Dark matter is slowly running out of places to hide. Two new looks at the gamma-ray sky suggest that if the mysterious matter is a particle, it is heavier than 40 gigaelectronvolts, about 44 times the mass of a proton.
That contradicts hints from three experiments on Earth that pointed to a lightweight dark matter particle weighing just a quarter as much, although some researchers say such featherweights are still in the running.
Dark matter makes up about 80 per cent of the matter in the universe, but no one is sure what it's made of.
The leading candidate is a WIMP, or weakly-interacting massive particle, that was produced in the big bang and has been clumping up and seeding structures such as galaxies ever since. Physicists know how much dark matter the universe contains in total, but not how much each individual WIMP weighs.
Direct detections
One way to find out is to wait patiently for a particle to smack into a detector buried deep underground to avoid spurious signals from ordinary particles raining down from space. Some of these detectors have yet to catch anything, but three of them ? CRESST II and DAMA, both in a mine in Italy, and CoGeNT, in a mine in Minnesota ? have reported tantalising hints of a particle weighing between 7 and 20 gigaelectronvolts (GeV).
Another way to probe the particles' properties is to look for the gamma-ray radiation produced when two WIMPs collide and annihilate each other, producing a cascade of particles and photons. Last year, Dan Hooper of Fermilab in Batavia, Illinois used data from the Fermi space telescope to show what could be radiation from a similarly lightweight dark matter particle coming from the centre of the Milky Way.
But now, two independent groups studying Fermi data say their studies point to a dark matter particle weighing at least 40 GeV.
Not enough radiation
Both groups searched for the gamma-ray glow from dark matter in dwarf galaxies orbiting the Milky Way. One, a group of Fermi researchers led by Johann Cohen-Tanugi of Montpellier University in France and Jan Conrad and Maja Llena Garde, both of Stockholm University in Sweden, looked at two years' worth of observations of 10 dwarf galaxies. The other, by Alex Geringer-Sameth and Savvas Koushiappas of Brown University in Providence, Rhode Island, looked at the gamma-ray output of seven galaxies over three years.
The groups used different statistical approaches to subtract out the gamma-ray emission from normal astrophysical sources such as pulsars and supernovae to hunt for a dark matter signal, and each arrived at the same conclusion ? that any gamma-ray light coming from dark matter must be generated by a relatively heavy particle.
Why? If each particle of dark matter was instead small and light, there should be a lot of them to account for the amount of dark matter indirectly detected in the universe by its gravitational pull on normal matter. If there were a lot of dark matter particles, there would be a lot of collisions between them and therefore a lot more gamma rays than are seen, the teams say.
"If the WIMPs were smaller, we should have seen them, but we don't," Koushiappas says. "This is the strongest limit to the mass that we have so far."
Not so fast
But the studies are not necessarily a death knell for lightweight dark matter particles, says Hooper. That's because they use a model of dark matter that decays into certain types of particles such as heavy quarks and travels at the same speed no matter what the temperature is. If, in reality, dark matter decays into particles that Fermi can't detect, or if it moved faster when the universe was younger and hotter, then it could still be as light as 10 GeV, Hooper says.
"They're ruling out certain kinds of particles, but they're not ruling out mine," he told New Scientist.
Still, the paper is an important step forward. "For the first time, we're really crossing off theoretically acceptable, well-motivated, attractive models," he says. "No one experiment is going to kill all dark matter models we can think of. It's going to take a lot of different approaches."
Journal reference: arxiv.org/abs/1108.3546 and arxiv.org/abs/1108.2914, both accepted to Physical Review Letters
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