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Particle Physicists Spot Hints of Dark Matter

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Lisa Winter

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2198 Particle Physicists Spot Hints of Dark Matter

The Alpha Magnetic Spectrometer (AMS) is an instrument that has been attached to the outside of the ISS since May 2011. AMS is a particle physics detector that analyzes antimatter within cosmic rays in order to find evidence of dark matter. After studying 41 billion cosmic ray events detected by the AMS, physicists believe they have uncovered clues about the existence dark matter. Samuel Ting of MIT is the principal investigator for AMS and led the massive international collaboration of particle physicists from over 60 countries. Two papers have been published in the journal Physical Review Letters, and a third paper will be published in the future.

“Dark matter is there,” co-author Paolo Zuccon of MIT said in a press release. “We just don’t know what it is. AMS has the possibility to shine a light on its features. We see some hint now, and it is within our possibility to say if that hint is true.”


Dark matter is believed to make up approximately 25% of the universe, but it can’t be measured directly. Instead, it must be studied indirectly based on other evidence. In this case, scientists were examining the spectra from electrons and their antimatter counterparts, positrons, in cosmic rays. Positrons have the same mass as electrons, but are positively charged instead of negatively charged.

AMS researchers tabulated over 10 million electron and positron detection events in cosmic rays and found that at higher energies, the positron flux levels off much slower than the electron flux. In standard models of cosmic ray particle collisions, the fraction of positrons to electrons should decrease with energy. The opposite was actually observed.

The imbalance in the positron flux compared to the electron flux above the 30 GeV energy range suggests that the positrons and electrons originated from different sources.

“The new AMS results show unambiguously that a new source of positrons is active in the galaxy,” Zuccon explained. “We do not know yet if these positrons are coming from dark matter collisions, or from astrophysical sources such as pulsars. But measurements are underway by AMS that may discriminate between the two hypotheses.”


From these data, scientists were also able to extrapolate for the first time ever that the positron fraction maximum—the energy at which the positron fraction stops increasing—is 275±32 GeV. This is significant in that this point indicates that the excess of positrons is the signature of dark matter annihilation events.

“This is the first experimental observation of the positron fraction maximum after half a century of cosmic rays experiments,” Ting told CERN. “Measurements are underway by the AMS team to determine the rate of decrease at which the positron fraction falls beyond the turning point.”

It has been theorized that dark matter is composed of weakly interacting massive particles (WIMPs) that can collide very violently. When this event occurs, the WIMPs are annihilated and release particles, such as an electron and a positron.

Even though the data in this study are consistent with dark matter particles in the one TeV range, the levels of positrons detected could be explained as a result of pulsars. AMS still has 10 more years of measurements to make, so there is plenty of time to clarify or confirm these findings.


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