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New T2K results shed light on the Universe

Feature | June 22, 2011 | By Kurt Riesselmann

The T2K experiment has detected six candidate events for oscillations of muon neutrinos into electron neutrinos. In this graphic, each colored dot shows the light detected when an electron neutrino hit a water molecule inside the Super-Kamiokande detector and created an electron that led to the emission of Cerenkov light.that allows scientists to better understand a phenomenon known as neutrino oscillations. For a long time scientists have suspected that the three known types of neutrinos can morph into each other. Several experiments previously found neutrinos to disappear. Image courtesy of T2K.

The T2K experiment in Japan has observed six particle events that indicate the oscillation of muon neutrinos into electron neutrinos, a long-sought signal that allows scientists to better understand a phenomenon known as neutrino oscillations.

For a long time scientists have suspected that the three known types of neutrinos can morph into each other. Several experiments previously found neutrinos to disappear. But the T2K experiment is the first experiment to report candidate events for the appearance of electron neutrinos in a muon neutrino beam.

The experiment analyses a muon neutrino beam that travels 295 kilometers through the earth from the Japan Proton Accelerator Research Complex (J-PARC) to the 50,000-ton Super-Kamiokande neutrino detector.

T2K uses LHC Computing Grid tools to maintain its own tiered system for data analysis, which makes use of resources worldwide. Scientists in different regions handle data in their own fashion; in Europe, for example, T2K has an EGI virtual organization, t2k.org, which they use to authenticate and distribute data. (Check out "A neutrino's journey" to read more about the computing behind T2K.)

Based on the analysis of the data the experiment collected between January 2010 and 11 March 2011, when the experiment was interrupted due to the magnitude 9 earthquake in eastern Japan, scientists found 88 neutrino events. Among these 88 events, they identified six candidate events as electron neutrino interactions.

The T2K result indicates that the last unknown neutrino mixing angle, called theta-13, is non-zero, with a significance of 2.5 sigma. If theta-13 were zero, the T2K experiment should only have found one or two electron neutrino events.

The result is good news for other neutrino experiments.

“This is a great result for NOvA,” said Mark Messier, of Indiana University and co-spokesperson of the NOvA neutrino experiment under construction at Fermilab. “It means that NOvA will be able to solve the question of the neutrino mass ordering, a question that no other neutrino experiment in operation or under construction can address. NOvA will let us know whether there are one light and two heavy neutrinos, or whether there are two light neutrinos and a heavy one.”

In addition, a non-zero value of theta-13 opens up the possibility that neutrinos violate the matter-antimatter symmetry and could be the reason that matter dominates over antimatter in our universe, a phenomenon that has puzzled scientists for a long time. The proposed Long-Baseline Neutrino Experiment is designed to look for this effect.

“This is a great time for neutrino physics,” Messier said.

A version of this article first appeared on Symmetry Breaking.