New Results of the AMS Experiment on the International Space Station ISS
Today, at CERN, the European Organization for Nuclear Research, Nobel Laureate and RWTH Honorary Doctor Professor Samuel C.C. Ting presented new results of the AMS Experiment on energetic cosmic ray electrons and positrons.
RWTH Aachen and Forschungszentrum Jülich significantly contribute to the AMS Project
These results are based on the first 41 billion events measured with the Alpha Magnetic Spectrometer (AMS). These results provide a deeper understanding of the origin and the nature of high energy cosmic rays and shed more light on the existence of dark matter. Under coordination of RWTH Professor Stefan Schael, several researchers from RWTH Aachen and the JARA-Fame section at Forschungszentrum Jülich have significantly contributed to the international research project.
Indications of a New Positron Source
About 10 million of the 41 billion cosmic ray events measured by AMS have been identified as electrons and positrons. AMS has measured the positron fraction, that is the ratio of the number of positrons to the combined number of positrons and electrons, in the energy range 0.5 to 500 GeV. The researchers observed that from an energy of 8 GeV onwards, this fraction starts to quickly increase and reaches a maximum at about 275 GeV. These results indicate the existence of a new source of positrons.
Precise measurement of the positron fraction is important for understanding the origin of dark matter. Dark matter collisions can produce an excess of positrons. Ordinary collisions between cosmic rays and the interstellar medium result in the positron fraction decreasing steadily with energy – this, however, is not consistent with the new results from AMS.
A New Physics Phenomenon
Depending on the nature of dark matter, the excess of the positron fraction has a unique signature. The new results from AMS, published today in the scientific journal “Physical Review Letters,” indicate a new physical phenomenon. They are consistent with the existence of a hypothetical dark matter particle, the neutralino. New AMS measurements are underway to determine whether the observed new phenomenon results from dark matter or from astrophysical sources such as pulsars. These measurements are to determine the rate of decrease at which the positron fraction falls beyond the maximum established by the new results, as well as to measure the anti-proton fraction in cosmic rays. Results of these measurements will be reported in future publications.
Significant Differences between Electrons and Positrons
Furthermore, AMS has facilitated precise measurements of the electron flux and the positron flux, that is intensities of cosmic ray electrons and positrons. These measurements demonstrate quantitatively that the behavior of electrons and positrons are significantly different from each other both in their magnitude and energy dependence.
The behavior of the flux as a function of energy is described by the spectral index and the flux was expected to be proportional to energy E to the power of the spectral index. The result shows that neither flux can be described with a constant spectral index. In particular, between 20 and 200 GeV, the rate of change of the positron flux is surprisingly higher than the rate for electrons.
This is important proof that the excess seen in the positron fraction is due to a relative excess of high energy positrons, as expected from dark matter collisions, and not the loss of high energy electrons. These results are published today in a separate article in the scientific journal “Physical Review Letters.” This new observation of the electron and positron fluxes also demonstrates that there is a fundamental difference between matter (electrons) and antimatter (positrons).
Over the last 50 years, there have been many experiments that measured the combined flux of electrons plus positrons in cosmic rays using non-magnetic detectors. Some of these experiments indicated the possible existence of structures at 300-800 GeV hinting at new physics effects. The new AMS results show a smooth combined flux and thus rule out such effects up to an energy range of 1000 GeV.
The AMS Detector
RWTH Aachen has significantly contributed to the development of the Alpha Magnetic Spectrometer (AMS-02). The seven-ton particle detector was built by an international collaboration made up of 60 research institutes from 16 countries. On its final mission in May 2011, space shuttle Endeavour delivered the AMS to the ISS. The AMS is expected to be in operation until 2024. In Germany, the AMS-related activities receive support from the German Aerospace Center, DLR.
Professor Stefan Schael, Chair of Experimental Physics at RWTH Aachen University, has coordinated the German contributions to AMS and, together with his team and a working group from Karlsruhe Institute of Technology (KIT) headed by Professor Wim de Boer, developed and built several AMS components. In collaboration with the Jülich Supercomputing Centre headed by Professor Thomas Lippert, the research groups have significantly contributed to the published results.
The heart of the spectrometer is a multiple-plane silicon tracker surrounded by a permanent magnet, which is toroidal in shape. Due to the magnetic field, charged particles passing through the detector will follow circular paths. From the curvature, the researchers can determine the particles' energy and the sign of their charge. On the top of the AMS, which works in multiple stages, is the so-called transition radiation detector, a device developed and built at RWTH Aachen. The radiation detector played a central role in making today’s research results possible.