When neutrinos switch type

Print edition : September 04, 2015

SCIENTISTS on the NOvA experiment saw their first evidence of oscillating neutrinos, confirming that the extraordinary detector built for the project not only functions as planned but is also making great progress towards its goal of a major leap in our understanding of these ghostly particles.

NOvA is on a quest to learn more about the mysterious particles called neutrinos, which flit through ordinary matter as though it was not there. While researchers know that neutrinos come in three types—electron neutrino, muon neutrino and tau neutrino— – they do not know the mass hierarchy of these, which is the heaviest and which is the lightest. Figuring out this ordering is one of the goals of the NOvA experiment, which could lead us to understand how the neutrino gets its mass. The measurement of the neutrino mass hierarchy is also crucial information for neutrino experiments trying to see if the neutrino is its own antiparticle.

Neutrinos also have the peculiar quantum mechanical property called neutrino oscillations by which they can change from one type to another as they travel in space. Oscillations of electron-type neutrinos from the sun, muon-type neutrinos produced by cosmic rays in the atmosphere, electron-type antineutrinos from reactors, and muon-type neutrinos from accelerators have been seen by several experiments, thus verifying this strange phenomenon.

NOvA is also an experiment designed to study the behaviour of the neutrino beam from the Fermilab accelerator. It has two detectors, a near detector close to Fermilab, Batavia, Illinois, and another massive far detector located about 800 km away in Ash River, Minnesota, weighing about 14,000 tonnes and measuring about 15 m x 15 m x 100 m in size.

The first NOvA results were released recentlythis week at the American Physical Society conference in Ann Arbor, Michigan. The neutrino beam generated at Fermilab passes through the underground near detector, which measures the beam’s neutrino composition before it leaves the Fermilab site. The particles then travel 810 km straight through the earth, oscillating (or changing types) along the way. The beam originating at Fermilab is made almost entirely of one type, muon neutrinos, and scientists measure how many of those muon neutrinos disappear over their journey and reappear as electron neutrinos.

If oscillations did not occur, experimenters predicted they would see 201 muon neutrinos arrive at the NOvA far detector in the data collected; instead, they saw a mere 33, proof that the muon neutrinos were disappearing as they transformed into the two other flavours. Similarly, if oscillations did not occur, scientists expected to see only one electron neutrino appearance (due to background interactions). But the collaboration saw six such events, evidence that some of the missing muon neutrinos had turned into electron neutrinos. NOvA will take data for at least six years.

The NOvA collaboration comprises 210 scientists and engineers from 39 institutions in the United States, Brazil, the Czech Republic, Greece, India, Russia and the United Kingdom.

R. Ramachandran

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