Neutrinos Take Center Stage in America’s Plan for Future Physics

Image: China Daya Bay Reactor Neutrino Experiment

Researchers prepare to install the detectors at the Daya Bay Reactor Neutrino Experiment Station at the Daya Bay Nuclear Power Plant in Shenzhen in south China's Guangdong province on Oct. 21, 2011. The Daya Bay Reactor Neutrino Experiment has begun its quest to answer some of the most puzzling questions about the elusive elementary particles known as neutrinos. The experiment's first completed set of twin detectors is now recording interactions of antineutrinos (antipartners of neutrinos) as they travel away from the powerful reactors of the China Guangdong Nuclear Power Group in southern China. Sun Zifa/ColorChinaPhoto / Color China Photo via ASSOCIATED PRESS

Building a machine to solve the mysteries of ghostly neutrinos should be America's next big project in particle physics, according to an influential report from a scientific panel

The report — issued by the Particle Physics Project Prioritization Panel, or P5 — is likely to determine how billions of dollars of federal money is spent over the next decade and beyond.

P5's analysis of research priorities was presented to the federal government's High Energy Physics Advisory Panel on Thursday, and if it's approved, it will serve as a guide for particle physics funding from the Department of Energy and the National Science Foundation.

The centerpiece of the 20-year plan is the creation of a $1.5 billion international project called the Long-Baseline Neutrino Facility, headquartered at Fermilab in Illinois. The neutrino project has been in the works for years, and calls for building a powerful particle-smasher at Fermilab to generate focused beams of neutrinos and antineutrinos. Those beams would be sent directly through 800 miles of Earth's crust to a detector in the Sanford Underground Research Facility in South Dakota.

Studying what happens to the neutrinos during the trip should provide insights into what makes neutrinos and antineutrinos different, and whether that difference could help explain the complete dominance of matter over antimatter in the observable universe.

LHC seen as model

The report calls for the United States to pay about $1 billion of the experiment's cost over a 10-year construction period, with international partners picking up the other $500 million. The model for funding and using the Long-Baseline Neutrino Facility, or LBNF, would be similar to the arrangement created for the $10 billion Large Hadron Collider at Europe's CERN physics lab on the Swiss-French border.

"If the world wants to do this science, then the world should organize, with Fermilab as the host, to solve the problems and make it happen," said P5 chairman Steven Ritz, a physicist at the Santa Cruz Institute for Particle Physics. Two dozen other physicists from around the world, including CERN, served on the panel.

Image: Long-Baseline Neutrino Facility
The plan for the Long-Baseline Neutrino Facility calls for shooting beams of neutrinos and antineutrinos in an 800-mile-long straight line from an underground accelerator in Illinois to an underground detector in South Dakota. Fermilab

The report lays out an agenda for addressing other mysteries in particle physics — including the implications of the discovery of the Higgs boson, the LHC's crowning achievement. The P5 report said an upgraded LHC should continue to take the lead role in the Higgs follow-up. It also prioritized a range of projects that could shed light on the nature of dark matter, dark energy and cosmic inflation.

Looking further ahead, panel members noted that Japan has expressed an interest in hosting an accelerator called the International Linear Collider. The ILC, which is seen as a complement to the LHC, would smash beams of electrons and antielectrons together at energies as high as 1 trillion electron volts. If that project proceeds, the panel said the United States should play a supporting role.

Why neutrinos are next

Ritz said the LBNF's construction phase could begin around 2018 and peak in the early to mid-2020s, just as LHC upgrades were ramping down. "This is something we can build that's affordable within our budget scenarios," he told NBC News. "We would love to do something more analogous to the LHC, but that's not something that's doable in the near future."

In some quarters, neutrinos are considered the next big frontier in physics after the Higgs boson. They interact only weakly with other subatomic particles. Trillions of neutrinos from the sun stream through our bodies every second, virtually unimpeded. Only recently did physicists discover that neutrinos actually have a tiny bit of mass — and that one "flavor" of neutrino can turn into a different flavor as it zips along.

That flavor-flipping behavior, known as neutrino oscillation, is particularly interesting to particle physicists. Is there a difference in the way neutrinos and antineutrinos change flavor? If so, that would represent an unexpected breakdown in the mirror symmetry between matter and antimatter.

"Theorists of my ilk can make very elegant theories in which neutrinos are responsible for the matter vs. antimatter non-symmetry," said Fermilab's Joe Lykken, a member of the P5 panel. Some evidence of the breakdown, known as CP violation, already has been seen in a Japanese neutrino experiment.

The LBNF could provide definitive evidence of the breakdown, just as the LHC provided definitive evidence of the Higgs boson's existence. That could bring physicists closer to understanding why matter won out over antimatter soon after the universe arose, even though the most straightforward theory suggests there should be equal measures of matter and antimatter popping in and out of existence.

Lykken said that unraveling the mystery of neutrino oscillation would be the "sweet spot" for the LBNF, but added that the experiment is likely to make other discoveries that can't be foreseen. "Anytime anyone does something with neutrinos, there's always a surprise," he said.

Getting physicists to agree on an agenda for future research can be more difficult than herding cats — but the consensus that was reached on the P5 report suggests that, at least in this case, "the cats are successfully herded," Lykken said.

"In the 20 years or so that I've been involved in these sorts of things, this is the most unified and excited that I've seen our community," he said. "This is something that's practical and can actually happen."

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