Particle physicists chasing ghosts

Wispy neutrinos could explain why matter dominates the universe

Particle physicists chasing ghosts

PROVIDENCE, R.I.?—?Two experiments on different continents have found hints that particles called neutrinos can shape-shift in an unexpected way.

This behavior may be the key to understanding why these particles are so weird, says neutrino physicist Jennifer Raaf of the Fermi National Accelerator Laboratory in Batavia, Ill., the nation’s largest particle physics lab. Raaf presented an overview of recent neutrino findings August 9 at a meeting of the American Physical Society’s Division of Particles and Fields.

The new results also bode well for future experiments with neutrinos that may one day help scientists understand why the universe contains vastly more matter than antimatter. These experiments are part of the changing landscape of particle physics in the United States. With Fermilab’s Tevatron, once the most powerful particle collider in the world, shutting down soon, the government laboratory is reconfiguring itself to focus on projects that require particularly intense beams and look for extremely rare events.

“Neutrinos will play a big role moving forward,” says Young-Kee Kim, deputy director at Fermilab.

In the bestiary of particle physics, neutrinos are the neutral counterparts to the three charged leptons: the familiar electron and the heavier and more exotic muon and tau. Neutrinos are loners by nature, rarely interacting with the rest of the universe. But they do occasionally change form. That process, called oscillation, may offer clues about why the universe contains so little antimatter.

In June the T2K experiment in Japan reported evidence that muon neutrinos occasionally oscillate into electron neutrinos. Six electron neutrinos appeared in a nearly pure beam of muon neutrinos traveling from an accelerator at the J-PARC facility to an underground detector 295 kilometers away.

Days later, physicists at the MINOS experiment announced finding traces of this oscillation in neutrinos traveling 735 kilometers from Fermilab to a mine in Minnesota. Those results, presented August 9 at the physics meeting, help to narrow T2K’s estimate of how often this changeup happens.

Taken together, the chance that both sightings are flukes is less than one in a thousand, according to a recent analysis by a team of physicists in Italy and Germany. That’s below the standard for claiming a discovery but good enough to warrant further study, says Ed Kearns, a neutrino physicist at Boston University and T2K team member.

“This helps us justify future experiments,” he says. “It makes a big difference in our confidence going forward.”

If confirmed, this oscillation will be a crucial piece of information for a neutrino experiment now under construction at Fermilab. The NOvA experiment, which is currently testing its first prototype detector, could help scientists work out the differences in the masses of the different kinds of neutrinos, a long-standing puzzle.

Another project, called LBNE (for Long-Baseline Neutrino Experiment), also hopes to extend this line of research. LBNE would send beams of neutrinos and antineutrinos from Fermilab to a detector 1,300 kilo?meters away, giving the particles more time to change identity — and the scientists a better shot at understanding whether neutrinos behave differently than their antimatter counterparts.

LBNE is still on the drawing board, though, and its future is clouded by uncertainties about funding. Last December, the National Science Foundation pulled out of a plan to build a laboratory in an old mine in South Dakota that would house not only a detector for the LBNE experiment but also a variety of other underground experiments. That decision ups the price tag for the Department of Energy, the experiment’s main funder. With the DOE’s budget uncertain, physicists are exploring different ways to try to lower the costs as much as possible.

“We’re expecting a clear decision about what’s right for LBNE by the end of this year,” William Brinkman, director of the DOE’s Office of Science, told a roomful of physicists during an August 11 forum at the physics meeting.

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