In 1916, Victor Hess risked his life to observe cosmic radiation by riding a balloon up to 17,500 feet without oxygen. Since then, scientists have been puzzled as to where high-energy cosmic rays come from and how they got accelerated to nearly the speed of light.

The mystery recently deepened when scientists noticed a discrepancy between the results of leading experiments that use different techniques to study ultra high-energy cosmic rays (UHECRs).

One of these experimental groups, the University of Utah’s High Resolution Fly’s Eye (HiRes), has been working hard to determine the source of this discrepancy. Their investigations have most recently led them to collaborate with the High Energy Laboratory Astrophysics (HELA) program here at SLAC, a new initiative led by Pisin Chen (ARD-A) and aimed at simulating astrophysical processes in the laboratory so that they can be studied at closer quarters.

Together, the two groups hope to not only shed some light on this new inconsistency, but also to gain a deeper understanding of how high-energy cosmic ray air showers behave.

We know that cosmic rays have a wide range of energies, beginning at the low end of high-energy physics. UHECRs stand apart from other high-energy cosmic rays because they can in principle be tracked to their origin by observing their trajectories. This is possible because their momentum is so great that, even if they carry charges, they are not deflected significantly by the magnetic fields they encounter as they travel through space.

Unfortunately, they are also very rare. At an energy of one hundred-billion-billion electron volts—two billion times greater than the highest energy particle SLAC’s accelerator can achieve, and the highest energy cosmic ray observed to date—only one such UHECR can hit a square kilometer on Earth per century, and there is no way of predicting where these rays will hit. High-energy cosmic rays do, however, cause something called an ‘air shower.’ A single high-energy cosmic ray can result in an air shower that is visible to detectors using air fluorescence technique from tens of kilometers away.

Upon entering the atmosphere, an UHECR would initially decay into hadrons, which shortly thereafter cascade into leptons, such as electrons. The air shower’s effect is similar to an avalanche’s progress—the more particles there are in the shower, the more interactions occur and the more showering particles result. By the time the shower reaches the ground the final particles are much lower in energy and have been spread wide.

Our atmosphere contains mostly nitrogen molecules. The high-energy leptons in an air shower interact with the electrons that are attached to those nitrogen molecules, imparting energy to them. This causes the nitrogen electrons to temporarily orbit further from the molecule’s nucleus.

As the electrons eventually return to their normal orbits, they emit photons in the ultraviolet range. This is what scientists called "air fluorescence," the same physical mechanism that makes fluorescent light bulbs work (but at different frequencies). Observing this fluorescence is useful in determining the initial energy of the UHECR, and is used by HiRes to detect high-energy cosmic rays.

Unlike HiRes, another leading experiment called the Akeno Giant Air Shower Array (AGASA) counts the number of shower particles that reach the earth. It compensates for the rarity of UHECRs by covering a large area with detectors of a different kind.

As both techniques have become more advanced, the two experiments have progressively increased their accuracies. It was only recently that the experiments’ resolutions were good enough for them to see the discrepancy between their results.

This unsettling discovery spurred Chen and Pierre Sokolsky, of the University of Utah and HiRes, to join forces. Together, with a team of scientists, they are attempting to confirm or correct the existing calibrations of air fluorescence technique by using SLAC’s electron beam to trigger and simulate air showers. Additionally, they hope to gain a better understanding of high-energy cosmic ray showers by cross checking the results of laboratory measurements against pre-existing computer simulations.

This improved calibration of the air fluorescence will help determine if it is inaccurate calibrations that have led to the mysterious discrepancy between HiRes and AGASA’s results.

Study of the simulations used to improve the calibration will also help scientists to better understand air showers. Chen commented, "It is a happy coincidence that the total energy of a typical SLAC beam is roughly equivalent to that of the highest energy cosmic ray observed to date."

The information gained will benefit not only the existing HiRes experiment, but also the next generation Pierre Auger Observatory, currently under construction in Argentina, as well as the next-next generation space based observatories such as the joint U.S.-European EUSO and the U.S.-lead OWL projects.

Said Sokolsky, "We’re going to be able to make a very good measurement at SLAC. This is exciting. SLAC is just perfect for this."