Did the PAMELA experiment see dark matter or didn’t it? That was one of the biggest physics mysteries of 2008, when preliminary results from the space-borne detector suggested that an anomaly in the ratio of high-energy positrons to electrons reaching Earth could be caused by annihilating dark matter. If true, the finding would be the most direct view yet of the elusive stuff that is thought to make up about 22% of the mass of the universe.

Others argued that the anomaly could have a more mundane explanation in that it is simply caused by positrons emitted by nearby pulsars. Now, however, the PAMELA team has published an analysis of different data — the anti-proton/proton ratio — which may give added weight to the pulsar explanation.

Launched in June 2006, the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite was designed by institutions in Italy, Russia, Germany and Sweden to examine the nature of antiparticles in cosmic rays. In November 2008 the PAMELA team reported preliminary results that suggested that the ratio of positrons to electrons at energies above 10 GeV is greater than predicted by theories that only take into account the production of positrons from interactions between cosmic rays and interstellar gas.

Pulsars or dark matter?

Although the PAMELA collaboration said at the time that the excess positrons could come from a nearby pulsar, they also suggested that it could be due to dark-matter particles annihilating.

The PAMELA team has published a paper on the ratio of anti-protons/protons in the cosmic ray flux, which should also be affected by dark matter annihilating to create anti-protons. PAMELA measured this quantity at energies between 1 and 100 GeV during 500 days of data collection (Phys. Rev. Lett. 102 051101).

The team found that ratio of anti-protons/protons rose from nearly zero at 1 GeV to a maximum value of about 0.0002 at around 10 GeV, before dropping back down again. According to the team, the data suggest that the anti-protons are produced when cosmic rays collide with instellar gas — with no need for dark matter annihilation.

An open question

So where does this leave the positron/electron results? PAMELA team member Wolfgang Menn of the University of Siegen in Germany told physicsworld.com that this remains an open question — and that dark-matter annihilation remains a possible explanation, along with pulsars.