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 (CERN
Courier September 2006 p8). 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