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Posts Tagged ‘Daya Bay’

A measurement to watch

Monday, October 12th, 2015

This article appeared in symmetry on Oct. 7, 2015.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Physics, perhaps more so than any other science, relies on measuring the same thing in multiple ways. Different experiments let scientists narrow in on right answers that satisfy all parties—a scientific system of checks and balances.

That’s why it’s exciting when a difference, even a minute one, appears. It can teach physicists something about their current model – or physics that extends beyond it. It’s possible that just such a discrepancy exists between a certain measurement of neutrinos coming out of accelerator experiments and reactor-based experiments.

Neutrinos are minuscule, neutral particles that don’t interact with much of anything. They can happily pass through a light-year of lead without a peep. Trillions pass through you every second. In fact, they are the most abundant massive particle in the universe—and something scientists are, naturally, quite keen to understand.

The ghostly particles come in three flavors: electron, muon and tau. They transition between these three flavors as they travel. This means that a muon neutrino leaving an accelerator at Fermi National Accelerator Laboratory in Illinois can show up as an electron neutrino in an underground detector in South Dakota.

Not complicated enough for you? These neutrino flavors are made of mixtures of three different “mass states” of neutrinos, masses 1, 2 and 3.

At the end of the day, neutrinos are weird. They hang out in the quantum realm, a land of probabilities and mixing matrices and other shenanigans. But here’s what you should know. There are lots of different things we can measure about neutrinos—and one of them is a parameter called theta13 (pronounced theta one three). Theta13 relates deeply to how neutrinos mix together, and it’s here that scientists have seen the faintest hint of disagreement from different experiments.

Accelerators vs. reactors

There are lots of different ways to learn about neutrinos and things like theta13. Two of the most popular involve particle accelerators and nuclear reactors.

The best measurements of theta13 come from nuclear reactor experiments such as Double Chooz, RENO and Daya Bay Reactor Neutrino Experiment based in China (which released the best measurement to date a few weeks ago).

Detectors located near nuclear reactors provide such wonderful readings of theta13 because reactors produce an extremely pure fountain of electron antineutrinos, and theta13 is closely tied to how electron neutrinos mix. Researchers can calculate theta13 based on the number of electron antineutrinos that disappear as they travel from a near detector to the far detector, transforming into other types.

Accelerators, on the other hand, typically start with a beam of muon neutrinos. And while that beam is fairly pure, it can have a bit of contamination in the form of electron neutrinos. Far detectors can look for both muon neutrinos that have disappeared and electron neutrinos that have appeared, but that variety comes with a price.

“Both the power and the curse of long-baseline neutrino oscillation is that it’s sensitive to all of neutrino oscillation, not just theta13,” says Dan Dwyer, a scientist at Lawrence Berkeley National Laboratory and researcher on Daya Bay.

With that in mind, we come to the source of the disagreement. The results coming out of accelerator-based experiments, such as the United States-based NOvA and Japan-based T2K, see just a few more electron neutrinos than researchers would predict based on what the reactor experiments are saying.

“The theta13 value that fits the beam experiments, that really describes how much electron neutrino you get, is somewhat larger than what Daya Bay, RENO and Double Chooz measure,” says Kate Scholberg, professor of physics at Duke University and researcher on T2K. “So there’s a little bit of tension.”

Many grains of salt

Data coming out of the accelerator experiments is still very young compared to the strong readings from reactor experiments, and it is complicated by the nature of the beam. No one is jumping on the discrepancy yet because it can be explained in different ways. Most importantly, the accelerator experiments just don’t have enough information.

“We have to wait for T2K and NOvA to get sufficient statistics, and that’s going to take a while,” says Stephen Parke, head of the Theoretical Physics Department at Fermilab. Parke, Scholberg and Dwyer all estimated that about five more years of data collection will be required before researchers are able to start saying anything substantial.

“There’s been a lot of pressure on Daya Bay to try to eke out as precise a measurement as we possibly can,” Dwyer says. “Every bit of increased precision we provide further improves the ability of NOvA and T2K and eventually [proposed neutrino experiment] DUNE to measure the other parameters.”

