ScienceCasts: A Whiff of Dark Matter on the ISS



Physics – Positrons Galore


The positron fraction in high-energy cosmic rays

Physics – Positrons Galore.

Antimatter is rare in the universe today. As far as we know, all relic antimatter produced in the big bang disappeared long ago in annihilation reactions with matter particles. What this means is that any antimatter particles that we can detect in the flux of energetic cosmic rays near Earth must have been created by “new” sources within our Milky Way Galaxy. (Antimatter particles from extragalactic sources are also conceivable, but they are exceedingly unlikely to make it to Earth before losing all their energy or annihilating.

The 400,000 positrons they have measured constitute the largest set of cosmic-ray positron data, increasing the total world sample a hundredfold. In addition, the range of the reported positron fraction extends out to a few hundred GeV, beyond the reach of previous experiments flown on high-altitude balloons [2] or space shuttles and satellites [3] (see 5 January 2012 Synopsis). 

First Result from the Alpha Magnetic Spectrometer Experiment

AMS-02 » Archive » First Result from the Alpha Magnetic Spectrometer Experiment.

– Extracts from the article –

The Alpha Magnetic Spectrometer (AMS) Collaboration announces the publication of its first physics result in Physical Review Letters. The AMS Experiment is the most powerful and sensitive particle physics spectrometer ever deployed in space. As seen in Figure 1, AMS is located on the exterior of the International Space Station (ISS) and, since its installation on May 19, 2011 until the present, it has measured over 30 billion cosmic rays at energies up to trillions of electron volts.

In the initial 18 month period of space operations, from May 19, 2011 to December 10, 2012, AMS analyzed 25 billion primary cosmic ray events. Of these, an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons. The 6.8 million particles observed in the energy range 0.5 to 350 GeV are the subject of the precision study reported in this first paper.

AMS has measured the positron fraction (ratio of the positron flux to the combined flux of positrons and electrons) in the energy range 0.5 to 350 GeV. We have observed that from 0.5 to 10 GeV, the fraction decreases with increasing energy. The fraction then increases steadily between 10 GeV to ~250 GeV. Yet the slope (rate of growth) of the positron fraction decreases by an order of magnitude from 20 to 250 GeV. At energies above 250 GeV, the spectrum appears to flatten but to study the behavior above 250 GeV requires more statistics – the data reported represents ~10% of the total expected. The positron fraction spectrum exhibits no structure nor time dependence. The positron to electron ratio shows no anisotropy indicating the energetic positrons are not coming from a preferred direction in space. Together, these features show evidence of a new physics phenomena.