MINOS serves up more neutrino evidence

The MINOS far detector is located in a cavern half a mile underground in the Soudan Underground Laboratory, Minnesota. The 100-foot-long MINOS far detector consists of 486 massive octagonal planes, lined up like the slices of a loaf of bread. Each plane consists of a sheet of steel about 25 feet high and one inch thick, with the last one visible in the photo. The whole detector weighs 6,000 tons.

The University of Minnesota Foundation commissioned a mural for the MINOS cavern at the Soudan Underground Laboratory, painted onto the rock wall, 59 feet wide by 25 feet high. The mural contains images of scientists such as Enrico Fermi and Wolfgang Pauli, Wilson Hall at Fermilab, George Shultz, a key figure in the history of Minnesota mining, and some surprises.

Image courtesy of Fermilab.

The evidence for the transformation of muon neutrinos into electron neutrinos is mounting: The Fermilab-based MINOS experiment announced June 24 that they had observed evidence for the rare phenomenon.

The MINOS (Main Injector Neutrino Oscillation Search) announcement came only ten days after the Japan-based Tokai-to-Kamioka (T2K) collaboration announced its own findings (read about it here). MINOS, which is analyzes data using grid computing, recorded a total of 62 electron neutrino-like events. The total would have been only 49 events if muon neutrinos didn’t transform into electron neutrinos at all. And if those transformations are as frequent as the T2K results suggest, MINOS would have observed 71 events.

The two experiments use different methods and analysis techniques to study neutrino transformation, which makes it possible for them to independently confirm results with a greater level of confidence.

To measure the transformation of muon neutrinos into other neutrinos, the MINOS experiment sends a muon neutrino beam 735 kilometers through the earth from the main injector accelerator at Fermilab to a 5,000-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 four hundredths of a second, giving the neutrinos enough time to change their identities.

The observation of electron neutrino-like events in the detector in Soudan allows MINOS scientists to extract information about a quantity called sin² 2θ₁₃ (θ₁₃ is the mixing angle which indicates the rate at which muon neutrinos transform into electron neutrinos). If muon neutrinos don’t transform into electron neutrinos, this quantity is zero.

The new MINOS result constrains sin² 2θ₁₃ to a range between 0 and 0.12, improving on results it obtained with smaller data sets in 2009 and 2010. The MINOS range is consistent with the T2K range for sin² 2θ₁₃, which is between 0.03 and 0.28. According to the T2K data, the most likely value is 0.11. The MINOS result prefers a value of 0.04, and its data indicates that sin² 2θ₁₃ is non-zero at the 89% confidence level. Image and caption courtesy of Fermilab.

The range of values for sin² 2θ₁₃ allowed by the latest MINOS measurement – 0 to 0.12 – overlaps with but is narrower than the range found by T2K (0.03 to 0.28); this new MINOS range of values improves on results obtained with smaller MINOS data sets in 2009 and 2010.

“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 2θ₁₃ 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.”

As MINOS’ data sets have grown, so have their computing needs. Over the last year, MINOS data analysis use of Open Science Grid has grown from 3.1 million core hours per year to about 5.6 million. MINOS also managed over four million file transfers in the last year. MINOS combines OSG resources with 180 terabytes of dedicated BlueArc (NFS mounted) file storage and several hundred cores of offsite computing at collaborating universities, where they do the occasional Monte Carlo generation.

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 2θ₁₃, are in the process of starting up.

“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.”