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Posts Tagged ‘T2K’

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


This article first appeared in symmetry breaking June 24.

The MINOS far detector is located in a cavern half a mile underground in the Soudan Underground Laboratory in Minnesota. The collaboration records about 1,000 neutrinos per year. A tiny fraction of them seem to be electron neutrinos. Photo: Peter Ginter

Step by step, physicists are moving closer to understanding the evolution of our universe.  Neutrinos — among the most abundant particles in the universe –  could have played a critical role in the unfolding of the universe right after the big bang. They are strong candidates for explaining why the big bang produced more matter than antimatter, leading to the universe as it exists today.

Scientists of the MINOS experiment at the Department of Energy’s Fermi National Accelerator Laboratory announced today the results from a search for a rare phenomenon, the transformation of muon neutrinos into electron neutrinos. If this type of neutrino transformation did not exist, neutrinos would not break the matter-antimatter symmetry, and a lot of scientists would be scratching their heads and wonder what else could have caused the dominance of matter of antimatter in our universe.

The MINOS result is consistent with and significantly constrains a measurement reported 10 days ago by the Japanese T2K experiment, which announced an indication of this type of transformation.

The observation of electron neutrino-like events allows MINOS scientists to extract information about a quantity called sin2 2θ13. If muon neutrinos don’t transform into electron neutrinos, sin2 2θ13 is zero. The new MINOS result constrains this quantity to a range between 0 and 0.12, improving on results it obtained with smaller data sets in 2009 and 2010. Credit: Fermilab

The Main Injector Neutrino Oscillation Search (MINOS) at Fermilab recorded a total of 62 electron neutrino-like events. If muon neutrinos do not transform into electron neutrinos, then MINOS should have seen only 49 events. The experiment should have seen 71 events if neutrinos transform as often as suggested by recent results from the Tokai-to-Kamioka (T2K) experiment in Japan. The two experiments use different methods and analysis techniques to look for this rare transformation.

To measure the transformation of muon neutrinos into other neutrinos, the MINOS experiment sends a muon neutrino beam 450 miles (735 kilometers) through the earth from the Main Injector accelerator at Fermilab to a 5,400-ton neutrino detector, located half a mile underground in the Soudan Underground Laboratory in northern Minnesota.  The experiment uses two almost identical detectors: the detector at Fermilab is used to check the purity of the muon neutrino beam, and the detector at Soudan looks for electron and muon neutrinos. The neutrinos’ trip from Fermilab to Soudan takes about one four hundredths of a second, giving the neutrinos enough time to change their identities.

For more than a decade, scientists have seen evidence that the three known types of neutrinos can morph into each other. Experiments have found that muon neutrinos disappear, with some of the best measurements provided by the MINOS experiment. Scientists think that a large fraction of these muon neutrinos transform into tau neutrinos, which so far have been very hard to detect, and they suspect that a tiny fraction transform into electron neutrinos.

The observation of electron neutrino-like events allows MINOS scientists to extract information about a quantity called sin2 2θ13. If muon neutrinos don’t transform into electron neutrinos, sin2 2θ13 is zero. The new MINOS result constrains this quantity to a range between 0 and 0.12, improving on results it obtained with smaller data sets in 2009 and 2010. Credit: Fermilab

The observation of electron neutrino-like events in the detector in Soudan allows MINOS scientists to extract information about a quantity called sin2 2 theta-13 (pronounced sine squared two theta one three). If muon neutrinos don’t transform into electron neutrinos, this quantity is zero. The range allowed by the latest MINOS measurement overlaps with but is narrower than the T2K range. MINOS constrains this quantity to a range between 0 and 0.12, improving on results it obtained with smaller data sets in 2009 and 2010. The T2K range for sin2 2 theta-13 is between 0.03 and 0.28.

“MINOS is expected to be more sensitive to the transformation with the amount of data that both experiments have,” said Fermilab physicist Robert Plunkett, co-spokesperson for the MINOS experiment. “It seems that nature has chosen a value for sin2 2 theta-13 that likely is in the lower part of the T2K allowed range. More work and more data are really needed to confirm both these measurements.”

The MINOS measurement is the latest step in a worldwide effort to learn more about neutrinos. MINOS will continue to collect data until February 2012. The T2K experiment was interrupted in March when the severe earth quake in Japan damaged the muon neutrino source for T2K. Scientists expect to resume operations of the experiment at the end of the year. Three nuclear-reactor based neutrino experiments, which use different techniques to measure sin2 2 theta-13, are in the process of starting up.

The MINOS far detector is located in a cavern half a mile underground in the Soudan Underground Laboratory in Minnesota. The collaboration records about 1,000 neutrinos per year. A tiny fraction of them seem to be electron neutrinos. Photo: Peter Ginter

“Science usually proceeds in small steps rather than sudden, big discoveries, and this certainly has been true for neutrino research,” said Jenny Thomas from University College London, co-spokesperson for the MINOS experiment. “If the transformation from muon neutrinos to electron neutrinos occurs at a large enough rate, future experiments  should find out whether nature has given us two light neutrinos and one heavy neutrino, or vice versa. This is really the next big thing in neutrino physics.”

