Antimatter surplus is not dark matter’s smoking gun
Antimatter enthusiasts will love it; dark matter hunters not so much. NASA’s FERMI satellite has confirmed a previous hint that there is more antimatter than expected coming from space. The bad news is that the result almost certainly rules out dark matter as the source.
The results were reported online by the FERMI Large Area Telescope Collaboration. They hit the web just in time for the Topics in Astroparticle and Underground Physics conference taking place in Munich, Germany, this week, where they were immediately incorporated into the first talks.
As far as antimatter is concerned, the results back up intriguing signals picked up in 2008 by the Russian-European PAMELA satellite. The result showed that there were more positrons – the antimatter counterpart of electrons – coming from space than were expected from known processes and sources.
“This is a powerful independent corroboration of the PAMELA result,” says Pasquale Serpico of the Annecy Le Vieux Theoretical Physics Laboratory, France, who is not part of the FERMI team.
Dark-matter theorists were quick to suggest that the positrons could be the debris from interactions of dark-matter particles with each other. One even suggested that the hunt for dark matter was coming to a successful end.
FERMI has also extended PAMELA’s finding to positrons with up to twice as much energy (200 gigaelectronvolts). The satellite clearly detected extra positrons at these higher energies too – and that’s the problem.
Dark-matter theorists had been expecting to find that the number of positrons would suddenly drop at some energy level. This cut-off would be the “smoking gun” of dark matter and would fix the mass of its particle – currently unknown – because the positrons could not be created with more energy than the dark-matter particle’s mass.
However, FERMI shows no such cut-off, driving up the mass of a putative dark-matter particle into realms that make theoreticians uncomfortable.
This is because the higher the dark-matter particle’s mass, the fewer particles are needed to provide the universe’s “missing mass” – the very thing that dark matter is meant to help explain. The fewer dark-matter particles exist, the more easily they must interact to create the numbers of positrons seen. At a mass of over 200 GeV, the dark matter is starting to become more interactive than experiments or calculations really suggest. Still-controversial dark-matter detections from the Dark Matter Collaboration (DAMA) instrument inside the mountain of Gran Sasso, Italy, point to a mass of around 100 GeV per particle. That puts them in clear conflict with the FERMI data.
“The FERMI result all but rules out a dark-matter interpretation for [the PAMELA] signal. I think that the positrons [seen by FERMI and PAMELA] must be astrophysical in origin,” says Serpico. In other words, the antimatter must be coming from as-yet unidentified celestial objects, but not dark matter: the intensely energetic environments of neutron stars have been suggested as a possible source. This conclusion does not mean, however, that dark matter does not exist – just that this particular signal has turned out to be a red herring rather than a smoking gun.
The results have been released just ahead of keenly awaited results from another dark-matter experiment, CRESST, which are due to be discussed at the Munich conference this afternoon. Stay tuned for our report.