Fermilab Physicists Present Their Reworked Plans for Giant Neutrino Experiment

Go west! Physicists would shoot neutrinos from Fermilab (above) through Earth to a detector in South Dakota. Options for reusing the existing beam line, which shoots neutrinos to Minnesota, were deemed less promising.

Reidar Hahn/Fermilab

Five months ago, the U.S. Department of Energy (DOE) asked physicists at the United States' sole laboratory specializing in particle physics to chop plans for their $1.9 billion flagship experiment for the next decade into more affordable chunks. Yesterday, officials from Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, presented their revised plan for the first stage of the Long-Baseline Neutrino Experiment (LBNE) to a federal advisory panel. The plan’s preferred option would cost $789 million—but U.S. planners say they could improve the project substantially if foreign governments are willing to chip in another $135 million.

"I think we are all pleased with the work that has been done to come up with a better approach that is not nearly as expensive as the first proposal," says William Brinkman, director of DOE's Office of Science, attended the presentation at a meeting of the High Energy Physics Advisory Panel in Rockville, Maryland. Young-Kee Kim, Fermilab's deputy director and chair of the LBNE Reconfiguration Steering Committee, and colleagues submitted a written report to Brinkman on 7 August.

Neutrinos are perhaps the least understood of the known fundamental bits of matter. They come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—that can morph into one another as the particles zip along at near-light speed. LBNE, at least as originally designed, would make definitive measurements of those "neutrino oscillations." It would also look for evidence that neutrinos and antineutrinos oscillate differently, a scenario called CP violation that could help explain how the universe evolved to contain so much more matter than antimatter. If built deep underground, the LBNE detector could also look for signs that protons decay, as predicted by some theories, and could detect neutrinos from supernovae and by cosmic rays hitting the atmosphere.

Kim presented the new plan for the first stage of LBNE. The preferred option calls for building a new beam line at Fermilab to fire neutrinos 1300 kilometers through Earth to a particle detector on the surface at the abandoned Homestake gold mine in Lead, South Dakota. That detector would contain 10,000 tonnes of frigid liquid argon. (The original LBNE design called for a larger, 34,000-tonne detector buried 1480 meters deep in Homestake and a smaller "near detector" at Fermilab.)

The steering committee considered alternative plans to continue to using Fermilab's existing neutrino beam, which currently fires neutrinos 735 kilometers to the Soudan Mine and 810 kilometers to a surface detector at Ash River, both in Minnesota. By upgrading the current beam, researchers estimated that they could save enough money to replace the existing detectors, building either a 30,000-tonne liquid argon detector at Ash River or a 15,000 tonne detector at a depth of 713 meters in Soudan. But the steering committee ultimately decided that building at Homestake would be a better first step toward a richer long-term program, Kim says.

Some physicists have expressed skepticism about placing the new detector on the surface. There, interference from cosmic rays and other sources of background radiation would swamp any signal from proton decays or supernova explosion. Researchers would still be able to detect the arrival of neutrino pulses from Fermilab by using timing systems, but the measurements would be less sensitive than they would be underground.

Fermilab officials believe they could solve that problem, however, if they can scrounge up another $135 million. That would allow them to build the 10,000-tonne detector underground at Homestake. And they're hoping that Europe, India, and other U.S. funding agencies might pony up that amount, Kim says. India has expressed interest in collaborating, she adds.

Attracting the extra money may not be far-fetched. European physicists already have their own neutrino experiments in which they fire neutrinos 732 kilometers from the European particle physics laboratory CERN near Geneva, Switzerland, to Italy's Gran Sasso National Laboratory below the Apennine Mountains. And some European researchers would like to build a bigger experiment, perhaps shooting neutrinos 2300 kilometers north to the Pyhäsalmi Mine in central Finland.

But paying for that new infrastructure could be difficult, because CERN is already committed to expensive upgrades to its Large Hadron Collider, the world's largest atom smasher. With such work on its plate, CERN would struggle to fit a new major project into its $1.2 billion annual budget, many physicists say. So a much cheaper buy-in on LBNE may be an attractive option. "The idea that with a contribution from Europe [the LBNE detector] could go underground and be competitive with other proposals, I'm sure it will attract interest in Europe," says Sergio Bertolucci, CERN's research director.

But before Europe considers chipping in, the United States will have to commit to the project, Bertolucci says. It's still not clear that the United States will. "We are considering how to fit [the new plan] into our budget going forward," Brinkman says. "There are a lot of demands on the high-energy physics budget."

The new plan will undergo its first DOE review at the end of October, Kim says. If it passes, researchers will get the go-ahead for so-called engineering design work. If DOE ultimately approves adding the project to its annual budget request to Congress—and lawmakers approve funding—lab officials hope to start construction in 2015 and have the experiment completed and running in 2021 or 2022.