Colorado Bound: The Pierre Auger Project

In southeastern Colorado, a land better known for crops and livestock may soon host the world’s largest astrophysical detector.

If an international consortium of scientists gets a green light from funding agencies, it will build an array of tanks of purified water—which make passing cosmic rays visible to sensors—across an area of Colorado nearly as large as the state of New Jersey.

The tanks are only 12 feet in diameter, but the network of 4400 of them placed 1.4 miles apart will cover 8,000 square miles (20,000 square kilometers). The result will be a massive “net” for catching the detritus of some of the highest-energy particles in the universe.

The project is a tremendous undertaking, not only because of its size. The network of tanks will crisscross through farms, ranches, towns and counties, and to succeed, the scientists need more than scientific ingenuity. Also integral is recruiting communities—including farmers and ranchers more interested in agriculture than astrophysics— to become partners in exploring the sky.

Bigger is Better

The Pierre Auger Cosmic Ray Observatory is dedicated to detecting and studying ultra-high energy cosmic rays, the most energetic particles in the universe. The observatory first built a detector array in Argentina, completed in 2008, called Auger South. This array in the southern hemisphere consists of 1,600 water tanks— each holding 3,000 liters of purified water—laid out on a triangular grid 1.5 kilometers on a side.

The proposed array in Colorado—Auger North—is more than the northern hemisphere complement to its Argentinean brother; it dwarfs Auger South, covering seven times the area. “As far as size goes, it’s really amazing,” says Angela Olinto, an astrophysicist who is part of that consortium, as well as a member of the Kavli Institute for Cosmological Physics. “You have to drive for hours to cross from one side to the other.”

Size is important because of the difficulty in detecting these particles. The highest-energy cosmic rays slam into the atmosphere with an energy of 10²⁰ eV, creating a cascade of secondary particles that spread over many square kilometers. Much more energetic than even the highest energy particles that will be collided at the Large Hadron Collider, these mysterious denizens of the cosmos might be able to tell scientists about some of the most energetic events in the universe.

Unfortunately, one of these extremely high-energy cosmic rays only reaches any given square kilometer of the earth once a century. The first ultra-high-energy cosmic ray was detected in Utah in 1991 by a detector array called the Fly’s Eye. Its energy was so high it challenged scientists’ assumptions about how high the energy of a cosmic ray could be and spawned the international Pierre Auger Collaboration.

There’s an intriguing difference between the common low-energy cosmic rays and the rare ultra-high-energy particles: the garden variety ones have been traveling around the universe for perhaps billions of years, bent this way and that by magnetic fields, while the rare gems are relatively local and haven’t lost their direction. That means they can point back to where they came from.

“For me that is the exciting part, it is essentially a new window,” says Case Western Reserve University astrophysicist Corbin Covault. “It’s like a new kind of telescope and a new energy or frequency band. It is just instead of looking at light or electromagnetic particles, you are looking at charged nuclear particles.”

So far, they seem to come from the directions of active galactic nuclei, says Auger collaboration co-spokesperson Paul Sommers, a physicist at Penn State University. These are “galaxies that have giant super-massive black holes in their centers that are actively consuming gas and dust and spewing out energetic particles.”

Auger South turned up an unexpected mystery. Before this detector was built, astrophysicists would have said that cosmic rays are almost universally protons—that is, hydrogen atoms stripped of their electron so that they are bare, charged protons traveling across the universe. But the cascade of particles — the showers — look quite different than the scientists expected.

“We get something that looks like iron or even gold,” says Olinto. “That’s weird. Astrophysically, why would iron or something even heavier than that be coming all the way from very far away?”

If the particles really are heavy ions, then that would be exciting and challenging for astrophysicists, says Sommers. If they are protons, then they are interacting in ways that challenge current models of particle physics.

Colorado Bound

When it came to planning Auger North, Colorado was chosen because it is relatively high, dry and flat, and the array can be expanded to the east if it ever needs to be made bigger. The southeastern part of the state is also sparsely populated, which isn’t necessarily an issue for the tanks, but the accompanying air fluorescence telescopes need dark skies to see the development of a cosmic-ray air shower.

The array is slated to be so much larger than Auger South because the highest energy cosmic rays are only detected by Auger South twice a month. For Auger North, tanks will be placed in a square array at the intersections of roads, minimizing the impact on landowners. Completed, the array will cover seven times the area of Auger South with fewer than three times as many tanks.

The project’s proposal is currently under review by the National Science Foundation and the Department of Energy. Around a third of the observatory’s $125 million budget is expected from the U.S., and the remainder from international collaborators. Tanks were on display at community centers around southeastern Colorado so farmers and ranchers could actually see what the scientists plan to build. (Credit: Pierre Auger Observatory)

While applying for funding, Auger Observatory scientists have also been recruiting community support. The scientists are keenly aware of the importance of the public perception of their project. “We want the people to love our tanks,” says Olinto, who helped select the Colorado site. “Our hope is to make them certainly interested in why we are doing this and the whole big picture of why does it matter.”

To earn this trust, the scientists are building on the ways Auger South was successfully integrated in Argentina and continues to be a good neighbor. For instance, in Malargue, Argentina, the Auger collaboration hosts an annual science fair and participates in a local parade. Visiting scientists give talks in the community, and children have helped name the tanks.

Because of the economic opportunities the observatory will create in the area, a southeastern Colorado economic development corporation paid the way for some county commissioners and a local newspaper reporter to travel to Argentina. “They got personal experience with how we operate,” says John Harton, an Auger Observatory scientist at Colorado State University who serves as a liaison with the local community, “and they were able to come back and tell their neighbors.”

Along with organizing and participating in a range of community meetings, scientists also set up tanks at community centers around southeastern Colorado so farmers and ranchers could actually see what the scientists plan to build. This way “they can kick it and they can see that it is smooth and it won’t hurt their livestock,” said Harton. “The more people know about it and the more they understand, the more they are positive about our project.”