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CPLC Seminar - Dr. Mikhail Tikhonov to Give on Friday, October 4, 2019

9/26/2019 2:54:50 PM

Modeling the interplay between plastic tradeoffs and evolution in changing environments

Performance tradeoffs are fundamental to evolutionary thinking, but in most models, are simply postulated. This approach is justified for tradeoffs enforced by rigid biophysical or biochemical constraints; unsurprisingly, the best-understood examples are in this class. However, experimental results suggest that many relevant tradeoffs are not rigid, but depend on genetic background and evolutionary history, and can themselves evolve. I will present a simple model capable of capturing the key feedback loop: evolutionary history shapes tradeoff strength, which, in turn, shapes evolutionary future. One consequence of this feedback is that genomes with identical fitness can have different evolutionary properties, shaped by prior environmental exposure. Another is that, generically, the best adaptations to one environment may evolve in another. These results extend previous work relating modularity and “evolvability” to a more general discussion of flexible tradeoff architectures and their impact on evolutionary dynamics.

3/8/2012 Jen-Chieh Peng and Celia Elliott

The Daya Bay Reactor Neutrino Experiment, a multinational collaboration operating in the south of China, today reported the first results of its search for the last, most elusive piece of a longstanding puzzle: how is it that neutrinos can appear to vanish as they travel? The surprising answer opens a gateway to a new understanding of fundamental physics and may eventually solve the riddle of why there is far more ordinary matter than antimatter in the universe today.

Members of the University of Illinois Daya Bay team include Professor of Physics Jen-Chieh Peng, who has been a member of the collaboration since its inception, postdoc Dawei Liu, and graduate students Ry Ely, Daniel Ngai, and En-Chuan Huang. The Illinois group has participated in the R&D, testing, installation, and commissioning of the photomultiplier-tubes (PMT) system, which is the primary device for detecting the neutrino signals.

"The value of the neutrino mixing parameter, θ13, has been one of the major 'unknowns' in neutrino physics," said Peng, leader of the Illinois Daya Bay group. "Observation of neutrino oscillation at the Daya Bay experiment allows the first unambiguous determination of this fundamental property of neutrinos. This finding should lead to better theoretical understanding of the origins of neutrino mixings and masses, and it will also have significant impact on the direction of future neutrino experiments."

Traveling at close to the speed of light, the three basic neutrino “flavors” – electron, muon, and tau neutrinos, as well as their corresponding antineutrinos – mix together and oscillate (transform), but this activity is extremely difficult to detect. From Dec. 24, 2011, until Feb. 17, 2012, scientists in the Daya Bay collaboration observed tens of thousands of interactions of electron antineutrinos, caught by six massive detectors buried in the mountains adjacent to the powerful nuclear reactors of the China Guangdong Nuclear Power Group. These reactors, at Daya Bay and nearby Ling Ao, produce millions of quadrillions of elusive electron antineutrinos every second.

The copious data revealed for the first time the strong signal of the effect that the scientists were searching for, a so called “mixing angle” named theta one-three (written θ13), which the researchers measured with unmatched precision. Theta one-three, the last mixing angle to be precisely measured, expresses how electron neutrinos and their antineutrino counterparts mix and change into the other flavors. The Daya Bay collaboration’s first results indicate that theta one-three, expressed as sin2 2 θ13, is equal to 0.092 plus or minus 0.017.

The data points are the ratios of the detected versus expected numbers of neutrinos at the three Daya Bay experimental halls (EH1, EH2, and EH3), assuming no neutrino oscillation. The red curve shows that the data can be explained by the presence of neutrino oscillation.

"This is a new type of neutrino oscillation, and it is surprisingly large,” says Yifang Wang of China’s Institute of High Energy Physics (IHEP), co-spokesperson and Chinese project manager of the Daya Bay experiment. “Our precise measurement will complete the understanding of the neutrino oscillation and pave the way for the future understanding of matter-antimatter asymmetry in the universe.”

Neutrinos, the wispy particles that flooded the universe in the earliest moments after the big bang, are continually produced in the hearts of stars and other nuclear reactions. Untouched by electromagnetism, they respond only to the weak nuclear force and even weaker gravity, passing mostly unhindered through everything from planets to people. The challenge of capturing these elusive particles inspired the Daya Bay collaboration in the design and precise placement of its detectors.

“Although we’re still two detectors shy of the complete experimental design, we’ve had extraordinary success in detecting the number of electron antineutrinos that disappear as they travel from the reactors to the detectors two kilometers away,” says Kam-Biu Luk of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley. Luk is co-spokesperson of the Daya Bay Experiment and heads U.S. participation. “What we didn’t expect was the sizable disappearance, equal to about 6 percent. Although disappearance has been observed in another reactor experiment over large distances, this is a new kind of disappearance for the reactor electron antineutrino.”

The Daya Bay experiment counts the number of electron antineutrinos detected in the halls nearest the Daya Bay and Ling Ao reactors and calculates how many would reach the detectors in the Far Hall if there were no oscillation. The number that apparently vanish on the way (oscillating into other flavors, in fact) gives the value of theta one-three. Because of the near-hall/far-hall arrangement, it’s not even necessary to have a precise estimate of the antineutrino flux from the reactors.

“Even with only the six detectors already operating, we have more target mass than any similar experiment, plus as much or more reactor power,” says William Edwards of Berkeley Lab and UC Berkeley, the U.S. project and operations manager for the Daya Bay Experiment. Since Daya Bay will continue to have an interaction rate higher than any other experiment, Edwards explains, “it is the leading theta one-three experiment in the world.”

The first Daya Bay results show that theta one-three, once feared to be near zero, instead is “comparatively huge,” Kam-Biu Luk remarks, adding that “Nature was good to us.” In coming months and years the initial results will be honed by collecting far more data and reducing statistical and systematic errors.

“The Daya Bay experiment plans to stop the current data-taking this summer to install a second detector in the Ling Ao Near Hall, and a fourth detector in the Far Hall, completing the experimental design,” says Yifang Wang.

Refined results will open the door to further investigations and influence the design of future neutrino experiments – including how to determine which neutrino flavors are the most massive, whether there is a difference between neutrino and antineutrino oscillations, and, eventually, why there is more matter than antimatter in the universe – because these were presumably created in equal amounts in the big bang and should have completely annihilated one another, the real question is why there is any matter in the universe at all.

“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,” says Steve Kettell of Brookhaven National Laboratory, the chief scientist for the U.S. effort. “This moment is exciting because we have finally observed all three mixing angles, and now the way is cleared to explore the remaining parameters of neutrino oscillation.”

“This is really remarkable,” says Wenlong Zhan, vice president of the Chinese Academy of Sciences and president of the Chinese Physical Society. “We hoped for a positive result when we decided to fund the project, but we never imagined it could come so quick!”

“Exemplary teamwork among the partners has led to this outstanding performance,” says James Siegrist, associate director for high energy physics at the U.S. Department of Energy’s Office of Science. “These notable first results are just the beginning for the world's foremost reactor neutrino experiment.”

The Daya Bay collaboration consists of scientists from the following countries and regions: China, the United States, Russia, the Czech Republic, Hong Kong, and Taiwan. The Chinese effort is led by co-spokesperson, chief scientist, and project manager Yifang Wang of the Institute of High Energy Physics, and the U.S. effort is led by co-spokesperson Kam-Biu Luk and project and operations manager William Edwards, both of Berkeley Lab and UC Berkeley, and by chief scientist Steve Kettell of Brookhaven.