Dark matter signal recedes into the shadows

From NewScientist.com

Dark matter seems to be receding further into the shadows. Last year, researchers thought they may have spotted its signature when a balloon-borne experiment called ATIC detected a bizarre spike in the number of high-energy electrons streaming in from space.

But now, NASA's Fermi space telescope finds no such spike – only subtle hints of a slight increase, suggesting that dark matter is not leaving any obvious trace in the charged particles detected from space.

Nobody knows exactly what dark matter is, but the leading theoretical model posits that it is made of up of particles called WIMPs (weakly interacting massive particles). When two WIMPS collide, the theory says, they annihilate, producing radiation and a cascade of particles, including electrons.

So researchers were excited last November, when a team studying data from two ATIC balloon flights over Antarctica reported finding many more electrons than expected at high energies around 600 gigaelectronvolts. Less exotic objects, such as pulsars and supernova remnants, also accelerate charged particles to high energies, so the ATIC data could potentially be explained by such garden-variety fare.

But the abundance and high energies of the 'extra' electrons detected, coupled with another unexpected cosmic ray result measured a few months earlier by a satellite called PAMELA, raised the tantalising possibility that dark matter – perhaps of an exotic type – might be responsible.

"Particle physicists have not had much to get excited about in the last 10 years – they were all ready for the Large Hadron Collider and then had a big setback" when it broke down, says Douglas Finkbeiner, a dark matter theorist at the Harvard-Smithsonian Center for Astrophysics. "Then PAMELA and ATIC came along with extra high-energy signals that could not be easily explained, and it was fun to think about."

Now, however, astrophysicists using the Fermi telescope say they don't see a dramatic spike in the number of high-energy electrons in space. "Our energy spectrum doesn't have prominent features," says Alexander Moiseev, a Fermi team leader at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Pros and cons

What could explain the discrepancy? For one thing, the two experiments have different strengths and weaknesses.

ATIC flew in Earth's atmosphere, which can create extra "noise" in its signal, while Fermi is a space mission. ATIC's balloon flights also lasted for no more than three weeks, while Fermi is constantly taking data from orbit.

Indeed, the Fermi team analysed more than 4 million high-energy electrons detected with the telescope over the course of about six months to arrive at their result, collecting hundreds of times more data at these energies than any previous measurement. "We are practically free of statistical errors," Moiseev told New Scientist.

But ATIC has a thicker calorimeter, an instrument at the bottom of its detector that incoming space particles strike, generating showers of other particles. "The deeper or thicker that calorimeter is, the less of that shower energy sneaks out the bottom," says ATIC team leader John Wefel of Louisiana State University in Baton Rouge.

"They can contain 68% of the energy, and we contain 85% of the energy," Wefel told New Scientist. As a result, he says there is more uncertainty in Fermi's measurement of the energy of incoming particles, which could broaden out any dramatic spikes like the one seen by ATIC.

"The reason they're not seeing that peak structure is because they have much poorer energy resolution in their instrument," says Wefel. He adds that his team has analysed a third balloon flight since the original ATIC announcement in November and finds the same sharp peak as before.
Instrumental effect?

The Fermi team acknowledges that it has a thinner calorimeter but says its detector is better in other ways – it boasts an instrument that tracks the path of incoming particles, for example – something that ATIC does not have. It has also run detailed computer algorithms that show its energy resolution is sharp enough to be able to see a spike in energetic electrons. "We would see an ATIC-like bump with huge confidence if it were there," maintains Moiseev.

Gregory Tarle, a physicist at the University of Michigan who is not affiliated with either team, agrees. The bump seen by ATIC "was probably an instrumental effect they hadn't compensated for", he says.

Both experiments have to grapple with the same basic challenge, Tarle explains – distinguishing between electrons and the much more abundant protons that pass through their detectors from space. Since neither experiment uses a magnetic field that could tell the two kinds of charged particles apart, the teams must try to do this by analysing the characteristics of the particle showers in their detectors.

