Hint of dark matter

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?

Composition of the Universe as measure by WMAP (“before Planck”) and as measured by Planck (“after Planck”). Normal matter like stars and galaxies comprise just 4.9% of the Universe, while dark matter, which is detected by its gravitational influence on ordinary matter, makes up 26.8% of the Universe. The remaining 68.3% is dark energy, an unknown force which accelerates the expanding Universe. (Credit: ESA & the Planck Collaboration)

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!

AMS (round instrument on the left) on the International Space Station, 31 May 2011. The AMS-02 experiment is a state-of-the-art particle physics detector. (Credit: AMS/NASA)

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.

Positron fraction as a function of energy from 0.5-350 GeV. The AMS results are shown in red (with error bars) and compared with previous published measurements. (Credit: Aguilar et al. 2013, Phys. Rev. Lett.)

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.

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[Sarah Maddison]

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One Response to Hint of dark matter

  1. Chris Flynn says:

    Fascinating follow-up work on this result has appeared very rapidly after the AMS team published their paper — within days!

    The AMS team announced in their paper that the “upturn” in the the positron to electron fraction as a function of energy could be dur to new physics — such as a yet unknown dark matter particle — or something more mundane — like pulsars. This is precisely what the followup work has looked at.

    Linden and Profumo (astro-ph 1304.1791, 2013), for example, show that the upturn can be neatly accounted for by either (or both) of two nearby puslars. The propose that they are sources of positrons at high energies — of order 1 TeV — which then diffuse through the magnetic field of the Milky Way galaxy to eventually reach the Earth and AMS, arriving in almost equal numbers in all directions (their directions of arrival have been totally scrambled in other words). A simple model of the energy distribution of the positrons and eletrons, and their subsequent journey through the Milky Way, leads to a quite good fit the the AMS data.

    Lei et al (astro-ph.HE arXiv:1303.0530v2) look at dark matter possibilities. They show that a dark matter particle, with a very large mass, of order 1 TeV, which decays into positrons/electrons, and then diffuse through the Milky Way’s magnetic field to eventually arrive at AMS. They would be mainly coming from the center of the Milky Way in this model, since that’s where the dark matter particles are most likely to interact and produce electron/positron pairs — so they have to diffuse a lot further than in the case of the pulsars. Such positrons would also be arriving at the AMS in very similar numbers from all directions, which is what AMS sees. These authors are able to produce good fits to the AMS data — jut as good as the pulsar modelers did — and even try both sources for good measure (the fits are of course just as good).

    The speed at which these papers came out indicate that the authors had a lot of the calculations ready and were just waiting for the AMS folks to publish — this happens quite often in science. Undoubtedly there’ll be more papers over the coming weeks and months!

    So – it might be pulsars, or it might be dark matter – or it might be both or neither — there are probably yet more possibilities for the upturn which need to be tried out.

    AMS will run its experiment for about two decades, so the data are going to get much better!

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