Researchers working on the AMS (Alpha Magnetic Spectrometer) experiment, which is attached to the International Space Station, have reported precision measurements of antiprotons in primary cosmic rays at energies never before attained. Based on 3.49 × 105 antiproton events and 2.42 × 109 proton events, the AMS data represent new and unexpected observations of the properties of elementary particles in the cosmos.

Assembled at CERN and launched in May 2011, AMS is a 7.5 tonne detector module that measures the type, energy and direction of particles. The goals of AMS are to use its unique position in space to search for dark matter and antimatter, and to study the origin and propagation of charged cosmic rays: electrons, positrons, protons, antiprotons and nuclei. So far, the collaboration has published several key measurements of energetic cosmic-ray electrons, positrons, protons and helium, for example finding an excess in the positron flux (CERN Courier November 2014 p6). This latter measurement placed constraints on existing models and gave rise to new ones, including collisions of dark-matter particles, astrophysical sources and collisions of cosmic rays – some of which make specific predictions about the antiproton flux and the antiproton-to-proton flux ratio in cosmic rays.

With its latest antiproton results, AMS has now simultaneously measured all of the charged-elementary-particle cosmic-ray fluxes and flux ratios. Due to the scarcity of antiprotons in space (being outnumbered by protons by a factor 10,000), experimental data on antiprotons are limited. Using the first four years of data, AMS has now measured the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays with unprecedented precision. The measurements, which demanded AMS provide a separation power of approximately 106, provide precise experimental information over an extended energy range in the study of elementary particles travelling through space.

In the absolute-rigidity (the absolute value of the momentum/charge) range 60–500 GV, the antiproton (p), proton (p), and positron (e+) fluxes are found to have nearly identical rigidity dependence, while the electron (e) flux exhibits a markedly different rigidity dependence. In the absolute-rigidity range below 60 GV, the p/p, p/e+ and p/e+ flux ratios each reach a maximum, while in the range 60–500 GV these ratios unexpectedly show no rigidity dependence.

“These are precise and completely unexpected results. It is difficult to imagine why the flux of positrons, protons and antiprotons have exactly the same rigidity dependence and the electron flux is so different,” says AMS-spokesperson Samuel Ting. “AMS will be on the Space Station for its lifetime. With more statistics at higher energies, we will probe further into these mysteries.”