELWEISS-II (the copper cryostat houses the detectors) / EDELWEISS-II Collaboration

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