A large value of sin2 2 theta-13 is welcome news for the worldwide neutrino physics community and a boon for the NOvA neutrino experiment, under construction at Fermilab. The experiment is designed to determine the neutrino mass hierarchy. It will find out whether there are one light and two heavy neutrinos, or whether there are two light neutrinos and a heavy one. Together with several nuclear physics experiments, such as EXO and Majorana, NOvA will help scientists determine what early-universe theories are the most viable ones.

To measure directly the matter-antimatter asymmetry hidden among the neutrino transformations, scientists have proposed the Long-Baseline Neutrino Experiment. It would send neutrinos on a 1,300-kilometer trip from Fermilab to a detector in South Dakota. This would give muon neutrinos more time to transform into other neutrinos than any other experiment. It would give scientists the best shot at observing whether neutrinos break the matter-antimatter symmetry and by how much. For more information about MINOS, NOvA and LBNE, visit the Fermilab neutrino website:

The MINOS experiment involves more than 140 scientists, engineers, technical specialists and students from 30 institutions, including universities and national laboratories, in five countries: Brazil, Greece, Poland, the United Kingdom and the United States. Funding comes from: the Department of Energy Office of Science and the National Science Foundation in the U.S., the Science and Technology Facilities Council in the U.K; the University of Minnesota in the U.S.; the University of Athens in Greece; and Brazil’s Foundation for Research Support of the State of São Paulo (FAPESP) and National Council of Scientific and Technological Development (CNPq).

Kurt Riesselmann


— by Nigel S. Lockyer, Director

The Canadian Association of Physicists (CAP) met last week in St. John’s, Newfoundland (a huge island it turns out), off the most eastern part of Canada. Newfoundland-Labrador (NL) (one of 10 Canadian provinces…joined the federation in March 31, 1949) is remote with a ruggedly beautiful coastline and—at this time of year—cool, rainy, and foggy. NL is famous for icebergs floating by the coast, pods of whales, schools of capelins, and millions of sea birds. Check out the maps, weather, and iceberg tracking.

Capelin fish.

A capelin is the fish the whales eat. They come to shore to spawn in June and July, followed by the whales, and everyone in town benefits, except of course the capelins. Looks like a “lycoptera” to me.

Rolf-Dieter Heuer, DG of CERN, attended the first day of the meeting and gave a public lecture in the evening to the conference delegates, university students, and local citizens. Standing-room only in an auditirium for at least 1,000!  It was a superb and captivating lecture for physicists outside of particle physics and for the public alike. Rolf is a tremendous spokesperson for CERN and particle physics.   During his daytime itinerary, Rolf participated in several sessions and panel discussions where he spoke passionately about the opportunity for Canada to work with CERN more closely as one of the first “associate members” from overseas.

One of the CAP meeting highlights was the T2K result, which reported a 2.5 sigma effect on theta-13, an angle that measures the degree to which flavours “1” and “3” of neutrinos change back and forth into one another. This result, if it holds, has major implications for the next-generation long-baseline neutrino experiments being discussed around the world. It is possible decisions will take place about proceeding to search for CP violation in the neutrino sector in the next 5 years…a billion dollar program wherever it is built.

Another conference highlight was the inaugural award of the CAP-TRIUMF Medal for Subatomic Physics named after Erich W. Vogt, one of the founders of TRIUMF and an early director of the laboratory.  Professor Vogt travelled to Newfoundland for the conference specifically for the purpose of handing the medal to David Sinclair (a professor at Carleton and a senior research scientist at TRIUMF) for his contributions to the SNO experiment.  It was a special moment as David acknowledged that he’d always seen Erich as a mentor.

Although most of the conference was work, we did get a few minutes to go outside and look around.  Touring the local historical sites was fun. Almost everyone visited Signal Hill (site of the first radio transmission across the Atlantic) and Cape Spear, the most eastern point in Canada. Cape Spear has the second oldest lighthouse in Canada. The tour, given by a young woman from Labrador (the first person I have met from Labrador), was fascinating. The lighthouse had been run for seven generations by the same family….yes seven. They hired a technician to keep the lighthouse maintenance up to snuff and to rewind the clockwork mechanism every three hours. This individual lived in a small room in the lighthouse, next to his minimal work shop.  The room was so cold in winter that our Labrador tour guide said the “contents of the pisspot froze” (FYI — “piss” is an acceptable word in haute-Canada).  I also learned about the big technological advance in lamps: when the wick on oil lamps was upgraded to be cylindrical rather than flat and a fluted glass chimney was attached. The round wick improved oxygen flow and most importantly increased light output versus a candle by a factor of seven and eliminated the smoke and hence the need to clean the glass chimneys and Fresnel lense so often. The Swiss physicist Argand is credited with this innovation in 1781.

Barrerl of sperm-whale oil.

The whales they caught provided the oil for the lamps. Barrels of sperm whale oil were stored next to the maintenance man’s bedroom and his piss-pot.

The final topic to share is the controversy over the Canadian sealing industry, strongly supported in NL. If you are inclined, check out http://www.ifaw.org/ifaw_canada_english/ or for the other side of the argument see http://speeches.empireclub.org/61890/data?n=20

I’ll remember this trip because I watched the Vancouver Canucks lose the seventh game of the Stanley Cup to Boston in a local St. John’s bar, a piss…, made worse by all the local Boston fans!  (just kidding)  It was a privilege to have our team in the finals.