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Target. A proposed physics experiment would fire neutrinos from Illinois to this former gold mine, now the Sanford Underground Research Facility, in Lead, South Dakota.

Target. A proposed physics experiment would fire neutrinos from Illinois to this former gold mine, now the Sanford Underground Research Facility, in Lead, South Dakota.

Matthew Kapust/South Dakota Science and Technology Authority

New Plan for U.S. Particle Physics: Go International

It's a gamble that leading U.S. particle physicists say they must take: To finally get started on building their next megaproject at the United States' sole particle physics lab—Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois—they want to stage it as an international collaboration—even if that means ceding direct control of the experiment to a council of member nations.

That recommendation comes from a draft road map for U.S. particle physics presented today to a federal advisory panel in Washington, D.C. Requested by the Department of Energy (DOE) and the National Science Foundation (NSF) 8 months ago, the report of the ad hoc Particle Physics Project Prioritization Panel (P5) largely embraces the same main projects that physicists have been mulling for years. But it strikes a decidedly more international tone than previous plans. "Pursue the most important opportunities wherever they are, and host unique, world-class facilities that engage the global scientific community," reads the first of its 29 recommendations.

"I think the case for proceeding this way is persuasive [because] collaboratively, we can address all the physics" originally envisioned for the Fermilab project, says Andrew Lankford, a particle physicist at the University of California (UC), Irvine, and chair of DOE and NSF's High Energy Physics Advisory Panel (HEPAP), which commissioned the report. "The question is, will the decision-makers in Washington appreciate it?"

Charting a course for the next decade, the report calls on Fermilab to build a so-called long-baseline neutrino facility. The megaproject would fire neutrinos 1300 kilometers to a gigantic underground detector filled with 40,000 tonnes of frigid liquid argon set 1480 meters down in an abandoned gold mine in South Dakota. It would study how the three types or "flavors" of neutrinos morph or "oscillate" into one another as they zing along at near light speed. Researchers are looking for an asymmetry between how neutrinos and antineutrinos oscillate, which could help explain how the evolving universe generated so much matter and so little antimatter.

Fermilab researchers already have a specific plan for such a project, called the Long-Baseline Neutrino Experiment (LBNE). But budget constraints have whittled it down so much that it's no longer worth doing, P5 says. It recommends starting over to build an international experiment even more ambitious than the original LBNE proposal. But DOE, the main funder of U.S. particle physics, would most likely have to cede control of it, says Fermilab’s director, Nigel Lockyer. "To me, this is a transformative moment," he says. "But the U.S. government has to accept that it has to give up something in the way they normally do things."

In going international, P5 has followed the lead taken by European particle physicists last May, when they revised their long-term strategy. The European report suggested a sharing of the field's biggest projects among Europe, the United States, and Japan. Europe would continue to run the world's highest energy atom smasher, the 27-kilometer-long Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, in Switzerland—which 2 years ago unearthed the long-sought Higgs boson. Europe would be willing to contribute to a massive neutrino experiment, perhaps in the United States, and to the proposed International Linear Collider (ILC). That 30-kilometer electron-positron collider would study the Higgs boson in great detail, and Japanese physicists hope to host it.

In keeping with that global vision, the P5 report puts a top priority on continued U.S. participation in the LHC and says physicists should work as planned to upgrade by 10-fold the rate at which the LHC smashes protons. More than 1200 physicists work on the two biggest experiments fed by the LHC, a plurality of the U.S. particle physics community. "The LHC upgrades are our highest priority near-term large project," Steven Ritz, a physicist from UC Santa Cruz and the P5 chair, told HEPAP. Similarly, if Japan goes forward with the ILC, then the United States should join that collaboration—if money allows.

Overall, the P5 report addresses the entire U.S. physics portfolio, including projects large and small. For example, informed by an earlier yearlong community survey known as the Snowmass process, it recommends increased funding for smaller underground experiments that strive to detect particles of dark matter—the mysterious stuff whose gravity binds the galaxies. Conversely, it calls for an end to current R&D efforts to develop a collider that smashes muons, heavier unstable cousins of electrons, and several other projects.

Physicists and DOE officials hope the plan will help unify the U.S. particle community, which they fear has become fragmented. Some prominent physicists had questioned the value of building the neutrino experiment and instead had pushed for greater effort on the ILC or in astrophysical research.

That problem was exacerbated by cuts to the LBNE plan. Fermilab physicists had envisioned building a neutrino detector filled with 34,000 tonnes of liquid argon in Homestake. Deep underground, the detector could have also looked for signs that protons in the liquid argon decay—a key prediction of some theories—and spot neutrinos from supernovas. But DOE officials balked at LBNE's $1.9 billion price tag. So, in August 2012, Fermilab physicists proposed a $789 million version with a detector less than one-third the size on the surface. It could have studied only neutrino oscillations, and only in a limited way.

There is a potential hitch in P5's global vision, however. The panel had to consider what to do under three scenarios: One in which DOE's particle physics budget, now $797 million, increases by 5% over 10 years, one in which it increases by 17% over 10 years, and one where it is unbounded. In either of the first two cases, the United States could not afford to contribute more than modest R&D money and perhaps some hardware to the ILC.

That may cause heartburn for Japanese backers of the ILC, who are anxious for other countries to commit to the project. And Japanese neutrino physicists, who historically have an edge over their U.S. counterparts, may then argue that their country would be better served by pursuing their own enormous neutrino experiment, already in the planning. "That's a challenge for the strategy," Lankford says, although he adds that the U.S. project might be able to go ahead without Japanese participation.

Ultimately, the success of the plan will depend on whether the community buys into it, Lankford says. "We believe it's a good program, and it's balanced," Lankford says. "We do think it's something that people can get behind." If they don't, researchers say, there isn't a plan B.

*Correction, 27 May, 3:53 p.m.: The story has been revised to correct the budget scenarios the panel considered.