Experimental Particle Physics Research Group


Sussex scientists observe ghostly antiparticles that could unlock secrets of the universe

NOvA experiment - electron antineutrino

Physicists from the University of Sussex have observed electron antineutrino appearance – oscillations of these ghostly antiparticles could hold the key to understanding some of the most fundamental mysteries of the universe. The team are part of the NOvA experiment – a global collaboration of scientists studying neutrino particles – who are announcing the first antineutrino findings from their experiment today, at the Neutrino 2018 conference in Heidelberg, Germany.  

For more than three years, scientists on the NOvA collaboration have been observing neutrinos as they oscillate from one type to another over a distance of 500 miles. In the results unveiled today, NOvA scientists saw strong evidence of muon antineutrinos oscillating into electron antineutrinos over long distances, a phenomenon that has never before been observed.

The University of Sussex is one of only two UK institutions participating on the NOvA experiment, along with University College London. Professor Jeff Hartnell, Professor of Physics at the University of Sussex comments on the findings:

“I've been researching these ghostly neutrino particles for 17 years and this is one of the most exciting results of my career. We've seen for the first time the appearance of electron antineutrinos, which was postulated over 50 years ago. More than just seeing this for the first time though, is what it says about the potential of our future measurements.

“Our ultimate goal is to improve our measurements to help answer questions such as why the universe is dominated by matter and not antimatter, and how galaxies cluster together on large scales. Over the next five years we will make further measurements with the NOvA experiment while we build the future DUNE experiment to really understand how these ghostly particles work.”

PhD student Diana Mendez, who won a Chancellors’ International Scholarship to study at the University of Sussex, was the first person in the world to see the muon antineutrino part of this new result. She adds:

“Being the first to see the data felt like opening a present, but much more intriguing and gratifying. I was very excited that day, so I could barely do anything else except wait to get permission to see the data. There was nothing I could compare it to because we had never seen this before; I was honestly surprised by the results.”

NOvA uses two large particle detectors – a smaller one at Fermilab in Illinois, and a much larger one 500 miles away in northern Minnesota – to study a beam of particles generated by Fermilab’s accelerator complex and sent through the earth, with no tunnel required. Based at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, NOvA is the world’s longest-baseline neutrino experiment. Its purpose is to discover more about neutrinos, ghostly yet abundant particles that travel through matter mostly without leaving a trace. The experiment’s long-term goal is to look for similarities and differences in how neutrinos and antineutrinos change from one type – in this case, muon – into one of the other two types, electron or tau. Precisely measuring this change in both neutrinos and antineutrinos, and then comparing them, will help scientists unlock the secrets that these particles hold about how the universe operates. 

The new result is drawn from NOvA’s first run with antineutrinos, the antimatter counterpart to neutrinos. NOvA began studying antineutrinos in February of 2017. Fermilab’s accelerators create a beam of muon neutrinos (or muon antineutrinos), and NOvA’s far detector is specifically designed to see those particles changing into electron neutrinos (or electron antineutrinos) on their journey.

If antineutrinos did not oscillate from muon type to electron type, scientists would have expected to record just five electron antineutrino candidates in the NOvA far detector during this first run. But when they analyzed the data, they found 18, providing strong evidence that antineutrinos undergo this oscillation.

Co-spokesperson of the NOvA collaboration, Peter Shanahan from Fermilab comments: “Antineutrinos are more difficult to make than neutrinos, and they are less likely to interact in our detector. This first data set is a fraction of our goal, but the number of oscillation events we see is far greater than we would expect if antineutrinos didn’t oscillate from muon type to electron.  It demonstrates the impact that Fermilab’s high-power particle beam has on our ability to study neutrinos and antineutrinos.”

The key to NOvA’s science programme is comparing the rate at which electron neutrinos appear in the far detector with the rate that electron antineutrinos appear. A precise measurement of those differences will allow NOvA to achieve one of its main science goals: to determine which of the three types of neutrinos is the heaviest, and which the lightest.

Neutrinos have been shown to have mass, but scientists have not been able to directly measure that mass. However, with enough data, they can determine the relative masses of the three, a puzzle called the mass ordering. NOvA is working toward a definitive answer to this question.

The NOvA collaboration includes more than 240 scientists from nearly 50 institutions in seven countries: Brazil, Colombia, Czech Republic, India, Russia, the UK and the U.S. For more information visit the experiment’s website at http://novaexperiment.fnal.gov.

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By: Alice Ingall
Last updated: Tuesday, 5 June 2018

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