BBC News - Alpha Magnetic Spectrometer zeroes in on dark matter
A $2bn experiment on the space station has made observations that could prove to be the first signs of dark matter, a mysterious component of the Universe.
The Alpha Magnetic Spectrometer (AMS) surveys the sky for high-energy particles, or cosmic rays.
It has seen evidence of what may prove to be dark matter colliding with itself in what is known as “annihilation”.
But scientists stress a precise description of this mysterious cosmic component is still some way off.
“It could take a few more years,” AMS deputy spokesman Roberto Battiston, a professor of physics at the University of Perugia, told BBC News.
Dark matter accounts for most of the mass in the Universe.
It cannot be seen directly with telescopes, but astronomers know it to be out there because of the gravitational effects it has on the matter we can see.
Galaxies, for example, could not rotate the way they do and hold their shape without the presence of dark matter.
The AMS - a kind of particle accelerator and nicknamed the “space LHC” in reference to the Large Hadron Collider here on Earth - has been hunting for some indirect measures of dark matter’s properties.
The AMS counts the numbers of electrons and their anti-matter counterparts - known as positrons - falling on its detectors.
Theory suggests that showers of these particles should be produced when dark-matter particles collide somewhere in space and destroy each other.
In a paper to be published in the journal Physical Review Letters, the AMS team reports the observation of a slight excess of positrons in the positron-electron count - an outcome expected of these dark matter annihilations.
The group also says the positrons fall on the AMS from all directions in the sky with no particular variation over time.
This is important because specific locations or timing variations in the signal could indicate a more conventional source for the particles, such as a pulsar (a type of neutron star) rather than dark matter.
The AMS was placed on the International Space Station in 2011. The longer it operates, the better its statistics will be and the more definitive scientists can be in their statements.
The Physical Review Letters paper reports the positron-electron count in the energy range of 0.5 to 350 gigaelectronvolts (GeV).
The behaviour of the positron excess across this energy spectrum fits with the researchers’ expectations. However, the “smoking gun” signature would be to see a rise in this ratio and then a dramatic fall. This has yet to be observed.
“At the moment, all we can say is that the (dark matter) particles could have a mass of several hundred gigaelectronvolts, but there is much uncertainty,” said Prof Battiston. (By way of comparison, a proton, the particle in the nucleus of every atom, has a mass of about 1 GeV).