Mistaken idenity

Dark matter comprises more than 85 percent of matter in the universe. Still, no direct hints about its particle nature have been conclusively detected.

Recent attempts to confirm a dark matter decay signal from the dwarf galaxy Draco came up empty handed, reported UC Santa Cruz physicists, Stefano Profumo and Tesla Jeltema.

The signal claims, by two separate research groups, were based on observations of an anomalous bright line among X-ray spectra emitted from clusters of galaxies considered to be the largest dark matter–dominated systems in the universe.

A bright line was found at 3.5 keV, a result predicted by one model in which a sterile neutrino would decay into a 3.5 keV photon and an ordinary neutrino. However, ordinary potassium ions can create similar spectral lines. A follow-up study, in an area without potassium, detected no bright line. This ruled out the sterile neutrino dark matter hypothesis at a 99 percent confidence level, explained Profumo.

“We’re back to the drawing board, but there’s no shortage of dark matter candidates,” he added.

“If a signal is found, and confirmed, then we’ll learn something fundamental about particle physics theory, which doesn’t include dark matter right now,” said Jeltema.