Second experiment hints at seasonal dark matter signal

Things just got a little less lonely for researchers who have been insisting for years not only that their experiment has found dark matter, but also that the dark matter signal varies with the seasons. Now a second experiment, called CoGeNT, is reporting similar findings, though both results are in conflict with two other teams' observations.

No one knows what dark matter is – astronomers merely detect its gravitational pull on normal matter, which it seems to outweigh by a factor of five to one. But many researchers believe it is made of theoretical particles called WIMPs, which interact only weakly with normal matter.

Since 1998, researchers running the DAMA experiment deep inside the Gran Sasso mountain in Italy have claimed to have found evidence of WIMPs.

DAMA uses an array of sodium iodide detectors to spot the rare moments when WIMPs slam into atoms in the detectors, producing flashes of light. The number of flashes ebbs and flows with the seasons, and DAMA team members argue that this is because Earth's velocity relative to the surrounding sea of dark matter changes as the planet orbits the sun. They say their observations could be explained by a WIMP weighing a few gigaelectronvolts.

Tense situation

However two other experiments have found no sign of dark matter with their detectors. One, called XENON100, uses 100 kilograms of liquid xenon deep below Gran Sasso mountain, and the other, called CDMS II, uses ultra-pure crystals of germanium and silicon housed in a deep mine in Soudan, Minnesota.

Both experiments are so sensitive that they should have seen dark matter if the DAMA result is due to WIMPs. "The situation has created tension," says Dan Hooper, a theorist at the University of Chicago in Illinois.

Now another dark matter experiment called CoGeNT has found a seasonal variation in its results, reports team leader Juan Collar, who presented an analysis of 442 days of observations at the American Physical Society meeting in Anaheim, California, on Monday.

Germanium crystal

"We tried like everyone else to shut down DAMA, but what happened was slightly different," Collar said during his presentation.

"The annual modulation is the closest thing to a smoking gun [for dark matter]," says theorist Jonathan Feng at the University of California, Irvine, who is not part of the CoGeNT team. "This is the first evidence we've seen it somewhere other than DAMA."

The CoGeNT detector is tiny compared with many other dark matter experiments. It comprises a 440-gram crystal of germanium. Still, dark matter is so abundant that 100 million particles of it are expected to pass through the CoGeNT detector every second.

About once a day, one of these will wallop a germanium nucleus, sending the nucleus careering through the crystal, where it rips electrons from neighbouring atoms. An electric field sweeps these electrons towards an electrode to produce a tiny electrical signal.

Background noise?

Previously, the CoGeNT team reported an excess of events when it ran its experiment in the Soudan mine for 56 days (Physical Review Letters, DOI: 10.1103/PhysRevLett.106.131301). Team members said the excess could be due to some kind of background noise that physicists don't understand, or potentially to WIMPs weighing 7 GeV.

The experiment kept running continuously until a fire in the Soudan mine on 17 March halted observations. This motivated Collar and his colleagues to look for a seasonal variation in the 442 days of observations they had already collected. "I hope this isn't the final data we have taken," says Collar, who has not yet been allowed to return to the Soudan mine to check for damage.

The CoGeNT team finds that their signal changes with the seasons in exactly the same way as the DAMA result does. And it is consistent with a low-mass dark matter particle, like that reported by DAMA.

Weird WIMP

Laura Baudis at the University of Zurich in Switzerland, who reported at the meeting on Monday that XENON100 still had seen no signs of dark matter, is not sure what to make of the results: "I need time to think about them."

Feng suggests that the discrepancy among all the experimental results may simply be due to the assumption that WIMPs interact the same way with protons and neutrons. If this is not the case, that could explain differences in the signals from xenon and germanium detectors, which each have a different ratio of protons to neutrons (arxiv.org/abs/1102.4331). "These experiments may look inconsistent, but a small theoretical tweak can bring everything in to line," he told New Scientist.

Both the CoGeNT and XENON100 teams are planning to enlarge their experiments. Approval has just been given to build the XENON1T experiment in the Gran Sasso mine, which will use 1 tonne of liquid xenon. And the CoGeNT team is planning to replace its single germanium crystal with four separate crystals, each weighing 1 kilogram, starting later this year.

Source New Scientist

Bruce Schneier: The security mirage

The feeling of security and the reality of security don't always match, says computer-security expert Bruce Schneier. At TEDxPSU, he explains why we spend billions addressing news story risks, like the "security theater" now playing at your local airport, while neglecting more probable risks -- and how we can break this pattern.

Spinstars: First Polluters of the Universe? Imprints of Fast Rotating Massive Stars in Milky Way's Bulge

From the analysis of the chemical composition of some of the oldest stars in our Galaxy, an international team of astronomers led by Cristina Chiappini from the Leibniz-Institut für Astrophysik Potsdam (AIP) and the Instituto Nazionale di Astrofisica (INAF) presents new clues on the nature of the first stellar generations in our Universe.

"We think that the first generations of massive stars were very fast rotators -- that's why we called them spinstars," explains Chiappini. Their findings will be published in a Nature article on April 28, 2011.
Massive stars live fast and furious, and hence the first generations of massive stars in the Universe are already dead. However, their chemical imprints, like fingerprints, can still be found today in the oldest stars in our Galaxy. These fossil records are thus the witnesses of the nature of the first stellar generations to pollute our Universe. "It is like if we tried to reveal the character of a cook from the taste of his dishes," says Prof. Georges Meynet, from the Geneva University.

Simulation of the formation of the first stars showing fast rotation. (Credit: A. Stacy, University of Texas / Figure adapted from Stacy et al, 2011, MNRAS 413,1, 543)

How were these first stars? Were they different from the stars we observe today? Soon after the Big Bang, the composition of the Universe was much simpler than at present as it was made of essentially only hydrogen and helium. The chemical enrichment of the Universe with other elements had to wait around 300 million years until the fireworks started with the death of the first generations of massive stars, polluting the primordial gas with new chemical elements, which were later incorporated in the next generations of stars.

Using data from ESO's Very Large Telescope (VLT), the astronomers reanalyzed spectra of a group of very old stars in the Galactic Bulge. These stars are so old that only very massive, short-living stars with masses larger than around ten times the mass of our Sun should have had time to die and to pollute the gas from which these fossil records then formed. As expected, the chemical composition of the observed stars showed elements typical for enrichment by massive stars. However, the new analysis unexpectedly also revealed elements usually thought to be produced only by stars of smaller masses. Fast-rotating massive stars on the other hand would succeed in manufacturing these elements themselves.

"Alternative scenarios cannot yet be discarded -- but -- we show that if the first generations of massive stars were spinstars, this would offer a very elegant explanation to this puzzle!," says Cristina Chiappini. Team member Urs Frischknecht, a PhD student at the Basel University, is already working on extending the stellar simulations in order to further test the proposed scenario.

The impact of having had an early generation of spinstars in the Universe is manifold. Fast rotation also affects other properties of a star, such as its colour, its lifetime and its luminosity. Spinstars would therefore also have strongly influenced the properties and appearance of the first galaxies which were formed in the Universe. The existence of spinstars is now also supported by recent hydrodynamic simulations of the formation of the first stars of the universe by an independent research group.

From  Science Daily