“THE 70-year-old dark matter puzzle is close to resolution,” says Michael Turner of the University of Chicago. And he’s not alone in thinking so. Rumour has it that a European space experiment has discovered a telltale signature of the dark matter that makes up 90 per cent of the mass in the universe.
Earlier this month, the team working on the experiment, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, released preliminary data at the International Conference on High Energy Physics in Philadelphia, Pennsylvania. PAMELA has spotted more antimatter than expected in our galaxy – one of the signs that dark matter particles are being annihilated. Though the team does not claim to have discovered dark matter, and will not discuss the results any further before they are published, other physicists are deeply intrigued, as they add to a growing list of satellite and balloon experiments that have found hints of a similar signature.
“PAMELA has seen more antimatter than expected – one of the signatures expected from the annihilation of dark matter”
Advertisement
Dark matter has frustrated cosmologists ever since the 1930s when they discovered that clusters of galaxies contain much more mass than can be found in stars, gas and dust combined. Computer simulations of the large-scale structure of the universe suggest that dark matter is most likely made of weakly interacting massive particles (WIMPs), whose mass can range from tens to thousands of gigaelectronvolts (GeV). These particles should accumulate at the centre of our galaxy, sucked in by the gravity of the supermassive black hole at its heart. If so, they would annihilate each other, spewing out other particles, including electrons, positrons and gamma rays.
The earliest sightings of such emissions came from NASA’s Energetic Gamma Ray Experiment Telescope, which flew from 1991 to 2000. EGRET saw more gamma rays than expected in the energy range of 1 to 10 GeV. Their source was near the galactic centre and some interpreted them as products of dark matter annihilation. But, “it is a very fuzzy, blurry picture”, says astrophysicist Dan Hooper at Fermilab in Batavia, Illinois.
Then came the High Energy Antimatter Telescope carried aloft by balloons in 1994, 1995 and 2000. HEAT was looking for positrons. When cosmic rays travel through space, they can smash into interstellar dust and generate positrons and antiprotons. For energies in the range of about 10 GeV, HEAT saw more positrons than could be explained from the action of cosmic rays alone. It was a strong hint that dark matter was being annihilated. But again, “it wasn’t a very detailed picture”, says Hooper. “We needed better data to know more.”
More recently, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, saw very bright emissions of photons at energies of 511 kiloelectronvolts (KeV). You expect to find such photons when electrons and positrons annihilate each other. Calculations showed that every second about 3 × 1042 positrons were being injected into the inner regions of the Milky Way – far more than is possible from cosmic ray interactions. Again, dark matter was suspected, although the mass of these hypothetical particles would have to be in the megaelectronvolt (MeV) range, much lighter than the WIMPs favoured by theory. “But we haven’t found any conclusive test to confirm or refute the dark matter hypothesis,” says Hooper.
NASA’s WMAP satellite, which was launched in 2001 and is busy measuring the cosmic microwave background, the radiation left over from the big bang, has also spotted similar signals. To help the WMAP team to see the faint CMB signals, Douglas Finkbeiner of Harvard University studied the known sources of microwaves from our galaxy, such as emissions from hot dust and synchrotron radiation from high-energy electrons spiralling around galactic magnetic fields. The idea was to subtract these signals to reveal the CMB. “But I couldn’t get things to fit,” he says. “I kept coming up with too many microwaves in the centre of the galaxy.”
Finkbeiner realised that the electrons and positrons produced by the annihilation of dark matter would also emit synchrotron radiation, and this could give rise to what he has dubbed the WMAP haze. “To me that was a compelling coincidence,” he says.
These experiments have primed physicists for PAMELA, one of the most sensitive instruments to be sent into space. “We expect to have 10 times better statistics than all the HEAT flights,” says Mirko Boezio of the National Institute of Nuclear Physics in Trieste, Italy, who presented the first results in Philadelphia.
Graciela Gelmini of the University of California, Los Angeles, attended the sneak preview. According to Gelmini, PAMELA has also seen more positrons in the energy range of 10 to 60 GeV than can be explained by cosmic rays. For energies of 40 to 60 GeV, the excess signal was as high as 10 per cent. This in line with data from HEAT, and is statistically more significant. The excess of positrons points towards dark matter annihilation. Gelmini is cautiously optimistic. “Cosmic rays are extremely difficult to predict,” she says. “This indication [of dark matter] has to be taken with a grain of salt.”
Hooper, however, is more excited. “This may be the thing that we have been contemplating for all these years, namely dark matter annihilations.” Finkbeiner agrees: “This is potentially some of the best evidence we have seen in many years that dark matter actually is a WIMP and is annihilating.”
“This may be the thing that we have been contemplating for all these years, namely dark matter annihilations”
Physicists are waiting for PAMELA’s full results, which will be published in a few months, says Boezio. The experiment can detect positrons with energies up to 270 GeV. If dark matter is responsible for the excess of positrons, then you would expect this excess to fall sharply at some higher energy. This cut-off point is related to the mass of the WIMP since there can be no more energy in the daughter particles than there is in original WIMPs. So no positrons in excess of those generated by cosmic rays should be seen above this energy. From the fuzzy HEAT data it appears that this cut-off lies at about 238 GeV. If PAMELA shows a clear fall-off of positrons at this energy, then it would be the clearest indication yet of the mass of dark matter particles. “It is a very important next step for PAMELA to actually show where this [excess] finishes,” says Gelmini.
However, PAMELA is not sensitive to the direction of positrons and cannot tell if they are coming from the centre of the galaxy. So all eyes are on the Gamma-Ray Large Area Space Telescope, which was launched on 11 June. Like EGRET, GLAST will be looking for an excess of gamma rays from the centre of the Milky Way, and could soon confirm PAMELA’s findings.
To settle the issue once and for all, Turner would like a triple strike: PAMELA and GLAST would look for indirect signs of dark matter while underground experiments would hunt for particles of dark matter (see “Where on Earth is the dark matter?). And the Large Hadron Collider, which is due to start operating next month near Geneva, Switzerland, should be able to produce dark matter. “That way we can convince even the most sceptical that most of the matter in the universe is not made of the star stuff that we are: a truly extraordinary and humbling fact,” he says.
Where on Earth is the Dark Matter?
In April, the Dark Matter (DAMA) collaboration reported an increase in the energy of particles hitting their detectors inside the Gran Sasso Mountain in Italy every June compared with December for the past 11 years. This annual fluctuation, they said, is a sign that the Earth is moving through the sea of dark matter particles in our galaxy (91av, 25 April, p 14).
But there’s intense scepticism about the results. “I would like to see an annual modulation experiment run from a different location, maybe using a different detector material,” says Dan Hooper of Fermilab in Batavia, Illinois. “Otherwise, I am not likely to believe that the DAMA signal was created by dark matter.”
The scepticism stems from the fact that the DAMA results hint at dark matter particles with a mass that’s less than 100 gigaelectronvolts.
But none of the many other direct detection experiments, such as the Cryogenic Dark Matter Search in Soudan, Minnesota, the Chicagoland Observatory for Underground Particle Physics near Chicago and XENON-10 in Gran Sasso, have seen anything in that energy range. These experiments are looking for evidence that dark matter particles have smashed into the nuclei of their detector material, such as xenon.
It is likely that dark matter particles will not interact well with the nuclei of normal matter. So it’s possible that the direct detection experiments may not yet be sensitive enough. But as they improve, “it’s very plausible that dark matter will show up in the next few years”, says Hooper.
