Intriguing results from the Alpha Magnetic Spectrometer aboard the International Space Station hints at evidence of annihilation from dark matter particles. Following from last week’s SAO astro news update from the Planck mission, the revised mass-energy budget of the Universe is 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Dark matter has been known about for over 80 years and is indirectly detected by its gravitational influence on ordinary matter. But can we directly detect dark matter particles?
Particle physicists and astronomers are trying three different approaches to directly detecting dark matter: by creating dark matter particles in particle accelerators like the Large Hadron Collider, by catching dark matter particles that whizz through the Earth in deep underground detectors, and by looking in space to find evidence of rare dark matter collision events.
When two dark matter particles collide, they will annihilate each other and transform their energy into high-energy photons and high-energy particles. These particles can be detected by the Alpha Magnetic Spectrometer (AMS), which was installed on the exterior of the International Space Station in May 2011 (in the last Space Shuttle Endeavour flight). The main aims of the AMS-02 experiment is to search for antimatter and dark matter, and it is constantly bombarded with high-energy particles, or cosmic rays. AMS uses a large, 3-foot magnetic ring to produce a strong magnetic field which deflects the path of the incoming charged particles as they pass through various detectors which measure the speed, energy and direction of the particles. To date AMS has measured over 30 billion cosmic rays!
This week the first results from the AMS team were released, which analysed 25 billion cosmic ray events (!) over the 18 months from May 2011 to December 2012. Of these cosmic ray events, 6.8 million were identified as electrons and their antimatter pair positrons with energies in the range 0.5 to 350 GeV. The team have measured the positron fraction, which is the ratio of the positron flux to the electron+positron flux across the 0.5-350 GeV energy range. They found that the positron fraction decreases with increasing energy, and then increases again from 10 GeV to ~250 GeV and finally appears to flatten beyond 250 GeV. This AMS data, which only represents about 10% of the expected data over the lifetime of the experiment, is of excellent quality when compared with pervious measurements of the positron fraction.
The team demonstrate that the positron fraction spectrum (the plot above) has no fine structure to it, nor does the spectrum vary with time. The positron-to-electron ratio is not anisotropic, which indicates that the positrons do not come from any preferred direction in space. The researchers conclude that they are seeing some new physical phenomena, either from particle physics or astrophysics. What is not yet known is whether this position fraction spectrum originates from dark matter particle annihilation or from pulsars in our Galaxy, which also produce electrons and positrons. Extending the spectrum to higher energies will resolve this issue.
For more information, see:
- First Result from the Alpha Magnetic Spectrometer Experiment, AMS-02 press release
- Scientists report hint of dark matter in first results from $2 billion cosmic ray detector, Phys.Org
- Shining light on elusive dark matter, ESA/DAMA mission press release
- First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV, Aguilar et al. (2013), Phys. Rev. Lett., 110, 141102 [Swinburne login required]