A joint Fermilab/SLAC publication

Fermilab experiment weighs in on neutrino mystery


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 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).

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