For decades, scientists have thought that the highest-energy cosmic rays—those packing up to a million trillion electronvolts—were almost exclusively protons. But data from the Pierre Auger Observatory in Argentina, the world's top facility dedicated to ultra-high-energy cosmic rays, could tell a startlingly different story. At the International Cosmic Ray Conference, held from July 7 to 15 in Lodz, Poland, Auger scientists are presenting data that raises the possibility that some of those super-speedy cosmic bullets could actually be iron nuclei.
"It would surprise a lot of people if some of these particles turned out to be iron," says Hank Glass, an Auger collaborator at Fermi National Accelerator Laboratory. While scientists can imagine mechanisms that would accelerate protons up to nearly the speed of light, they have no idea where ultra-energetic iron nuclei could originate.
"Then again," Glass adds, "nature is full of surprises."
Scientists at Auger combine two methods to investigate these mysterious extragalactic invaders. An array of 1600 water tanks, spread over 1200 square miles of Argentine grassland, acts as a giant particle detector, tracking the showers of secondary particles (see animation) that high-energy cosmic rays generate when they hit air molecules in Earth's atmosphere. A set of fluorescence telescopes picks up the faint ultraviolet glow of those secondary particles interacting with atmospheric nitrogen. Merging data from both sources, scientists can determine the energy, trajectory, and—ideally—identity of the primary particles.
One telltale statistic is the altitude at which the cosmic rays interact. Based on data from human-made particle accelerators, theorists have developed models for the interactions of various particles in Earth's atmosphere. Protons, with their small mass, should penetrate relatively deeply before they start to shower. Heavier nuclei, like iron, should generate showers further from Earth's surface.
At Auger, scientists measure a cosmic ray's "shower maximum"—the height at which it produces the most secondary particles. They have observed an average shower maximum that is farther from the Earth's surface than predictions for pure protons but closer than for pure iron. You might suppose this is the signature of intermediate-mass nuclei like carbon. But because such nuclei are less tightly bound than iron, most shatter in collisions with Cosmic Microwave Background photons before they reach Earth. A more plausible explanation is that Auger sees a mixture of protons and iron.
Plausible—but still highly contentious.
"There is no consensus within the Auger collaboration," says Glass.
For one, models for very-high-energy cosmic-ray interactions rely on substantial extrapolations from particle collider data.
"It might be that those extrapolations are just wrong, and we aren't seeing what we think we are seeing," says Eun-Joo Ahn, a postdoctoral researcher at Fermilab. Input from high-energy collisions at the Large Hadron Collider will help refine the models, she says, but even those events will fall below the energy range of the highest-energy cosmic rays.
Accompanying that uncertainty is the possibility, common to all particle physics experiments, that the detectors are not operating perfectly. Auger scientists are confident about their measurements, but they continue to monitor their detectors and scrutinize all aspects of their data.
But the sharpest clarifications, predict both Ahn and Glass, will likely come from more observations. "At this point, the data has raised more questions than it has answered," Glass says. "About all we can do is collect more data and see if a more coherent picture evolves."
In the meantime, Glass expects a flurry of theoretical papers to follow the ICRC conference, proposing origins for the highest-energy cosmic rays. Inventing the right model will be a tough task. Scientists suspect that ultra-high-energy protons originate in the accretion disks surrounding super-massive black holes, but those disks contain mostly hydrogen and helium. Iron does turn up in supernovae, but scientists do not think that exploding stars have enough energy to rev nuclei up to the energies seen at Auger.
Bottom line: no definite news yet. Still, says Glass, "in science, it's always better to show the data you have and let people talk about it, rather than to just sit tight."
by Rachel Carr