When the US National Science Board nixed plans for an underground lab in 2010, multiple potential experiments were left homeless, and the US physics community was in a kerfuffle. Now, 40 leading theoretical physicists, including three Nobel Prize winners, have written to the US Department of Energy (DOE) urging it build an underground facility to study subatomic neutrinos that would compensate to some degree for the lab’s absence.
“We … are writing this letter to note the pressing scientific need for having a large underground detector, linked to a long baseline intense neutrino beam,” say the signatories, who include Nobelists Steven Weinberg of the University of Texas at Austin, Sheldon Glashow of Boston University in Massachusetts and Frank Wilczek of the Massachusetts Institute of Technology in Cambridge.
Their letter, dated 19 January, is a boost for the Long Baseline Neutrino Experiment (LBNE), an estimated US$1.3-billion complex with detectors housed in the Homestake mine in South Dakota and, 1,300 kilometres away, at Fermilab in Batavia, Illinois, where particle accelerators would generate beams of neutrinos and antineutrinos (see graphic). The letter argues that an underground facility is needed to search for matter–antimatter asymmetry using neutrinos and for proton decay, a process that, if seen, would confirm the theoretical unification of the forces of nature at scales beyond that which could be probed by the Large Hadron Collider at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland.
The letter is a welcome boost at a difficult time for LBNE, which faces a Go/No-Go decision from DOE later this year, probably between May and September. Budget constraints have recently forced the collaboration to choose between two detection technologies: a liquid-argon detector, in which neutrinos would be detected as they interact with argon nuclei, and a Cerenkov detector, in which neutrinos would be seen as they pass through water. It had previously been hoped that both might be built. Whereas an external review committee convened by DOE favoured liquid argon, a later committee comprising many members of the collaboration settled on the Cerenkov option. “We thought it was less expensive and less risky,” says LBNE co-spokesman Robert Svoboda of the University of California, Davis. But project manager James Strait of Fermilab went with the liquid argon. That leaves about 70 people who had been working on the water–Cerenkov option at a loose end. Svoboda says that he’s encouraging them to take three months to make a decision about how to proceed — presumably by either leaving the experiment or switching to the liquid-argon effort. He expects that the majority will decide that they will be able to do the physics they want to do in liquid argon and stay. In their letter to the DOE, the theoretical physicists say that either technology should be able to fulfill the physics goals, with liquid argon being slightly more sensitive to proton decay.
Pran Nath, a physicist at Northeastern University in Boston who co-signed the letter, says that the signatories worked on it for several months following the decision not to build the underground lab, experiments in which were strongly endorsed in a 2011 report from the US National Academy of Sciences. Nath says that the theorists’ hope is that once an underground facility is up and running, it might be able to house other experiments as well.
In addition, the closure of the Tevatron, Fermilab’s particle collider, in 2011, means that the United States risks being left without a large-scale particle-physics experiment, and young experimentalists will have to go to Europe to work on the Large Hadron Collider. “A country as large as the US needs a large experiment to be at the forefront of physics,” says Nath.
Image: LBNE/Symmetry Magazine