The mysterious origin of ultra-high energy cosmic rays is, it seems, still a mystery. Two years ago, scientists at the Pierre Auger Observatory in Argentina thought they had it solved. They published a paper in Science, based on two dozen particles, that there was a correlation with the location of Active Galactic Nuclei — supermassive black holes that accelerate jets of material at near-light speed throughout the universe. At the time of the announcment, there was some doubt: The Hi-Res project, which scans the northern sky like Auger does the south, found no such correlation.

And now, today, Stefan Westerhoff, an Auger scientist from the University of Wisconsin at Madison, said that, based on new particle detections — they have more than 50 now — the correlation no longer holds. “The signal strength is certainly considerably weaker now,” he told his audience. “This is certainly a disappointment.”

But the correlation isn’t so weak that they can give up. The 70% correlation between the cosmic rays and the AGN at the time of the Science publication has now dropped to about 40% — considerably less, but not enough to support the null hypothesis. What could cause some particles to come from AGN, but not others? Westerhoff says it might have something to do with their composition. Maybe the protons come from the AGN, whereas higher mass cosmic rays, say iron nuclei, do not.

Westerhoff says this will be sorted out as they track more particles — which can only come with more time and bigger detectors. If Pierre Auger is the size of Rhode Island, the proposed Pierre Auger North, not too far from Denver, would be the size of Massachusetts — and Westerhoff showed a slide how they can get the necessary statistics in a decade or two. No offense to the lovely state of Colorado, but I say keep the cow pasture free of Cerenkov detectors, and give folks like JEM–EUSO a chance to stick their 2.5-meter camera on the space station.

Image: Pierre Auger