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Brookhaven | Long Island, NY | USA

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Ghost Hunters: an international team tracks a disappearing particle

Born in the hearts of stars and nuclear reactors, almost undetectable, nearly as fast as light, able to pass unhindered through everything from planets to people, and confirmed shapeshifters. That role call describes what makes the particles known as neutrinos both exciting and perpetually challenging for physicists on the hunt.

A series of brilliant experiments designed and executed since the 1950s have managed to detect these slippery subatomic wonders, revealing much about their origins, travels, and presence as one of the most abundant particles in the cosmos.

Earlier this week, an international collaboration led by China and the United States at the Daya Bay Reactor Neutrino Experiment in the south of China pinpointed the action behind one of the neutrino’s signature magic tricks: its ability to seemingly vanish entirely. The disappearing act is the product of neutrino oscillations, and the Daya Bay team calculated the final unknown transformation type. The 5-sigma discovery not only helps demystify the neutrino, but it will also guide future experiments in exposing more fundamental mysteries – such as how we exist.

Photomultiplier tubes on the Daya Bay walls.

Sensitive photomultiplier tubes line the Daya Bay detector walls, designed to amplify and record the faint flashes that signify an antineutrino interaction. (Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

“It’s surprising and exciting that this result came so quickly and precisely,” said Brookhaven Lab’s Steve Kettell, who is Chief Scientist for the U.S. at Daya Bay. “It has been very gratifying to be able to work with such an outstanding international collaboration at the world’s most sensitive reactor neutrino experiment.”

Understanding the new discovery by the experimental team at Daya Bay, operated adjacent to six powerful nuclear reactors, requires a (very) brief look at the neutrino’s history in particle physics. If it didn’t seem tricky enough, the particle actually comes in three distinct flavors: electron, muon, and tau. The discovery of the muon neutrino by a team at Brookhaven in 1962, proving the existence of multiple types, earned the 1988 Nobel Prize in Physics.

A later experiment, conducted by Brookhaven’s Ray Davis (winning the 2002 Nobel Prize), detected that roughly two-thirds of the neutrinos generated by the sun’s fusion reactions seemed to vanish en route to earth. This in turn led to the deduction that the vanishing act was actually the product of oscillations – spontaneous transformations from one flavor into another.

Once the flavorful shapeshifting was exposed, a major question lingered: How do the transformations happen? The two so-called “mixing angles” that characterize transformations of solar neutrinos and those neutrinos produced in the earth’s atmosphere are both large and have been precisely measured. Meanwhile, the final measurement was much smaller and remained just out of reach.

Cue Daya Bay, with six massive detectors in three separate halls buried beneath the mountains adjacent to powerful nuclear reactors. The six China Guangdong Nuclear Power Group reactors churn out millions of quadrillions of electron antineutrinos every second, giving the collaboration an abundant stream of the elusive particles. Theory suggests that while the matter and antimatter neutrino twins are not strictly identical, their oscillations should be. So the Daya Bay team tallied the electron antineutrinos observed in the detectors nearest the reactors, and then predicted the number expected in the far hall (2 kilometers away) if no oscillations were to occur. The large number of antineutrinos that then actually vanished on the way, transforming into other flavors, provided the difference needed to calculate the final oscillation angle, called θ13, or theta one-three, as 8.8 degrees, plus or minus 0.9.

All of that may sound dizzying – mixing angles of oscillations, flavor transformations, and antimatter twins. But the fact that this final transformation was not zero means that neutrinos and antineutrinos could exhibit a dynamic bit of symmetry-breaking known as charge-parity (CP) violation, which may ultimately explain why the Big Bang embraced matter and shunned its anti-twin. The origin of that cosmic imbalance is one of the greatest mysteries in physics.

Daya Bay will continue to hone these important results with the installation and operation of two additional detectors this summer. Though the neutrino just lost a bit of its mystery, additional experiments such as the Long-Baseline Neutrino Experiment are more essential than ever to pin down the few secrets remaining to the ghostly particle and answer fundamental questions.

“We finally know all three mixing angles that describe neutrino oscillation, and now the road is paved for future experiments,” said Kettell. “The need for the next generation of neutrino exploration becomes even greater.”

So, the work of the ghost particle hunters continues. In fact, the most exciting conclusions about how neutrinos fit into the cosmic puzzle have yet to be drawn.

The original press release for the discovery can be seen here.

–Justin Eure, BNL Media & Communications

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