1 December 2011

Strongest limit set on dark matter’s mass

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Dark matter must have a mass greater than 40 giga-electronvolts, say scientists who claim to have set the strongest limit yet on the mass of dark matter.
 dark matter's mass

Physicists have set strongest limit on the mass of dark matter yet, according to a new study. Brown physicists studied seven dwarf galaxies, some shown here circled in white. Their observations indicate these galaxies are full of dark matter because their stars’ motion cannot be explained by their mass alone. Credit: NASA/DOE/Fermi-LAT Collaboration/Koushiappas and Geringer-Sameth/Brown University

PARIS: Dark matter must have a mass greater than 40 giga-electronvolts, say scientists who claim to have set the strongest limit yet on the mass of dark matter.

A pair of physicists from Brown University in the U.S. used publicly available data from NASA’s Fermi Large-Area Telescope to investigate the mysterious substance thought to make up nearly a quarter of the known universe.

Using a new statistical approach, the researchers determined that dark matter must have a mass greater than 40 giga-electron volts in dark matter collisions involving heavy quarks, which are an elementary particle and a fundamental constituent of matter. The pair constrained the mass of dark matter particles by calculating the rate at which the particles are thought to cancel each other out in seven dwarf galaxies that appear to be full of dark matter.

“What we find is if a particle’s mass is less than 40 GeV, then it cannot be the dark matter particle,” said co-author Savvas Koushiappas of the paper published in the current issue of Physical Review Letters.

Underground detectors

Scientists believe that normal matter (which makes up all planets, stars and galaxies) accounts for only 5% of the universe. The rest is made up of dark matter (23%) and dark energy (72%). However, dark matter cannot be ‘seen’ and can only be inferred by the gravitational pull that it exerts on normal matter in galaxies.

There are several candidates for dark matter. These include MACHOs (massive compact halo objects) – huge gas balls made up of normal matter that emit little or no radiation and RAMBOs (robust associations of massive baryonic objects) – dark clusters of brown dwarfs. The most common suspects, however, are the hypothetical WIMPs (weakly interacting massive particles).

WIMPs are thought to be present everywhere in our galaxy and despite having a mass similar to that of an atomic nucleus, their extremely weak interaction with normal matter enables them to travel undetected as they pass through the Earth and indeed the entire galaxy.

To spot these WIMPs directly, researchers have built detectors in underground labs where the low background noise should allow any signals to stand out. The biggest of these labs is under Gran Sasso, a mountain in central Italy, where various dark-matter experiments such as DAMA, XENON and CRESST are being performed. Such dark-matter detectors work when a WIMP collides with an atomic nucleus, which then recoils, producing a trademark scintillation, or flash.

Constraining WIMP mass

WIMPs can also be detected less directly using satellites or balloon-based instruments. Because of the effects of their own gravity, the number of WIMPs at the centres of galaxies should be very high – high enough that they begin to collide with one another.

When this happens they annihilate each other, producing large numbers of heavy quarks and leptons, and in turn generate gamma-ray photons. An excess of such photons coming from the centres of galaxies is the ‘signature’ of dark matter that satellites such as Fermi are searching for.

Koushiappas and his colleague Alex Geringer-Sameth studied gamma-ray emission data collected over the last three years for seven dwarf galaxies in the Milky Way. They compared the emission from these galaxies to the emission from the area surrounding each dwarf. They analysed the gamma-ray data to estimate the number of annihilations that might be taking place. This allowed them to constrain the mass of WIMPs.

The results indicate that if a particle’s mass is less than 40 GeV, then it cannot be a dark matter particle, said Koushiappas. In contrast, DAMA, XENON and CRESST measurements suggest that the mass of dark matter lies between just 7 to 12 GeV.

“If the WIMP mass is around 10 GeV, as suggested by the previous underground experiments, then we should already have seen a signal in the Fermi telescope data, but we don’t,” he said.

Light WIMPS still possible

However, Koushiappas is also cautious: “The disparate figures come from the fact that we have two experimental techniques (indirect and direct) that approach the dark matter hunt from two very different directions, and the two are not consistent in the simple generic case,” he explained. “It is possible that the solution to this discrepancy comes from the underlying assumptions of what the physics of dark matter actually is.”

“These are exciting and interesting new results: the limit presented here adds to the scepticism regarding the recent low-mass ‘detections’ by direct search experiments such as DAMA/LIBRA, CoGeNT and most recently, CRESST,” commented Alexander Murphy of Edinburgh University in Scotland. “Results from other such experiments, such as CDMS and ZEPLIN all exclude the low-mass results with fairly high confidence. Koushiappas’ and Gringer-Sameth’s work is important in that it uses a completely independent technique – satellite observations of the radiation that would be produced from annihilations of WIMPs in nearby dwarf galaxies.”

Neal Weiner of the Centre for Cosmology and Particle Physics at New York University added, “This is a really nice result but it doesn’t really affect my thinking about whether light WIMPs are viable. Although I think the claims of what to take away are too much, I do believe that these researchers have actually started pushing into the range relevant for detecting ‘conventional’ WIMPs.”

The ‘light’ explanation of WIMPs may be just as alive today as it was last week, he said.

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