The satellite experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has made a new measurement of the antiproton-to-proton flux ratio in cosmic rays with energies up to 100 GeV. The results, which represent a great improvement in statistics compared with data published previously, provide significant constraints on exotic sources of cosmic antimatter.
The PAMELA experiment has been in low Earth-orbit on the Resurs-DK1 satellite since its launch in June 2006. During 500 days of data collection it has identified 1000 antiprotons with energies in the range 1–100 GeV, including 100 antiprotons with an energy above 20 GeV. This is a larger data sample at higher energies than any other experiment has obtained.
Cosmic antiprotons can be made in particle (mainly proton) collisions with interstellar gas but they could also have more exotic origins, for example, in the annihilation of dark-matter particles. Finding out more about the actual production mechanisms requires detailed studies of the antiproton energy spectrum over a wide energy range, which in turn depend on data with good statistics, as PAMELA now provides.
Analysis of the data from PAMELA show that the antiproton-to-proton flux ratio rises smoothly to about 10 GeV, before tending to level off. The results match well with theoretical calculations that assume only secondary production of antiprotons by cosmic rays propagating through the galaxy. This places limits on contributions from other, more exotic sources.
O Adriani et al. 2009 Phys. Rev. Letts. 102 0511011.