The Department of Energy’s next generation neutrino experiment has passed its latest round of reviews and is moving towards construction. The Long Baseline Neutrino Experiment, or LBNE, passed the department’s“Critical Decision 1” review on December 10, which outlined the project’s budget and overall plan.
The experiment will push limits of existing technology by deploying a new generation of detectors to attempt to unravel the mass hierarchy of neutrinos. However, budget compromises mean that the project will have to take on a limited scientific scope unless outside investment can be secured to move the detectors deep underground.
The Sanford Underground Research Facility, located at the former Homestake gold mine in Lead, South Dakota, will be home to the LBNE. Fermilab will shoot a beam of neutrinos through 1300 kilometers of rock to the detectors at Homestake. Located on the surface of the underground research facility, the detectors will look for the hierarchy of neutrino masses and evidence of charge parity violation in hopes of finding clues to why matter won out over antimatter in the early universe.
The planned experiments have been significantly scaled back from their original scope. When it was first conceived, the LBNE was to be a part of the Deep Underground Science and Engineering Laboratory, or DUSEL, run in conjunction with the National Science Foundation. However in December of 2010 the NSF backed out of the project and the scope of experiments at the mine had to be reduced. The plan shrank from dozens of underground multidisciplinary scientific projects to three physics experiments, a dark matter detector, a search for neutrinoless double-beta decay, and the LBNE.
The designers of the LBNE proposed a budget of about $1.7 billion, but the DOE demanded more reductions. They dropped the proposed near detector that would have measured neutrinos at the beam’s Fermilab origin. After a review that included evaluating other sites and other kinds of detectors, the LBNE commission recommended building the next-generation liquid argon detectors at the surface of the Homestake mine to keep the project under its $850 million budget.
“The issue is that the cost is higher if you build things deep underground than if you build them on the surface,” said Jim Siegrist, Associate Director for High Energy Physics in the Department of Energy’s Office of Science. “Surface construction is so much cheaper.”
The review essentially changed the experiment into a multi-staged project. The first phase would be built above ground, and future phases would be built below, using the first phase’s infrastructure.
However building the detectors on the surface puts limitations on the science. Project managers were able to save the project’s experimental next-generation liquid argon detectors, though they were reduced from 34,000 gallons to 10,000. The DOE wanted to keep argon, with its potential to be more sensitive than established water Cherenkov detectors, to push the boundaries of current technology. But because the detectors won’t be shielded by nearly a mile of solid rock, they will be susceptible to cosmic rays and other background radiation. This shouldn’t be enough to affect the hunt for the mass hierarchy of neutrinos, but will likely eliminate the ability to find rarer particle events.
“The chance to look for astrophysical supernovae or the decays of protons is precluded,” Siegrist said. Proton decays, if they take place at all, are extremely rare and their signature would be totally lost amongst the background of cosmic ray interference. Scientists had also hoped to look for neutrino spikes that corresponded with supernovae.
The project’s administrators remain hopeful that there might still be a way to move the detectors down into the mine below.
“One of the reasons to choose the Homestake site is to keep the possibility open for putting the detector underground,” said Milind Diwan, a physicist at Brookhaven and spokesperson for LBNE. “It is certainly our desire to have the first-phase detector put underground. But that desire requires additional funds.”
The additional cost to locate the detectors underground in the mine is estimated to be between $130 and $140 million, which would have to be raised in about two years.
“Things are arranged so that there is time for foreign contributions to come in,” Siegrist said. “We’re some years away from construction… it has sort of a long lead time so we can get these issues figured out.”
He added that they had already been in talks with several foreign partners in hopes of bringing them on board. He said that the CD-1 approval shows the DOE’s commitment to the project, and would help encourage investment from abroad.
“There’s some interest from Asia and we’re trying to get our European colleagues interested,” Siegrist said.
He noted that the DOE’s flat budget over the last three years has forced the agency to make tough choices about priorities.
“If we’re going to raise the amount we're putting into construction, then we have to lower the amount we're putting into research and facility operations,” Siegrist said. “Our operations costs aren’t spiraling out of control while we’re building these other projects, so that’s a good thing.”
Scientists working on LBNE fought hard to keep funding for the underground detectors in the project’s budget. At the August High Energy Physics Advisory Panel meeting, scientists strongly urged the Department of Energy to spring for the underground detectors. Many voiced the concern that the US might miss out on important discoveries by not locating the detectors underground.
The shielded underground detectors offered scientists the best hope of detecting decaying protons for the first time. If the United States doesn’t build underground detectors until a later phase of the project, some years from now, another country could in the interim make the discovery.
However Diwan said that because proton decay was part of the original design of the detectors, the United States is far ahead of any other nation that might be hunting for the elusive process.
“I do not think we have competition for quite some time,” Diwan said. “Compared to other regions we are quite far ahead. That is because we have a team in place. That is very important. We have a design for a project that is generally accepted as a sound design by everyone and we have a site selection.”
Japan right now is the leader in neutrino detectors, but would still need to upgrade. In addition, CERN has the particle beams, several potential sites and has commissioned studies to look into the possibility of hunting for proton decays.
“They certainly could do this,” Diwan said, “It’s a question of priorities and funds.”
The next step in the process is for Fermilab to reconfigure its beamlines, which should start around 2015. LBNE is expected to pass CD-2, its next round of approval, the following year which will settle on a final cost estimate and construction schedule. If all goes according to plan, the experiment should start taking data in 2023.
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Staff Science Writer: Michael Lucibella