Cosmic rays are high-energy electrons that are found throughout space and are accelerated when stars explode in supernovae. But a balloon-borne detector flying over Antarctica called the Advanced Thin Ionization Calorimeter (ATIC) has detected 70 more high-energy electrons than the normal background level attributed to supernova blasts.The experiment consisted of three flights around Antarctica at an altitude of about 23 miles above earth's surface, carried by an enormous helium balloon that expands until it would be big enough to fill a football stadium.
The detector uses the principle of ionization calorimetry: several layers of the scintillator bismuth germanate emit light as they are struck by particles, allowing to compute the cosmic rays' energy. A silicon matrix is used to determine the cosmic rays' electrical charge.
In November 2008, researchers published in Nature, the finding of a surplus of high energy electrons. During a 5-week observatory period in 2000 and 2003, ATIC counted 70 electrons with energies in the range 300-800 GeV; these electrons were in excess of those expected from the galactic background. The source of these electrons is unknown, but it is assumed to be relatively close, no more than about 3000 lightyears away, since high energy electrons rapidly lose energy as they travel through the galactic magnetic field and collide with photons.The electrons could come from a nearby astrophysical object, such as a pulsar, that lies within 3000 light years from Earth. But the team has spent four years trying to fit the signal to such an object and has yet to find a good match.
The alternative is that the electrons were produced when two dark matter particles met and destroyed(annihilated) each other.That hypothesis is strengthened by the electrons' observed energies, which range from 300 to 800 gigaelectronvolts.
According to Wefel,"There is nothing that we know of in high-energy physics or astrophysics that happens in this energy range".
What's more, the signal peaked at 650 GeV and then rapidly declined to the background level at 800 GeV. According to Wefel, this is the kind of signature you would expect if a type of exotic particle known as a Kaluza Klein particle was the dark matter culprit, with the peak at 650 GeV corresponding to its mass. This type of particle is a WIMP (weakly interacting massive particle), one of the most promising candidates for dark matter, and comes from theories in which the universe has extra spatial dimensions. These extra dimensions can only be detected by observing WIMPS that have leaked into the four dimensions (three of space and one of time) that are familiar to us.
In 2007, NASA's WMAP satellite, which measures the big bang's afterglow, picked up an excess of microwaves from around the centre of our galaxy. This 'WMAP haze' could be radiation produced when dark matter particles collide.A few months ago, another group found tantalising hints of dark matter in antimatter measurements taken by a detector known as PAMELA.
So how do the results from ATIC fit in with these?
Though the data from PAMELA cover a different energy range from the ATIC signal, Wefel believes that "there is no contradiction between ATIC and PAMELA, at least to within the uncertainties on the presently available data.It is possible that we may be observing the same source."
But ATIC has detected 200 times more potential dark matter than WMAP did at the galactic centre. "We need a boost factor of 200 for the results to be compatible," says Wefel. "So either the WMAP haze is wrong, the theory is wrong, or dark matter is not uniformly distributed(this is how u speculate :)) all over the place."
With so many unanswered questions, will we ever be able to say conclusively that dark matter has been spotted? Experiments such as the recently launched Fermi Gamma Ray Space Telescope should continue to discover new possible sources of dark matter. These sources will need to be studied in other wavelengths and with other instruments in order to determine their properties.Physicists eventually hope that the LHC(Large Hadron Collider) is able to create dark matter particles when it finally gets into the running condition.Then it can be seen if any of them have the capability to produce the electron signal that ATIC observed.
How long before the search is given up?
Until Fermi and other instruments run out of new source discoveries,this elusive search will continue.Meanwhile other experiments will try to study the electrons in more detail to see if they can 'pin down' the signature of dark matter annihilation.
However the pieces fit together, other experts say the ATIC discovery is intriguing. That's because there are still some questions about what accelerates electrons and other charged particles in space, called cosmic rays.
Even if all this proves not to be dark matter, the puzzle of how these very high-energy cosmic rays are produced will still be a mystery and further work based on these rather intriguing ideas would continue to delve the human mind for years to come.