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Fermilab | Batavia, IL | USA

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Japanese T2K neutrino observation a boon for U.S. physics

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

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