What did PAMELA see? | University of Oxford
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What did PAMELA see?

Pete Wilton

Could we already have detected dark matter?

That was the question posed by the news earlier this year that the PAMELA satellite has observed more high energy positrons than expected.

One thing that could have generated these positrons are quantum collisions between dark matter particles - called annihilations.

Now Philipp Mertsch and Subir Sarkar from Oxford University's Department of Physics have come up with a way of testing whether these positrons really come from dark matter or from other phenomena such as pulsars. A report of their work has just been published in Physical Review Letters.

I asked Philipp and Subir about their ideas and how they could lead to a dark matter detector...

OxSciBlog: How do we think dark matter collisions generate high energy positrons?
Philipp Mertsch:
If dark matter is made of new elementary particles (and antiparticles) created in the Big Bang then these will occasionally annihilate with each other in the dense environ of our Galaxy. Most of the energy released like this goes into neutrinos or gamma-rays but a small fraction is released as energetic positrons into the cosmic radiation.

OSB: What are the challenges involved in detecting these positrons and tracing them back to their source?
Subir Sarkar: Since everything is made of matter (rather than antimatter) the positrons are outnumbered by electrons in the cosmic radiation. A detector with a powerful magnet is necessary to separate their trajectories and measure the individual fluxes. Care must be taken to not confuse the more abundant protons (also positively charged!) with positrons.

Tracing charged particles back to their sources is impossible as they are deflected by the magnetic fields in the Galaxy and execute a random walk rather than travelling in a straight line.

OSB: How would your test discriminate between the different possible sources?
Philipp Mertsch:
Of the various proposals to create the positrons, neither dark matter annihilations nor nearby pulsars can create heavy nuclei. So there should be no deviation from the usual expectation for the abundances of secondary nuclei like lithium, beryllium and boron in the cosmic radiation which are created by spallation of heavier more abundant nuclei like carbon, nitrogen and oxygen.

However the model in which the positrons are created by the acceleration of cosmic rays in a nearby supernova shock wave also predicts an associated increase in the secondary nuclei according to our calculations.

OSB: What now needs to be done to put your ideas into practice?
Subir Sarkar: Measuring the ratio of the flux of boron nuclei compared to carbon nuclei can therefore discriminate between the dark matter and pulsar source models on the one hand and the nearby cosmic ray accelerator model on the other hand. This ratio is currently being measured with unprecedented accuracy by the same PAMELA satellite which also detected the positron excess so we are looking forward to their results.

An even bigger and more powerful experiment AMS is scheduled to fly on the International Space Station in a year's time. So the test we have proposed will soon be carried out. 

UPDATE: Read this related article on PhysOrg.com