This has been covered widely today, here is a sampling of stories and reference:

From the BBC:

Antimatter belt around Earth discovered by Pamela craft

A thin band of antimatter particles called antiprotons enveloping the Earth has been spotted for the first time.

The find, described in Astrophysical Journal Letters, confirms theoretical work that predicted the Earth’s magnetic field could trap antimatter.

The team says a small number of antiprotons lie between the Van Allen belts of trapped “normal” matter.

The researchers say there may be enough to implement a scheme using antimatter to fuel future spacecraft.

The antiprotons were spotted by the Pamela satellite (an acronym for Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) – launched in 2006 to study the nature of high-energy particles from the Sun and from beyond our Solar System – so-called cosmic rays.

These cosmic ray particles can slam into molecules that make up the Earth’s atmosphere, creating showers of particles.

Many of the cosmic ray particles or these “daughter” particles they create are caught in the Van Allen belts, doughnut-shaped regions where the Earth’s magnetic field traps them.

Among Pamela’s goals was to specifically look for small numbers of antimatter particles among the far more abundant normal matter particles such as protons and the nuclei of helium atoms.

‘Abundant source’

The new analysis, described in an online preprint, shows that when Pamela passes through a region called the South Atlantic Anomaly, it sees thousands of times more antiprotons than are expected to come from normal particle decays, or from elsewhere in the cosmos.

Read more here

From CERN:

PAMELA pins down cosmic antiproton flux

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 exotic sources

From http://arxiv.org/abs/1107.4882

The discovery of geomagnetically trapped cosmic ray antiprotons

(Submitted on 25 Jul 2011)

The existence of a significant flux of antiprotons confined to Earth’s magnetosphere has been considered in several theoretical works. These antiparticles are produced in nuclear interactions of energetic cosmic rays with the terrestrial atmosphere and accumulate in the geomagnetic field at altitudes of several hundred kilometers. A contribution from the decay of albedo antineutrons has been hypothesized in analogy to proton production by neutron decay, which constitutes the main source of trapped protons at energies above some tens of MeV. This Letter reports the discovery of an antiproton radiation belt around the Earth. The trapped antiproton energy spectrum in the South Atlantic Anomaly (SAA) region has been measured by the PAMELA experiment for the kinetic energy range 60–750 MeV. A measurement of the atmospheric sub-cutoff antiproton spectrum outside the radiation belts is also reported. PAMELA data show that the magnetospheric antiproton flux in the SAA exceeds the cosmic-ray antiproton flux by three orders of magnitude at the present solar minimum, and exceeds the sub-cutoff antiproton flux outside radiation belts by four orders of magnitude, constituting the most abundant source of antiprotons near the Earth.

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