New AMS Results – hints of TeV Dark Matter?

September 20, 2014 at 3:16 pm 1 comment

Yesterday the AMS Collaboration released updated results on the positron excess. The press release is available at the CERN press release site. (Unfortunately, the AMS web site is down due to syntax error – I’m sure this will be fixed very soon.)

The Alpha Magnetic Spectrometer was installed three years ago at the International Space Station. As the name implies, it can measure the charge and momenta of charged particles. It can also identify them thanks to a suite of detectors providing redundant and robust information. The project was designed and developed by Prof. Sam Ting (MIT) and his team. An international team including scientists at CERN coordinate the analysis of data.

AMS installed on the ISS. Photo from bowshooter blog.

There are more electrons than positrons striking the earth’s atmosphere. Scientists can predict the expected rate of positrons relative to the rate of electrons in the absence of any new phenomena. It is well known that the observed positron rate does not agree with this prediction. This plot shows the deviation of the AMS positron fraction from the prediction. Already at an energy of a couple of GeV, the data have taken off.

AMS positron fraction compared to prediction.

AMS positron fraction compared to prediction.

The positron fraction unexpectedly increases starting around 8 GeV. At first it increases rapidly, with a slower increase above 10 GeV until 250 GeV or so. AMS reports the turn-over to a decrease to occur at 275 ± 32 GeV though it is difficult to see from the data:

AMS positron fraction.  The upper plot shows the slope.

AMS positron fraction. The upper plot shows the slope.


This turnover, or edge, would correspond notionally to a Jacobian peak — i.e., it might indirectly indicate the mass of a decaying particle. The AMS press release mentions dark matter particles with a mass at the TeV scale. It also notes that no sharp structures are observed – the positron fraction may be anomalous but it is smooth with no peaks or shoulders. On the other hand, the observed excess is too high for most models of new physics, so one has to be skeptical of such a claim, and think carefully for an astrophysics origin of the “excess” positrons — see the nice discussion in Resonaances.

As an experimenter, it is a pleasure to see this nice event display for a positron with a measured energy of 369 GeV:

AMS event display: a high-energy positron

AMS event display: a high-energy positron

Finally, AMS reports that there is no preferred direction for the positron excess — the distribution is isotropic at the 3% level.

There is no preprint for this article. It was published two days ago in PRL 113 (2014) 121101″

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Entry filed under: Astronomy.

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