Finding meaning in neutrinos

If the accelerator experiments gather more data and if a clear discrepancy emerges—a big if—what does it mean?

Turns out there are lots of reasons to love theta13. It’s one of the fundamental parameters that can define our universe. From a practical standpoint, it helps design future experiments to better understand neutrinos. And it could help physicists learn something new.

“We don’t expect things not to agree, but we kind of hope that they won’t,” says André de Gouvêa, professor of physics at Northwestern University. “It means that we’re missing something.”

That something could be CP violation, evidence that neutrinos and antineutrinos behave differently. CP violation has never been seen in neutrinos before, but if researchers observed it with accelerator experiments, it could help explain why our universe is made of matter rather than equal parts of matter and antimatter.

Figuring out if CP violation is occurring means nailing down all of the different neutrino mixing parameters, which in turn means building more powerful, next-generation experiments such as Hyper-K in Japan, JUNO in China and the Deep Underground Neutrino Experiment in the United States. DUNE will build on oscillation experiments like NOvA but will be able to better separate background noise from neutrino events, see a broader energy spectrum of neutrinos and find other neutrino characteristics.

DUNE, which will be built in a repurposed gold mine in South Dakota and detect neutrinos passed 800 miles through the Earth from Fermilab in Illinois, will be one of the best ways to see CP violation and rely on expertise gained from smaller neutrino experiments.

“Developing these types of experiments is very complicated,” de Gouvêa says. One of the major challenges of physics experiments is making sure you are measuring what you think you are measuring. “That’s part of the reason why we have a significant number of neutrino oscillation experiments.”

Ultimately, the neutrino puzzle is still missing many pieces. A variety of experiments are ramping up to fill in the gaps, making it an exciting time to be a neutrino physicist.

“We have to untangle the mysteries of the neutrino, and it’s not easy,” Parke says. “The neutrino doesn’t give up her secrets very easily.”

Lauren Biron


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.” (more…)


Neutrinos could tell us why matter formed in the early universe.

The Japan-based experiment T2K Tuesday gave scores of U.S. particle hunters a license to ready their detectors and take aim at the biggest question in the universe: How everything we see came to exist.

“It’s our hunting license,” said Fermilab physicist and University of Rochester professor Kevin McFarland, who works on T2K and neutrino experiments at Fermilab.

The observation by T2K affects what the Fermilab neutrino experiments NOvA and the proposed Long Baseline Neutrino Experiment, LBNE, can expect to discover and how quickly. It also makes the experiment McFarland serves as co-spokesman on, MINERvA, more important than ever in the international neutrino-research field.

Physicists working with T2K recorded six muon neutrinos changing into electron neutrinos across a long distance, a transformation called theta 13 in physics circles. Physicists had predicted that they should observe only 1.5 of these transformations as background events rather than the six they did observe, so the probability of the existence of an electron neutrino appearance is estimated to be 99.3 percent. While the T2K observation doesn’t rise to the level of “discovery” in the science community, it is far enough beyond the expected statistical error bar to make people shout for joy and start revising plans for their own particle hunts.

“Because neutrino science is so hard, scientist don’t get a lot of exciting days,” McFarland said the day of the T2K announcement. “But this is a very exciting day.”

The T2K observation also was statistically large enough that it quells a long-standing fear that this transformation would be statistically too small, much less than one percent, to observe. At that level, modern technology wouldn’t be able to use the observation as a stepping stone to move to the next research phase in figure out how matter came to dominate antimatter in the universe.

The quarry:

Something, possibly neutrinos, tipped the scales to have more matter than antimatter in the universe allowing for life. Credit: symmetry magazine

Physics predicts that the three types of neutrino particles can change back and forth into one another across long distances. Previous solar and reactor neutrino experiments had observed two types doing just that, but the third switch – muon neutrino into electron neutrino – had remained elusive.

T2K’s recording of this transformation, the first of its kind, means that physicists will have the tools to track down the next two potential discoveries on the path to the ultimate trophy. After the Big Bang, equal amounts of matter and antimatter should have annihilated each other leaving nothing but free-floating energy. But we’re here and antimatter isn’t, so that didn’t happen. Something tipped the scales in matter’s favor, allowing particles to join together and form planets, plants and people. Physicists think neutrinos could be that tipping-point particle.

