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Europe's Updated Particle Physics Plan Weighty With Global Implications

31 May 2013 3:35 pm
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In line. New European physics report suggests a future international path for building a linear collider, shown above in a 2005 artist’s conception.

It sets the course only for European physicists, but an updated strategy for European particle physics released yesterday hints at a possible global strategy for the whole field.

Presented in Brussels, the updated plan builds on an original strategy formulated in 2006. Predictably, its first priority is full exploitation of the world's biggest atom smasher, the Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, near Geneva, Switzerland. The LHC started taking data in 2010 and last year coughed up the long-sought Higgs boson, the particle that's key to physicists' explanation of how other fundamental particle get their mass.

More significantly, the updated strategy explicitly states for the first time that Europe is willing to join in other megaprojects, even if they're built in North America or Asia. "We are offering the other regions that Europe is willing to participate once they have a clear plan," says Rolf-Dieter Heuer, director-general of CERN and a member of the European Strategy Group, the panel that produced the update.

That statement could have outsized implications. Although there are smaller experiments, particle physicists' plans currently center on three mammoth projects. The first is the LHC, which researchers plan to run until 2030 in hopes of discovering other new particles. Second, for more than a decade, physicists have hoped to build a 30-kilometer-long straight-shot International Linear Collider (ILC) that would study in detail in its clean electron-positron collisions the new particles the LHC would unearth in its messy proton-proton collisions. Third, physicists aspire to build a far-bigger version of current experiments in which they shoot a beam of elusive particles called neutrinos through Earth to a gargantuan underground detector hundreds of kilometers away to study how the three types of neutrinos morph into each other. Known as neutrino oscillations, that morphing could help explain how the universe generated so much more matter than antimatter.

But physicists haven't agreed on who should build what. U.S. researchers had hoped to host the ILC, but in 2008 the U.S. Department of Energy (DOE) said that it couldn't afford a 50% share in the project, estimated to be roughly $7 billion. After that, many scientists thought that CERN was the likely site for the machine. However, last year, Japanese officials expressed interest in hosting the machine, which would be paid for in part by funds for rebuilding after 2011's Tohoku tsunami. The situation in neutrino physics is even murkier, as physicists in Japan, Europe, and the United States all hope to build a huge neutrino experiment.

Two passages in the updated European strategy could help clarify these situations. Regarding the linear collider, the document says, "The initiative from the Japanese particle physics community to host the ILC in Japan is most welcome. … Europe looks forward to a proposal from Japan to discuss a possible participation." Regarding neutrino physics, the report says, "Europe should explore the possibility of major participation in leading long-baseline neutrino projects in the US and Japan."

That willingness to participate in projects abroad could sow the seeds for a broader global approach to particle physics. Connecting the dots, one could envision, for example, that Japan builds the ILC and the U.S. builds the big neutrino experiment. Of course, so far there's no guarantee that the Japanese government will actually put up the billions of dollars needed to host the ILC or that Japanese neutrino physicists won't insist on building their own huge experiment.

Nevertheless, the strategy statement could be a big shot in the arm for U.S. physicists' plans to build their Long-Baseline Neutrino Experiment (LBNE), which would fire neutrinos from Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, 1300 kilometers to a detector filled with frigid liquid argon at the abandoned Homestake gold mine in Lead, South Dakota. Until recently, researchers had envisioned LBNE as the definitive neutrino oscillation experiment, with a 34,000 tonne detector 1480 meters underground.

However, in March 2012, DOE balked at the $1.9 billion cost of the experiment and asked for a plan to build it in cheaper stages. Physicists now plan to start with a 10,000 tonne detector on the surface at Homestake, at a total cost of $867 million, according to Milind Diwan, a physicist at Brookhaven National Laboratory in Upton, New York, and co-spokesperson for the LBNE collaboration. But to really make the experiment sing, the detector would need to be built underground to shield it from cosmic rays, and LBNE researchers and Fermilab officials are looking for contributions from other countries, such as India, to raise the more than $100 million needed to do that.

Europe's participation and the resources that come with it could make LBNE a world beater. However, LBNE researchers may still find themselves in a bind. Europe will participate only if the U.S. commits to a top-flight experiment, Heuer says. "The ball is in the U.S.'s court," he says. "LBNE has to be a world-class project that will do the physics that you want to do—and that means it has to go underground." Diwan agrees. "The Europeans want Americans to play for something big," he says. "They won't participate if it is an attenuated, miserly experiment. They want America to be America."