"It's hard to make these measurements – very hard," says Tarle. But he says Fermi's calorimeter is better suited for the analysis than ATIC's. It is made of atoms that have a higher number of protons, which do not readily interact with protons coming in from space. That causes "less contamination in the electron

So if the ATIC bump isn't real, what does that mean for dark matter?

Tarle says it means that high-energy electron detectors such as ATIC and Fermi do not show any evidence for dark matter. "There's nothing in their data that could indicate new physics," he says.

But other researchers say Fermi's data does show what may be a subtle sign of dark matter. If they look at the data in the most conservative way, Fermi team members do not see this potential signature – they say the electron energy spectrum they measure is smooth, without any wiggles that might indicate 'extra' electrons.

If they are not as conservative, however, Fermi team members say they see a slight bump in the number of electrons at higher energies – though nothing as dramatic as ATIC's.

That gentle bump, they say, might be due to a slight theoretical underestimation of how many high-energy cosmic rays are produced in objects such as pulsars – an idea Tarle favours.

"The most likely explanation of the excess electrons at high energy seen by Fermi is that the theoretical estimates are wrong," Tarle says. "There is no reason to believe that these theoretical predictions based on lower energy data are valid in the high-energy regime of Fermi."
'Hard to fit'

Alternatively, it might be due to one or more nearby sources that are pumping out energetic electrons. The sources are thought to be nearby because high-energy electrons lose energy as they travel through space, so for them to arrive at the energies that Fermi detects, they must have come from somewhere within about 3000 light years of Earth.

The nearby sources could be pulsars, but "dark matter is not ruled out" as a possible source, says Moiseev.

Finkbeiner agrees. Last year, he and colleagues came up with a new model of dark matter that could account for both the PAMELA and ATIC signals.

After the Fermi team released its results at a physics meeting earlier this week, Finkbeiner said his inbox was flooded with emails saying, "So, annihilating dark matter is dead, right?" he says. "Nothing could be further from the truth."

"It was always a little bit hard to fit the ATIC bump," he says, explaining that such a sharp spike hints that dark matter might be annihilating straight to electrons – a process that is theoretically forbidden.
'Less information'

His and other dark matter models instead argue that annihilating dark matter particles create intermediate particles – such as pions – before producing electrons.

"It's hard to make a sharp feature but easy to make a broad, smooth feature" like the one Fermi may be seeing, he says, adding that the same is true for electrons produced in astrophysical sources such as pulsars.

"In a way, it's a relief we don't have to make the ATIC bump, but if ATIC is real, it would really be telling us something," Finkbeiner told New Scientist. "We're not likely to learn as much about dark matter from [Fermi's electron spectrum] – basically, we have less information than we had before."
Tricky observation

If further observations with Fermi suggest there is not even a gentle rise in the number of high-energy electrons it detects, that will make any annihilating dark matter difficult to observe – but it will necessarily not rule it out, says Finkbeiner.

"Before the ATIC and PAMELA results, the expected annihilation signal for the leading dark matter candidate, the WIMP, was much smaller, so failure to find a signal with Fermi does not in any way rule out conventional WIMP annihilation," he says.

"Of course, there could be no signal at all: dark matter could just sit there and gravitate and do absolutely nothing else," he adds. "That's kind of the most boring scenario: we can never learn what kind of particle it is."
Tracing the source

The Fermi team hopes to shed light on the issue by continuing to collect electron data from all over the sky. It's difficult to trace the source of electrons that fall into its detector because the charged particles are diverted by magnetic fields in space. But if Fermi detects even a slight excess of electrons in one region of the sky, it might point to their source, says Moiseev.

Fermi is also hunting for possible signs of dark matter in the distribution of gamma-ray photons in the sky. Gamma rays are thought to be produced by annihilating dark matter and unlike electrons, are not affected by intervening magnetic fields (see Where will new Fermi telescope find dark matter?).

Future experiments might also provide a cross-check of both ATIC and Fermi. One, called the Alpha Magnetic Spectrometer, may fly to the International Space Station before the shuttles are retired in 2010. It uses a magnetic field to separate charged particles and has a calorimeter a little thicker than Fermi's.

Journal reference: Physical Review Letters (vol 102, p 181101)