Following the tracks:

The first step in finding out if they are right is T2K’s observation. Plugging this observation into the research equation, physicists on NOvA, an experiment under construction in Minnesota, will be able to tease out the details of what is called the neutrino mass hierarchy. The pattern of this hierarchy essentially will tell physicist if neutrinos behave like other particles, in a pattern of light, heavy and very heavy, or neutrinos behave oddly in a pattern of light, heavy and heavy.

This pattern of masses is important to know because it provides a clue to help physicists understand what causes neutrinos to have masses that are so much lighter than other particles and why neutrinos aren’t massless as predicted by the Standard Model, the playbook for how the world works at the subatomic level.

Physicists think the origins of neutrino masses are closely tied to subatomic processes that took place right after the big bang. Determining which neutrino types are heaviest and lightest—the neutrino mass ordering—is a first step toward revealing these processes. Credit: symmetry magazine

NOvA is ideally situated to do discern this pattern because its particle beam will travel three times as farther than T2K’s, allowing researchers see how the material in the Earth alters the change from muon to electron neutrinos. T2K’s observation of half a dozen muon neutrino to electron neutrino changes points to the relatively high rate of the change, so NOvA should have a lot of data to work with to speed up the discovery of the mass hierarchy.

Step three is combining what NOvA learns about the mass hierarchy with more precise data from the LBNE experiment to look for differences in the neutrino and antineutrino probabilities of changing from muon to electron neutrino types. After accounting for the effect of the earth and the mass hierarchy, any remaining difference would point to a fundamental difference between matter and antimatter neutrinos. Differences between matter and anti-matter are nearly non-existent in nature and these differences are precious clues about why matter dominated antimatter to survive in today’s universe.

The three types of neutrinos mix across long distances enabling physicists to see them to change type if the distance is long enough. Credit: symmetry magazine

LBNE, proposed for South Dakota, sits even farther away from the Fermilab neutrino source, making it well-suited to make this comparison of antineutrinos, which are rarer and harder to detect than neutrinos. T2K’s observation of a large change signal means LBNE will have better statistics to create precise comparisons.
The level of precision could mean the difference between getting an answer or not, depending on how subtle the difference is between neutrinos and antineutrinos.

Bringing out the rifle scope:

Short-baseline experiments can’t compete in the hunt for why matter dominated antimatter, which requires tracking neutrinos across great distances, but they can provide the precision measurements that work like a rifle scope for the particle hunters. MINERvA at Fermilab and the neutrino reactor experiments Daya Bay in China and Double Chooz in France will provide the data to allow NOvA and LBNE to zoom in on the minute details of mass hierarchy and how neutrinos change types.

The reactor-based experiments with detectors near to neutrino spewing reactors were designed to be experts at finding the neutrino change T2K found. Ideally, they will find a cleaner neutrino transformation signal, without the data complications, such as the effects of Earth material on the transformation that come with T2K and NOvA being multi-purpose experiments. Cleaner reactor experiment measurements provide a baseline for the measurements of NOvA and LBNE.

MINERvA will provide data to help NOvA and LBNE map the type and amount of background events that can obscure their search. This will enable physicists to put the trophy deer-like potential discovery in their analysis cross-hairs and discount the imposter trees and hunters dressed in brown that cloud the view of their data. While MINERvA was built for this job and currently aids neutrino experiments across the globe, including T2K, with this variable-removing research information, T2K’s observation makes MINERvA’s unique skill more important. The large T2K signal means a lot of data and the ability to do precision analysis if MINERvA can tell researchers what variables to discount.

“There is always an exchange of data, and one experiment builds on another,” McFarland says.

Previously data from the MINOS experiment at Fermilab told T2K how to tune the energy of its particle beam. Now T2K is returning the favor with an observation that will help Fermilab experiments.

“Experiments building on one another,” he says, “that is what makes it exciting.”

Related information:

Symmetry breaking: Japan’s T2K experiment observes electrion neutrino appearance